The potential for abiotic organic synthesis and biosynthesis at seafloor hydrothermal systems

Abstract

Geofluids (2010) 10, 161–192

Abstract

Calculations are presented of the extent to which chemical disequilibria are generated when submarine hydrothermal fluids mix with sea water. These calculations involve quantifying the chemical affinity for individual reactions by comparing equilibrium compositions with the compositions of mixtures in which oxidation–reduction reactions are inhibited. The oxidation–reduction reactions that depart from equilibrium in these systems provide energy for chemotrophic microbial metabolism. Methanogenesis is an example of this phenomenon, in which the combination of carbon dioxide, hydrogen and methane induced by fluid mixing is far from equilibrium, which can be approached if more methane is generated. Similarly, the production of other organic compounds is also favorable under the same conditions that permit methanogenesis. Alkanes, alkenes, alcohols, aldehydes, carboxylic acids and amino acids are among the compounds that, if formed, would lower the energetic state of the chemical composition generated in mixed fluids. The resulting positive values of chemical affinity correspond to the thermodynamic drive required for abiotic organic synthesis. It is also possible that energy release accompanies biosynthesis by chemotrophic organisms. In this way, hydrothermal ecosystems differ radically from familiar ecosystems at Earth’s surface. If captured, the energy released may be sufficient to drive biosynthesis of carbohydrates, purines, pyrimidines and other compounds which require energy inputs.

Introduction

Twelve years ago, evidence supporting the ideas that abiotic synthesis of organic compounds could occur in hydrothermal processes, and that biosynthesis of organic compounds benefited from hydrothermal conditions, came from theoretical calculations of the geochemical and thermodynamic consequences of mixing a submarine hydrothermal fluid with sea water (McCollom & Shock 1997; Amend & Shock 1998; Shock & Schulte 1998). These studies showed that mixing produced states of disequilibrium in which metabolic and organic synthesis reactions became thermodynamically favored. As an example, it was shown quantitatively that reductive metabolisms, such as methanogenesis and sulfate reduction, would release energy at higher temperatures, and that oxidative metabolisms, such as methanotrophy and sulfide oxidation, would release energy at lower temperatures (McCollom & Shock 1997). It was also shown that present conditions where marine hyperthermophiles thrive are far more conducive to synthesis of all 20 protein‐forming amino acids than is synthesis in surface sea water, and that overall synthesis reactions are exergonic (energy releasing) for 11 of them (Amend & Shock 1998). Exergonic biosynthesis reactions are not at all what humans experience at the Earth’s surface where formation of any organic compound in a 20% oxygen atmosphere requires input of energy. Exploration of abiotic organic synthesis during mixing of hydrothermal fluids and sea water on the early Earth and Mars, with sea water compositions far less oxidized than at present, revealed thermodynamic drives for the formation of many classes of organic compounds (Shock & Schulte 1998), and set the stage for further speculations about the emergence of life in hydrothermal systems (Shock 1997; Shock et al. 1998; Russell & Martin 2004; Russell et al. 2005; Martin & Russell 2007). These advances were driven by a single model of the mixing of sea water and a submarine hydrothermal fluid composition based on measurements made on fluids from 21°N on the East Pacific Rise (EPR).

Over the intervening years, several lines of evidence from experiments and observations of nature show that abiotic organic synthesis is indeed possible at hydrothermal conditions. At the same time, the ability to consider biosynthesis in more detail through theoretical mixing calculations has improved owing to the expansion of the number and variety of biomolecules that can be included. These developments provided the impetus for the present study, which involved a comparison of seven submarine hydrothermal fluids from all of the major recognized biogeographic zones.

Recent observations of natural submarine hydrothermal systems show the presence of dissolved organic compounds that may be the products of abiotic organic synthesis. As an example, Proskurowski et al. (2008) provide concentration, stable isotopic and radiocarbon evidence that C₁–C₄ hydrocarbons have an abiotic source in the fluids venting from Lost City at 30°N on the Mid‐Atlantic Ridge. The Lost City hydrothermal field occurs in ultramafic rocks, and is characterized by highly basic fluids (pH 9–11) emanating at relatively low temperatures (28–90°C). These authors prefer a model of light hydrocarbon generation through a Fischer–Tropsch‐type synthesis (see McCollom & Seewald 2007) involving mineral surfaces to catalyze the reduction of CO₂(aq) at H₂(aq)‐enriched conditions. The dissolved hydrogen is a product of the reduction of H₂O as ferrous silicates are oxidized to yield magnetite (Fe₃O₄). Overall, the process involves coupled oxidation of iron‐bearing minerals and reduction of H₂O, accompanied or followed by reduction of CO₂(aq), argued to be mantle derived (Proskurowski et al. 2008), to methane and other light hydrocarbons catalyzed by mineral surfaces. Analogous processes are invoked to explain the synthesis of hydrocarbons, carboxylic acids and other organic compounds in higher temperature seafloor systems hosted in ultramafic rocks undergoing serpentinization (Charlou et al. 2002; Konn et al. 2009).

Experimental studies have also demonstrated the potential for organic synthesis at conditions meant to mimic those in natural hydrothermal systems. Hydrocarbons were the focus of a study by Foustoukos & Seyfried (2004) who conducted Fischer–Tropsch‐type experiments at 390°C and 400 bars. Experiments that included the simultaneous formation of the mineral chromite (FeCr₂O₄) yielded the highest concentrations of methane, ethane and propane, suggesting a catalytic role for chromium. Synthesis was confirmed by using ¹³C‐labeled NaHCO₃ as the carbon source. These experiments were designed to test the idea that abiotic hydrocarbon synthesis happens at high‐temperature ultramafic‐hosted seafloor systems like those at Rainbow and Logatchev on the Mid‐Atlantic Ridge. Nevertheless, there could be similarities in the overall abiotic synthesis processes happening in cooler systems like Lost City, especially if conductive cooling is as extensive as some recent models predict (Foustoukos et al. 2008).

While progress was being made in hydrothermal abiotic organic synthesis through experiments and analysis of natural samples, several breakthroughs allowed the expansion of thermodynamic data for aqueous organic compounds at hydrothermal conditions. All of these developments were built on the revised Helgeson–Kirkham–Flowers equation of state for aqueous species (Shock et al. 1992). Data and parameters for carbohydrates were added by Amend & Plyasunov (2001), allowing the first thermodynamic analysis of metabolism of sugars at elevated temperatures and pressures. Group contribution algorithms and other estimation methods made it possible to revise data for amino acids and to make predictions for polypeptides and unfolded proteins (Dick et al. 2006), and for purines, pyrimidines, nucleotides, nucleosides and many other biomolecules involved in energy and electron transfer and the formation of DNA and RNA (LaRowe & Helgeson 2006a,b). These advances make it possible to consider biosynthesis of organic compounds by microbes in hydrothermal systems, together with abiotic organic synthesis that may be occurring in the same systems.

As several lines of evidence have advanced the notion that abiotic synthesis of organic compounds can occur in hydrothermal systems, it seems timely to investigate the energetic state of overall organic synthesis reactions that may be occurring, so that greater detail about pathways of abiotic organic synthesis might be suggested. At the same time, identifying novel conditions that are energetically favorable for abiotic organic synthesis can help in predicting new biosynthetic couplings that lower overall energy costs. The appearance of new data and parameters for biomolecules presents an opportunity to begin exploring the overall energy requirements of biosynthesis in thermophilic microbes that populate mixing zones of submarine hydrothermal systems.

Diversity of submarine hydrothermal fluids

Hydrothermal systems are known to occur on ridges and in back‐arc basins regardless of spreading or convergence rates. Fluids venting from diverse tectonic settings vary in composition in large part because of differences in the composition of the rocks that host the hydrothermal systems. Early discoveries along the EPR and the Mid‐Atlantic Ridge revealed hydrothermal systems hosted in basalt, and resulting fluids are sometimes considered to be typical of the hydrothermal input to the oceans. Basalt‐hosted fluids are relatively reduced, mildly acidic, sulfidic and enriched in metals. Subsequently, it was learned that submarine hydrothermal systems can be hosted in ultramafic rocks, leading to fluids that are more reduced than their basaltic counterparts (Wetzel & Shock 2000; Wetzel et al. 2001). Fluids can also be more acidic than from basalt‐hosted systems, notably those from andesitic rocks of back‐arc spreading centers. Sediments are present in some fluid‐recharge zones and in some cases cover hydrothermally active ridges. Reactions involving sediments and fluids can yield greater concentrations of carbon dioxide and ammonia than unsedimented basalt‐hosted fluids. Compositions of fluids are listed in Table 1 from several seafloor settings including mid‐ocean ridges and back‐arc basins from the locations shown in Fig. 1. These fluids were chosen to represent a large proportion of the diversity encountered in submarine hydrothermal systems. They also are from each of the major recognized biogeographic zones of seafloor hydrothermal ecosystems.

Table 1. Compositions of hydrothermal vent fluids and sea water used in model calculations.

	BSW	Rainbow	Guaymas	Endeavour	Lau	9°N	TAG	Kairei
		MAR	Basin	Juan de Fuca	Basin	EPR	MAR	CIR
System type		MOR	MOR	MOR	BAB	MOR	MOR	MOR
Host rock	–	Ultramafic	Basalt	Basalt	Basalt/ande‐site	Basalt	Basalt	Basalt/gab‐bro?
Sediments	–	No	Yes	May be	No	No	No	No
T (°C)	2	350	315	345	334	350	350	349
pH (25°C)	7.8	2.8	5.9	4.5	2.	2.6	3.1	3.48
O2(aq)	0.1	–	–	–	–	–	–	–
H2(aq)	–	16	3.4	0.62	0.5	1.8	0.37	8.19
∑H2S(aq)	–	1.2	59.8	20	13.1	8.71	6.7	3.97
CH4(aq)	0.0000003	2.5	63.4	3.4	0.06	0.01	0.147	0.191
∑CO2	2.3	16	61.1	22	20	11.4	3.4	5.13
∑	27.9	∼0	∼0	–	–	–	–	1.56
∑	0.00075	–	–	–	–	–	–	0.00139
	0.0003	–	–	–	–	–	–	–
	–	0.1	15.6	1.0	5.0	0.01	0.65	0.0388
SiO2(aq)	0.16	6.9	13.8	17	14.5	20	22	16.7
Na^+	464	533	513	319	590	683	584	511
Cl^−	546	750	637	235	489	846	659	595
Br^−	0.84	1.18	5.3	0.895	1.14	1.35	1.045	0.95
K^+	9.8	20.4	48.5	29	79	41.5	18	14.5
Al^3+	0.00002	–	0.0079	0.0019	0.006	0.0052	0.01	0.00528
Ca^2+	10.2	66.6	41.5	4.3	41.3	45.6	26	30.2
Mg^2+	52.7	∼0	∼0	∼0	∼0	∼0	∼0	∼0
Fe^3+	0.0000015	–	–	–	–	–	–	–
Fe^2+	–	24.1	0.18	1.36	2.5	12.1	5.17	4.6
Ba^2+	0.00014	0.008	0.053	0.005	0.04	0.008	0.02	0.0425
Mn^2+	0	2.25	0.236	0.55	7.1	3.28	0.71	0.829
Cu^2+	0.000007	0.16	0.0011	0.021	0.034	0.035	0.13	0.245
Zn^2+	0.00001	0.185	0.04	0.035	3	0.106	0.083	0.0895
Pb^2+	∼0	–	0.000652	0.00051	0.0039	0.000308	0.00011	0.00047

• All concentrations in mmolal. Bottom Seawater (BSW), Mid‐Atlantic Ridge (MAR), East Pacific Rise (EPR), Central Indian Ridge (CIR), mid‐ocean ridge (MOR), back‐arc basin (BAB). BSW composition from Bruland (1983) and McCollom (2007, 2008). Rainbow data from Charlou et al. (2002). Guaymas data from Von Damm et al. (1985b, 2005) and Campbell et al. (1988b), and from compilation by Gamo (1995) where original sources are: Piepgras & Wasserburg (1985), Lupton (1983), Campbell et al. (1988b), Welhan & Lupton (1987) and Gieskes et al. (1988). Endeavour vent fluid from Lilley et al. (1993) and Butterfield et al. (1994) in Gamo (1995) and Seewald et al. (2003); Al³⁺, Ba²⁺ and Pb²⁺ from Juan de Fuca Ridge S. Cleft Segment, used in the absence of readily available data for Endeavour, are from Hinkley & Tatsumoto (1987), Trefrey et al. (1994), Philpotts et al. (1987), Von Damm & Bischoff (1987) and Evans et al. (1988). Lau Basin vent fluid composition from Fouquet et al. (1991a,b, 1993), and Charlou et al. (1991); methane, ∑CO₂, H₂(aq), ∑H₂S from maximum values presented in Tivey (2007) for back‐arc basins; Al³⁺ from White Lady vent in north Fiji back‐arc basin (Ishibashi et al. 1994a,b). EPR 9°N compositions from Von Damm et al. (1991, 1997), Shanks et al. (1991), Lilley et al. (1991) and Lupton et al. (1991); Al³⁺, Ba²⁺, Zn²⁺, Pb²⁺, NH₄⁺, Cu²⁺ concentrations from EPR 21°N OBS (Von Damm et al. 1985a) used in the absence of readily available data. TAG data from Campbell et al. (1988a), Douville et al. (2002) and Charlou et al. (1996, 2002); NH₄⁺ from maximum values presented in Tivey (2007) for mid‐ocean ridges. Kairei data from Gallant & Von Damm (2006) and Kumagai et al. (2008).

Figure 1

Geologic and tectonic setting of seafloor hydrothermal systems modeled in this investigation. Color coding is maintined in subsequest figures. Data from Baker et al. (1995), Van Dover et al. (2002), German & Von Damm (2004), Hannington et al. (2005), Koschinsky et al. (2006), Tivey (2007) and German et al. (2008).

While our understanding of biogeographic zones is far from complete, studies of the evolutionary history of vent and seep invertebrates has allowed the identification of six different biogeographic zones (Van Dover et al. 2002). Many of these tend to correlate well with ocean basins and degrees of isolation along the mid‐ocean ridge system (Tunnicliffe & Fowler 1996). Oceanic circulation patterns and Cenozoic tectonic history were important in defining biogeographic zones and patterns that are recognized today (Van Dover et al. 2002). An example of this is evident in the differentiation exhibited by communities on the EPR that are separated by many hundreds of kilometers from those of the north‐east Pacific ridge system. These communities have been diverging since the formerly continuous ridge system was split ∼28 Ma by the over‐riding North American Plate (Tunnicliffe 1988). By contrast, on the Mid‐Atlantic Ridge there are differences among communities that are relatively close to one another. It is argued that emplacement of the Azores plateau ∼20 Ma may have served to isolate biological communities at hydrothermal systems to the north and south (Cannat et al. 1999; Van Dover et al. 2002).

East Pacific Rise (9°N)

The EPR exhibits fast spreading with a rate of ∼11 cm year⁻¹ (Klitgord & Mammerickx 1982; Carbotte & Macdonald 1992). High‐temperature hydrothermal vent systems at EPR were first discovered in 1979 (Macdonald et al. 1980; Spiess 1980), and since then the area along 9–11°N has become a major focus site for research including towed camera surveys, use of Argo I, ocean bottom seismometers and over 300 Alvin dives, the first of which occurred in March/April 1991 as part of an extensive fluid sampling program (Fornari et al. 1990, 1998a,b; Haymon et al. 1991, 1993; Lilley et al. 1991; Lupton et al. 1991; Shanks et al. 1991; Von Damm et al. 1991, 1997; Wright et al. 1995; Gregg et al. 1996; Sohn et al. 1998; Kurras et al. 2000; Engels et al. 2003; Von Damm 2004).

The 9°N vent field lies at a depth of ∼2500 m, and evidence from multi‐channel seismic data shows the presence of a thin crustal magma chamber ∼1.5 km beneath the ridge axis (Detrick et al. 1987). The axial summit trough is approximately 60 m across and is the site of active hydrothermal venting with the entire system experiencing extensive volcanism (Fornari et al. 2004). Seafloor eruptive events occurred in 1991 and 1992 (Haymon et al. 1993; Shank et al. 1998), as well as in the months preceding April 2006 which elicited a series of response cruises sponsored by the Ridge 2000 program to verify the recent activity (Lilley et al. 2006; Von Damm et al. 2006; Cowen et al. 2007). Continued volcanism throughout the life span of the ridge at EPR, in conjunction with the high spreading rate and entrainment of fluid and precipitates upward into plumes, is responsible for the relatively diminutive size of the vent deposits (Tivey 2007). However, the continued volcanic activity has also added to the diversity of venting as both black and white smokers dot the vent field and low‐temperature diffuse flow is observed exiting from the cracks and crevices in the basaltic floor (Haymon & Kastner 1981). At these locations of diffuse flow there is mixing between cool sea water and hot hydrothermal fluid within the crust. Some chemical species do not behave conservatively during mixing, indicating that there may be subsurface microbial communities consuming H₂S, H₂ and CO₂ while at the same time producing other sulfur and carbon species (Von Damm & Lilley 2004).

The high‐temperature (330–405°C) black smokers do not seem to entrain much sea water because the Mg concentrations in samples are very low (Von Damm 2004). All black smoker vents within the hydrothermal system at 9°N are acidic and have Cl concentrations indicating that phase separation occurs within the crust (Von Damm 2004). These fluids are enriched in CO₂ owing to phase separation and/or magmatic degassing on this section of the ridge (Lilley et al. 2002; Von Damm & Lilley 2004), which may also account for some of the low pH values and elevated sulfide concentrations. The EPR 9°N system has experienced considerable activity over the course of its existence and it is thought that the changing fluid compositions reflect magma migration within the crust on shorter time scales than have been observed in other hydrothermal vent systems (Von Damm 2004).

Trans‐Atlantic Geotraverse

The Trans‐Atlantic Geotraverse (TAG) location hosts the single largest known vent deposit on any spreading center (Tivey 2007). TAG has been active for the last ∼140 Kyr (Lalou et al. 1995) and possesses both black and white smokers as well as zones of diffuse venting. The large sulfide deposit, known as TAG mound, is the focal point for high‐temperature activity (Kleinrock & Humphris 1996; Humphris & Tivey 2000) and measures almost 200 m in diameter with a height of almost 50 m (Humphris et al. 1995). Venting at TAG was first sampled during an Alvin expedition in 1986. The typical exit temperature for vent fluids on the mound are in excess of 360°C (Campbell et al. 1988a; Chiba et al. 2001; Parker & Von Damm 2005).

Newer near‐bottom magnetic data seem to imply that there is crustal thinning from long‐lived extension on a normal fault (Tivey et al. 2003; deMartin et al. 2007) at the site of TAG. This in turn would indicate that TAG is in fact perched on the hanging wall of an active detachment fault, meaning that it is the tectonic setting of the site rather than the volcanism that is more crucial to the long‐lived hydrothermal activity and circulation, which has allowed TAG to grow to such a large size (deMartin et al. 2007). Over the course of its activity, continuous sea water entrainment has instigated precipitation of anhydrite, chalcopyrite and pyrite within the mound and remobilized metals (Edmond et al. 1995; Tivey et al. 1995). A sequence of pyrite, anhydrite, silica and chloritized basalt breccias and stockwork beneath the mound were revealed during the Ocean Drilling Program’s recovery of rock cores (Humphris et al. 1995). These characteristics prove to be closely matched by those of the Cyprus massive sulfide deposit (Hannington et al. 1998).

Endeavour

Located near the northern end of the Juan de Fuca Ridge, the 90‐km‐long Endeavour Segment has an intermediate spreading rate of ∼6 cm year⁻¹ (Riddihough 1984). In the middle of this segment is a 25‐km volcanic high split by a 75‐ to 200‐m‐deep, 0.5‐ to 1‐km‐wide, steep‐sided axial valley (Glickson et al. 2007). As the valley continues south to the end of the segment it attains a width of approximately 3 km. The Main Endeavour Hydrothermal Field (MEF) was discovered in 1982 (Tivey & Delaney 1986; Delaney et al. 1992), and is located at 47°57′N and 129°05′W. Hydrothermal activity at MEF extends along ∼400 m of the western wall of the ∼800‐m‐wide axial valley (Seewald et al. 2003). Bounding normal faults provide conduits for fluids to reach the seafloor and vent locations tightly correlated with these faults (Tivey & Delaney 1985; Delaney et al. 1992). Unlike typical mid‐ocean ridge systems, the Endeavour systems feature quite large hydrothermal deposits with steep sides and an abundance of amorphous silica and flanges (Kelley et al. 2002). Some of these structures attain heights of up to 20 m and diameters of 30 m (Seewald et al. 2003). Also out of the ordinary are the high concentrations of CH₄, NH₄, Br and B, as well as light CH₄ carbon isotopic compositions (δ¹³C < −45‰), which are consistent with the interaction between sedimentary organic matter and hydrothermal fluids (Tivey & Delaney 1986; Delaney et al. 1992; Lilley et al. 1993; Butterfield et al. 1994; You et al. 1994). Diffuse venting is commonly found along faults and fissures of this basaltic system (Delaney et al. 1992).

For some time it was thought that this system was in a tectonically dominated phase of a volcano‐tectonic cycle (Kappel & Ryan 1986); however, thinking changed after the discovery of a seismic reflector, imaged at ∼1.9–4.0 km beneath the central third of the ridge, which was interpreted as the roof of an axial magma chamber extending beneath all vent fields in the system (Detrick et al. 2002; Van Ark et al. 2007). Further exploration led to the discovery of the High Rise, Salty Dawg, Mothra and Sasquatch high‐temperature hydrothermal fields. All of these sites are located along a 15‐km section of the central ridge segment making the Endeavour segment one of the most active hydrothermal fields ever discovered (Robigou et al. 1993; Kelley et al. 2001a,b). The high density of vent fields and the intensity of their venting has enabled research into the linkages between biological, chemical, geological and geophysical processes associated with this segment of the ridge (e.g. Delaney et al. 1992; Lilley et al. 1993; Sarrazin et al. 1999; Kelley et al. 2002; Tivey & Johnson 2002; Schrenk et al. 2003; Wilcock 2004; Wilcock et al. 2009).

Steep gradients in vent fluid composition and temperature across the MEF are interpreted as stemming from supercritical phase separation (Delaney et al. 1992; Kelley et al. 2002). Studies by Delaney et al. (1992, 1997), Lilley et al. (1993) and Butterfield et al. (1994, 1995) identified highly variable chlorinities (40–540 mmol kg⁻¹) and temperatures along or near the boiling curve for sea water, reflecting both sub‐ and supercritical processes. In 1999 a magnitude 5.0 earthquake perturbed the system and vent fluids with higher temperatures and salinity indicated the possibility of a minor release of the sequestered brine (Lilley et al. 2000). The long‐lived nature of the MEF hydrothermal system and organic‐rich composition of its fluids support a diverse and prosperous microbial community with sulfide edifices housing between 10⁵–10⁹ cells g⁻¹ within the chimney structures (Schrenk et al. 1998, 1999).

Guaymas

Sedimented ridge systems are uncommon because they require the rate of deposition to exceed that of spreading. However, this is just the case at the Guaymas Basin, located at 27°N, 111.5°W in the central Gulf of California. Overall, the Guaymas Basin is a site of active seafloor spreading associated with the extensional tectonics of the EPR (Simoneit et al. 1992). It is comprised of two north‐east‐trending grabens known as the Northern and Southern Troughs. The Northern Trough is ∼40 km long, while the Southern is ∼20 km long and both are 3–4 km wide (Peter & Scott 1988). Lonsdale & Becker (1985) identified them to be a pair of en echelon, axial rift valleys that overlap at a non‐transform offset. Biogenic and terrigenous sediments at this locale are at least 500 m thick (Lonsdale & Lawver 1980). New crust forming at the spreading axis intrudes into overlying sediments as sills and dikes (Einsele et al. 1980; Einsele 1982).

Ferromanganese‐encrusted sulfide and talc deposits were first discovered in 1977 during submersible dives in the Northern Trough (Lonsdale 1978; Lonsdale et al. 1980). Elevated ³He values, 65–70% higher than atmospheric levels, were interpreted as input from mantle sources (Lupton 1979). Subsequently, in August 1980, extensive hydrothermal deposits were mapped by deep‐tow side‐scan sonar and photography (Lonsdale 1980), marking the discovery of the Southern Trough hydrothermal area. Samples of hydrothermal precipitates were collected by dredge in 1980, and their mineralogy was described by Koski et al. (1985). In 1982 a series of nine dives by the DSV Alvin was conducted in the Southern Trough within the area of high heat flow (Lonsdale & Becker 1985).

The unusually high sedimentation rates of 1–2 m (10³ years)⁻¹ (Calvert 1966) help shape the biogeochemical processes inherent to this ridge system. Magmatic activity and sill injection produce hydrothermal fluids and secondary solid phases that differ in chemical and mineralogical composition from those that occur at open‐ocean, sediment‐starved spreading axes, owing to interaction with the overlying sediments (Kastner 1982; Stout & Campbell 1983; Von Damm et al. 1985a,b; Gieskes et al. 1988). The terrigenous component is derived predominantly from Tertiary volcanics of the Mexican mainland. Sediments accumulating in this shallow basin are hemipelagic, diatomaceous, organic‐rich material of low density and high permeability (Curray et al. 1982). Compared with sediment‐starved ridges, Guaymas Basin fluids have higher pH, alkalinity, H₂S, NH₄ and CH₄ concentrations, and are depleted in dissolved metals such as Fe, Mn, Cu and Zn (Gieskes et al. 1982a,b, 1991; Von Damm et al. 1985a,b; Welhan & Lupton 1987). Another consequence of the setting that this ridge inhabits is that hydrothermal alteration of sediments produces petroleum products on timescales of tens to thousands of years (Simoneit & Lonsdale 1982; Simoneit 1983a,b, 1985; Peter et al. 1990, 1991). As a consequence, the composition of the vent fluids, sedimentary substrate, and active petroleum generation combine to produce a habitat where a plethora of unusual bacteria, archaea and eukaryotes can thrive (Teske et al. 2002).

Lau basin

Of the ∼65 000 km of global ridge crest, less than ∼10% has undergone systematic exploration for the presence and location of high‐temperature venting (Baker & German 2004). Of that entire length, back‐arc spreading centers constitute less than 7000 km (Bird 2003) but offer a great variety of hydrothermal fluid compositions, host rock alteration and volatile compositions that is difficult or even impossible to study at mid‐ocean ridge systems (German & Von Damm 2004). In this respect, some have argued that the Lau Basin is an ideal study site (German et al. 2006) because of its tectonophysics (Zellmer & Taylor 2001) and the potential for integration of geochemical and geophysical investigations (Turner & Hawkesworth 1998). The Lau Basin hydrothermal systems also attract biological studies (German et al. 2006) owing to the differences between Lau and EPR vent communities (Desbruyeres et al. 1994) and the similarities seen between the Lau and Central Indian Ridge vent communities (Van Dover et al. 2001).

Hydrothermal activity in the Lau basin was first discovered and sampled during the 1989 Nautilau cruise aboard the R/V Nadir between 17 April and 10 May 1989 using the submersible Nautile. During that time 22 dives were performed along the Valu Fa back‐arc ridge behind the Tonga–Kermadec trench between latitudes 21°25′S and 22°40′S in water ∼2000 m deep (Fouquet et al. 1991a,b, 1993). The dive sites were selected on the basis of results from cruises of the R/V Jean Charcot and R/V Sonne (Von Stackelberg and Shipboard Party 1988) and yielded samples from three major vent fields: White Church, Vai Lili and Hine Hina (Fouquet et al. 1991a,b, 1993; Taylor et al. 1996). In addition, seismic imaging indicated the existence of a magma chamber ∼3.2 km beneath the Central Valu Fa Ridge (Morton & Sleep 1985; Collier & Sinha 1992). More recently, studies were undertaken to explore the full length of the Valu Fa Ridge and the East Lau Spreading Center (Baker et al. 2005, 2006; Ishibashi et al. 2006; Martinez et al. 2006). These investigations demonstrated that hydrothermal activity progressively increases from south to north (Baker et al. 2005, 2006).

The Vai Lili hydrothermal vent field lies at a depth between 1680 and 1740 m and covers an area of approximately 100 × 400 m² consisting of at least 10 discrete clusters of black and white smokers. This area also contains a massive sulfide mound measuring ∼50 × 200 m² with a height of ∼15 m (Herzig et al. 1998). The host rock associated with the Vai Lili sulfide deposit is predominantly basaltic andesites and rhyodacitic differentiates (Fouquet et al. 1991b) with mineralization being controlled by a normal fault running subparallel to the ridge (Herzig et al. 1993). Portions of the stockwork zone have also been exposed by block rotation (Herzig et al. 1993), which may indicate an advanced stage of tectonic activity (Fouquet et al. 1993). On top of the massive sulfide mound there are active high‐temperature black smokers (320–342°C) and lower temperature white smokers (250–320°C) with 25°C pH measurements as low as 2. These vents also have unusually high concentrations of dissolved metals (e.g. up to 7 mm Pb, 3 mm Zn, 35 mm Cu, etc.), which are thought to result from the reaction of andesitic basement rocks with sea water under greenschist conditions (Fouquet et al. 1991b).

Kairei

The Kairei vent field was the first hydrothermal system discovered and sampled in the Indian Ocean (Gamo et al. 2001; Hashimoto et al. 2001). In 2000 the ROV Kaiko located the black smoker chimneys on the south‐western flank of an off‐axis knoll ∼24 km north of the Rodriguez Triple Junction on the Central Indian Ridge (CIR) (Gamo et al. 2001; Hashimoto et al. 2001). The CIR has a spreading rate of ∼50–60 mm year⁻¹ (DeMets et al. 1990). Active vents at Kairei are located on the south‐western flank of a sulfide mound, called Hakuho Knoll, with venting focused in an area approximately 80 m × 40 m (Hashimoto et al. 2001).

The black smokers of this basalt‐hosted system are similar to others, with some exceptions. Chloride, temperature and silica measurements indicate a complex trajectory of water through the system. The model proposed by Gallant & Von Damm (2006) suggests that there is a high‐temperature source fluid that last equilibrated with quartz at supercritical conditions before undergoing phase separation during the portion of its ascent where it is still above the critical point for sea water. During the remaining part of the ascent conductive cooling is believed to occur. The other major difference is the unusually high H₂(aq) concentration in conjunction with a moderately low abundance of CH₄(aq).

Recently, Nakamura et al. (2009) have proposed that the serpentinization of troctolites in the subsurface is responsible for the low CH₄/H₂ ratio and high Si concentrations observed. This suggests that the deepest high‐temperature subsurface reactions occurring at Kairei involve sea water reacting with constituents of deep oceanic crust or portions of the crust/mantle boundary (Dick et al. 2000; Nakamura et al. 2009). Given the unique geology of the system and its impact on the chemistry of the vent fluids, Karei was proposed to host a hydrogen‐based hyperthermophilic subsurface lithoautotrophic microbial ecosystem (HyperSLIME; Takai et al. 2004). Microorganisms in this system would be solely dependent on CO₂ and H₂ for their primary carbon and energy needs, and this type of ecosystem is proposed to be an analogue for life on Earth before photosynthesis (Takai et al. 2006).

Rainbow

Hydrothermal activity was discovered along the Mid‐Atlantic Ridge in the late 1970’s leading to the sampling of vent fields such as TAG, Lucky Strike, Menez Gwen, Broken Spur and others. All of these are hosted in mafic rock systems and share characteristics similar to those of basaltic systems. The later MICROSMOKE cruise (1995) identified several slow‐spreading on‐axis ultramafic systems (Konn et al. 2009). The Rainbow hydrothermal field, first discovered in 1997 (Fouquet et al. 1997) shares characteristics with two other ultramafic hydrothermal systems: Logetchev (Sudarikov & Roumiantsev 2000) and Ashadze (Mozgova et al. 2008).

The Rainbow hydrothermal field is located south of the Azores on the Mid‐Atlantic Ridge at 36°14′N, 33°54′W and a depth of ∼2300 m (Charlou et al. 2002). The system is at the intersection of a non‐transform fault with the ridge faults, and extends ∼250 m west of the ridge with a width of ∼60 m (Simoneit et al. 2004). The intensity of the hydrothermal activity, as well as the height of the mounds, increases from west to east and is thought to be related to propagation of fluid channels toward the east caused by cementing and sealing of veins in talus and sediment cover on the west (Simoneit et al. 2004). Within the field are nearly a dozen groups of black smokers distributed across the entire region, all with peridotite host rock compositions and vent fluids issuing large amounts of CH₄ and H₂ (Charlou et al. 1998, 2002) at ∼360°C (Fouquet et al. 1997).

Rainbow is also characterized by low 25°C pH fluids (3–4) with high chlorinity (780 mmol kg⁻¹), high metal concentrations (e.g. total Fe 24 mmol kg⁻¹), high alkaline earth cation levels (Donval et al. 1997; Douville et al. 2002) and low dissolved SiO₂ concentrations (∼7 mmol kg⁻¹). The low silica concentrations are less than half that of other high‐temperature basalt or gabbro‐hosted vent systems (Von Damm et al. 1985a; Campbell et al. 1988a; Von Damm 1995). The high Fe concentration suggests a relatively low in situ pH (Ding & Seyfried 1992). Low pH and high dissolved Fe and SiO₂ concentrations are not typically associated with alteration processes in ultramafic systems (Janecky & Seyfried 1986; Berndt et al. 1996; Wetzel & Shock 2000). Serpentinization of peridotite host rock may be responsible for the concentrations of dissolved methane and hydrogen associated with these systems where levels may be as high as 16 mmol kg⁻¹ for dissolved hydrogen, and 2.5 mmol kg⁻¹ for dissolved methane (Charlou et al. 1998, 2002). The latter is nearly two orders of magnitude greater than basalt‐hosted systems with similar dissolved chloride concentrations (Baross et al. 1982; Lilley et al. 1982; Welhan & Craig 1983). Hydrocarbons have also been reported in vent fluids from Rainbow (Holm & Charlou 2001) raising the possibility of organic synthesis. It has been argued that abiotic organic synthesis through Fischer–Tropsch‐type processes occurs at Rainbow, driven by the large amounts of hydrogen generated by serpentinization (Konn et al. 2009).

Mixing of submarine hydrothermal fluids with sea water

Examination of the data in Table 1 reveals that, in addition to being at very different temperatures, sea water and hydrothermal fluids differ radically in chemical composition, especially for elements that occur in more than one oxidation state, including H, O, C, N, S and Fe. The reduced forms of these elements predominate over the oxidized forms in hydrothermal fluids, and the situation in sea water is reversed. As hydrothermal fluids mix with sea water all of these elements in differing oxidation states are brought together, which generates disequilibria that exceed any oxidation–reduction disequilibrium states present in the end‐member solutions. If there were no kinetic inhibitions to oxidation–reduction reactions, then these mixtures could re‐equilibrate rapidly. However, in every case there are mechanistic complexities to transferring electrons between oxidized and reduced forms of these elements, and these kinetic barriers tend to increase as temperature decreases. As these oxidation–reduction reactions do not easily re‐equilibrate, disequilibria persist long enough for microbial processes to operate, and for microbes to use various forms of disequilibria as sources of chemical energy.

In the calculations conducted in this study, oxidation–reduction equilibria were suppressed for all reactions except H₂(aq) + O₂(aq) = H₂O, which is required to keep the calculations from departing unrealistically from the range of H₂O stability. Although this reaction is also kinetically inhibited during mixing of sea water and hydrothermal fluids, allowing H₂(aq) and O₂(aq) to equilibrate simulates the transition from reduced conditions at high temperatures to oxidized conditions at lower temperatures as mixing occurs. In a crude way, maintaining this single oxidation–reduction equilibrium simulates the ecosystem transitions that occur as fluids mix and microbes take advantage of the emerging energy supplies. It also provides a lower limit to the activity of H₂(aq) in the mixed fluids. One way to think of this is that the calculated fluid mixtures may have less H₂(aq) than their natural counterparts, which means that the resulting thermodynamic analyses represent conservative estimates of the energy available from oxidation–reduction disequilibria, especially at higher temperatures. This same approach was taken by McCollom & Shock (1997), Shock & Schulte (1998) and Amend & Shock (1998).

One goal of the present study was to consider a suite of fluids representative of the diversity of submarine hydrothermal fluid compositions from basalt‐hosted, ultramafic‐hosted, sedimented and back‐arc spreading centers, and to compare their potential for supporting abiotic organic synthesis and biosynthesis. Another was to expand upon the variety of organic compounds included in such calculations, especially so that more of the monomers needed for synthesis of biological polymers (proteins, RNA, DNA and lipid membranes) were considered. The EQ3/6 software package (Wolery & Jarek 2003) was used in the present study with a set of equilibrium constants derived from the slop07 database¹1 The slop07 database includes standard state data and parameters published through 2007, and is available from the GEOPIG website (http://geopig.asu.edu/).
for the Supcrt92 (Johnson et al. 1992) code. Mixing is simulated through a titration model in which successive amounts of cold sea water are added to an initial kilogram of hot hydrothermal fluid. The bottom sea water data listed in Table 1 were used to characterize sea water in all of the calculations conducted in this study.

Many authors report pH values for submarine hydrothermal fluids measured at 25°C, usually onboard ship soon after the samples are collected. These are the pH values listed in Table1. Calculating the consequences of mixing between hydrothermal fluids and sea water requires that the pH and speciation of the hydrothermal fluids at the measured temperature be assessed, which is done by calculating the consequences of heating the fluid to the original measured temperature. Once this is accomplished, the resulting fluid composition is used for the mixing calculations. As a consequence, the initial pH values for the high‐temperature hydrothermal fluids differ from those reported at 25°C. These initial values plot at the high‐temperature ends of the curves shown in Fig. 2. It can be seen that the initial temperatures of the hydrothermal fluids vary, with some at or near 350°C and others at somewhat lower temperatures. These results show that the diversity of submarine hydrothermal fluids exhibit a large range in high‐temperature pH values, spanning from nearly seven down to almost two. It can be seen that the variations in hydrothermal fluid compositions drive considerably different trajectories of pH during mixing with sea water. In some cases, the curves exhibit plateaus and inflections (Kairei, Rainbow, EPR 9°N, Lau and Endeavor), and some results show minima in pH during mixing (Guaymas and Endeavor). These changes in pH stem from the changing speciation of multiple elements during mixing, with major contributions from carbonic acid and hydrogen sulfide.

Figure 2

Calculated values of pH resulting from mixing of sea water with each of the hydrothermal fluids listed in Table 1. Initial pH values of the hydrothermal fluids were re‐calculated at 250 bars and the temperature reported for each fluid in this study, and do not correspond to the 25°C 1‐bar values listed in Table 1. Note that the high‐temperature ends of the curves correspond to the reported temperatures in Table 1, and the low‐temperature ends converge on the value for sea water listed in Table 1.

Changes in activities of CO₂(aq), H₂(aq), , CH₄(aq), H₂O and H₂S(aq) for the seven mixing calculations are shown in Fig. 3. The distribution of the curves at high temperatures derives from the differences in the compositions of the hydrothermal fluids. The results for the activity of H₂(aq) all show the effects of maintaining equilibrium with the reaction H₂(aq) + O₂(aq) = H₂O. The individual curves plunge to large negative log activity values at the stage of mixing where the O₂(aq) content of sea water overwhelms the H₂(aq) content of the vent fluid. The other solutes are unaffected by this behavior as all other oxidation–reductions have been suppressed in the calculations. Trends for the activities of other solutes as mixing with sea water proceeds and temperatures decrease depend increasingly on the composition of sea water, and many reflect perturbations induced by the shifts in pH shown in Fig. 2. As an example, inspection of Table 1 shows that the Guaymas hydrothermal fluid contains the largest concentration of total dissolved carbon dioxide, but the upper left‐hand plot in Fig. 3 shows that the activity of CO₂(aq) is most positive in the mixture derived from the Lau Basin fluid. As shown in Fig. 2, the pH of the Lau mixture is the lowest of all the seven calculations, and the effects of the low pH on the speciation of carbonic acid in solution drive the activity of the actual CO₂(aq) species to greater values, despite the lower total abundance of dissolved carbon dioxide. An analogous explanation applies to the activities of H₂S(aq) shown in the lower right‐hand plot in Fig. 3. Total sulfide is the greatest in the Guaymas fluid, but the higher pH causes the activity of H₂S(aq) in the fluid mixture to be lower than many other calculated values. The fluid that ranks second in total sulfide (Endeavour) has the highest activity of H₂S(aq). It can also be seen in Fig. 3 that activities of CH₄(aq) and are the highest in the Guaymas mixture, but the changing pH in this case affects the speciation of relative to NH₃(aq). The other mixtures maintain low enough pH values that the speciation of aqueous ammonia has limited impact on the activity. Finally, the activities of H₂O depicted in the lower left‐hand plot do not stray far from unity in these calculations, but the values are slightly different owing to salinity variations that affect ionic strength. The H₂O activities shown were used explicitly in the affinity calculations discussed below.

Figure 3

Selected results of mixing calculations. Activities of CO₂(aq) (upper left), H₂(aq) (upper right), (middle left), CH₄(aq) (middle right), H₂O (lower left) and H₂S (lower right) as functions of temperature at 250 bars as each hydrothermal fluid mixes with sea water. Results for H₂(aq) show the effects of maintaining equilibrium with the reaction H₂(aq) + O₂(aq) = H₂O throughout the mixing calculation, and plunge toward large negative values at the points where the dissolved oxygen in sea water is calculated to overwhelm the dissolved hydrogen in the hydrothermal fluid.

Quantifying disequilibria

The extent to which a chemical system departs from equilibrium can be assessed by evaluating the chemical affinity of various possible reactions within that system. The chemical affinity of a reaction (A_r) is defined as the partial derivative of the change in the overall Gibbs energy of the system (ΔG) with respect to the progress of that reaction (ξ_r) given by (Helgeson 1979)

(1)

where P and T stand for pressure and temperature, and ξ_k represents the progress of all other reactions. As the Gibbs energy decreases for spontaneous reactions, it follows that the chemical affinity is positive for favorable reactions that are not at equilibrium. The overall Gibbs energy of a reaction can be expressed as

(2)

in which Q_r stands for the activity product given by

(3)

where ν_i, r stands for the stoichiometric reaction coefficient of the ith chemical species in the reaction, which is positive for products and negative for reactants, and Δ_rG⁰ indicates the standard Gibbs energy of the reaction, given by

(4)

which implies that the equilibrium constant, K_r, is a function of temperature, pressure and the choice of standard states.²2 Standard states used in this study are unit activity for pure minerals and pure H₂O at any temperature and pressure, unit fugacity of pure gases at 1 bar and any temperature, and, for aqueous species, unit activity of a hypothetical 1 molal solution referenced to infinite dilution at any temperature and pressure.
These relations can be combined to yield

(5)

If K_r > Q_r, A_r is positive, and energy is released as the reaction proceeds. By keeping oxidation–reduction reactions from equilibrating in the calculations pursued in this study, values of the chemical affinity for numerous reactions can be evaluated across the entire range of sea water–hydrothermal fluid mixing ratios.

The connection between chemical affinity and the potential for a reaction to take place can be illustrated with the example of the overall reaction that occurs during autotrophic methanogenesis. This reaction is given by

(6)

for which the logarithmic form of Eqn (3) is

(7)

An autotrophic methanogen can only gain energy by making methane when the activity of aqueous methane is below the equilibrium value consistent with the coexisting activities of aqueous carbon dioxide and hydrogen. It is precisely at these same conditions where the chemical affinity of reaction (6) is positive. So, the first step to finding conditions at which autotrophic methanogenesis pays as a metabolic lifestyle is to determine where positive values of the chemical affinity occur. This can be done by combining the activities of CH₄(aq), CO₂(aq), H₂(aq) and H₂O shown in Fig. 3 to evaluate Q₆ (Eqn 7), and combining the result with independently calculated values of the equilibrium constant (K₆) to yield values of A₆ using Eqn (5). Equilibrium constants for reaction (6) can be calculated using equations, standard state thermodynamic data and equation‐of‐state parameters from Shock et al. (1989) and Shock & Helgeson (1990).

Resulting chemical affinities for autotrophic methanogenesis from the seven mixing calculations conducted in this study are shown in Fig. 4. It can be seen that chemical affinities for reaction (6) become positive in all cases over at least some of the temperature range of each mixing calculation. Keeping in mind that these calculated values are conservative, owing to the simultaneous equilibration of H₂(aq) and O₂(aq) with H₂O, the results in Fig. 4 suggest that autotrophic methanogens should be present in the mixing zones around all of these seafloor hydrothermal sites. The common occurrence of thermophilic and hyperthermophilic methanogens at submarine hydrothermal vent locations (Takai et al. 2004; Nercessian et al. 2005; Reed et al. 2009) corroborates these results. As illustrated in Fig. 4, the most positive values of the chemical affinity for reaction (6) are attained by the calculations for mixing between sea water and the fluid compositions from Rainbow and Kairei. It follows that these locations are the most conducive to supporting autotrophic methanogens of the locations considered in this study. Note that the positive values of affinity at the highest temperatures indicate that these fluids start with insufficient CH₄(aq) compared with what would be in equilibrium with the measured CO₂(aq) and H₂(aq) abundances. By contrast, hydrothermal fluid compositions from Lau, TAG and Endeavor have more methane than equilibrium with CO₂(aq) and H₂(aq) requires, yielding negative values of the affinity at the highest temperatures. It can also be seen in Fig. 4 that all curves plunge to negative values of affinity as temperature decreases, corresponding to the conditions at which sea water, which is far too oxidized to support methanogenesis, begins to overwhelm the ever‐diminishing contribution to the mixture from the hydrothermal fluid.

Figure 4

Calculated affinities for the overall methanogenesis reaction (6). Positive values indicate temperature ranges where methanogenesis would release energy. Results depend, in part, on the activities of CO₂(aq), H₂(aq) H₂O and CH₄(aq) shown in Fig. 3, and reflect the plunge in H₂(aq) activities. Suppression of equilibrium between dissolved hydrogen and oxygen would permit affinity values to attain more positive values.

Comparison of Fig. 4 with the appropriate four plots of Fig. 3 provides insights into the ranking of the results, and why the various curves take the shapes that they do. At high temperatures the ranking of affinities in Fig. 4 is Rainbow > Kairei > EPR 9°N > Guaymas ≈ Lau >Endeavor > TAG. At low temperatures, many of the curves cross, but they plunge to negative values at temperatures that decrease in the order TAG > Lau ≈ Endeavour > EPR 9°N > Guaymas > Kairei ≈ Rainbow. These rankings are similar to the relative activities of H₂(aq), with the exception that the activity of H₂(aq) is greater at Guaymas than at EPR 9°N. The ranking of methanogenesis affinities for these two mixing calculations is inverted because of the high activities of CH₄(aq) at Guaymas. It is evident that the activity of H₂(aq) exerts greater control over the ranking of affinities than do the other activities in reaction (6). The key is in the stoichiometric reaction coefficients, which are 1 and −1 for CH₄(aq) and CO₂(aq), respectively, but −4 for H₂(aq). All activities of H₂O are not far from 1, and do not contribute significantly to the differences in these curves despite its stoichiometric reaction coefficient of 2.

Affinities for abiotic organic synthesis

Positive values of affinity for autotrophic methanogenesis means that the mixtures of hydrothermal fluids and sea water are poised to favor the production of methane. This raises the possibility that the synthesis of other organic compounds may be favored during fluid mixing, which can be tested by evaluating chemical affinities for overall organic synthesis reactions. If concentrations of these compounds were known in sea water and hydrothermal fluids, then activities in mixed fluids could be evaluated, and calculations similar to those described above for methanogenesis could be done. However, compositional data of this type are largely lacking. In the absence of analytical data we can make comparisons between calculated affinities by setting the activities of the organic solutes to some arbitrary value. In this study, an activity of 10⁻⁶ was selected to make these comparisons. If activity coefficients are close to unity, then this activity is roughly equivalent to a micromolal concentration, which should be detectable by currently available analytical methods. In addition, such concentrations would be relatively large for intercellular fluids, making the comparisons for biosynthesis reactions compelling. Affinities for arbitrarily selected activities of organic solutes can be assessed from the mixing calculations conducted in this study provided that values of K_r can be calculated. Standard state thermodynamic data for aqueous organic compounds, leading to equilibrium constants for reactions in which they participate, exist for hundreds of organic compounds (Shock & Helgeson 1990; Shock 1992, 1993, 1995; Schulte & Shock 1993; Amend & Helgeson 1997a,b, 2000; Amend & Plyasunov 2001; Dick et al. 2006; LaRowe & Helgeson 2006a,b), and estimation methods permit the inclusion of thousands more (Plyasunov & Shock 2001a,b, 2003; Plyasunov et al. 2004, 2006a,b; Plyasunova et al. 2005). Examples for several groups of aqueous organic compounds are described here starting with some of the smaller representatives of these groups.

C₁ compounds

Reduction of CO₂(aq) can lead to the formation of other compounds in addition to methane. Affinities for forming activities of 10⁻⁶ of methanol, formaldehyde, CO(aq) and formic acid are shown in Fig. 5, corresponding to the reactions

(8)

(9)

(10)

and

(11)

Figure 5

Chemical affinities calculated for the synthesis of the C₁ aqueous compounds methanol, formaldehyde, CO(aq) and formic acid corresponding to reactions (8–11) as functions of temperature during mixing between hydrothermal fluids and sea water at 250 bars.

By choosing the same activity for each of these organic compounds, and setting the ranges of the plots to be the same, it should be easier to compare the thermodynamic drives for these synthesis reactions. Note that, as in the case of methanogenesis shown in Fig. 4, affinities become most positive as the temperature decreases. Temperature is controlled by the ratio of sea water to vent fluid; so, as temperature decreases the contribution from the vent fluid is being progressively diluted.

As shown in the upper left‐hand plot in Fig. 5, affinities are negative for the methanol synthesis reaction (8) in all of the high‐temperature (>300°C) vent fluids, indicating that these fluids would have equilibrium activities of methanol less than 10⁻⁶. However, as fluids from Rainbow, Kairei, Guaymas and EPR 9°N mix with sea water, and the temperature of the mixture decreases, thermodynamic drives emerge that are more than sufficient to generate methanol activities of 10⁻⁶. Affinities for forming this activity of methanol are barely positive at Lau and Endeavor, and only over a narrow range of temperature between ∼100 and ∼200°C. The mixture resulting from TAG fluid and sea water never attains positive affinities for a methanol activity of 10⁻⁶, which means that only lower activities would be possible if reaction (8) was to proceed toward equilibrium.

In contrast to methanol, affinities never turn positive for 10⁻⁶ activities of formaldehyde (reaction 9) in any of the mixing calculations conducted in this study (Fig. 5 upper right‐hand plot). If formaldehyde forms in these mixtures, its activity is likely to be less. Note that the set of curves for formaldehyde is more tightly grouped than the corresponding set for methanol, or that for methane in Fig. 4. In addition, the curves for formaldehyde increase more gently with decreasing temperature than the corresponding curves for methanol or methane. The carbon in formaldehyde is more oxidized than the carbon in methanol or methane, which means that the stoichiometric reaction coefficient for H₂(aq) is correspondingly lower. As the contribution of to Q_r decreases, the resulting curves of affinity versus temperature flatten and become closely spaced.

This trend of flattening and bunching of affinity curves reaches its extreme for the cases of CO(aq) and formic acid (reactions 10 and 11), as shown in the lower two plots in Fig. 5. Formic acid can be considered to be a hydrated version of CO(aq) according to the reaction

(12)

which helps to explain why the two sets of curves look so similar. At lower temperatures, formic acid is slightly more stable than CO(aq) in aqueous solution, and comparison of the two sets of affinity curves shows that those for formic acid tend to be slightly more positive at temperatures <200°C than the corresponding curves for CO(aq). The observations that the affinity curves for CO(aq) are slightly more positive at higher temperatures can be explained by the fact that dehydration in aqueous solution becomes increasingly favorable as temperature increases (Shock 1993).

C₂ compounds

As the number of atoms in organic molecules increases, so do the possibilities for new structures and arrangements. In this study, affinities for 8 two‐carbon compounds were calculated for the seven mixing calculations. These simplest forms of organic compounds with a carbon–carbon bond include hydrocarbons, acids, an alcohol and an aldehyde. As in the case of the C₁ compounds in Fig. 5, activities for C₂ compounds were all set to 10⁻⁶ to facilitate comparisons. Affinities were calculated for the reactions

(13)

(14)

(15)

(16)

(17)

(18)

(19)

and

(20)

and the results are plotted in Figs 6 and 7. The ranges of the axes are the same in all of the plots shown in these figures, which should help comparisons among the affinity results.

Figure 6

Calculated affinities for the synthesis of aqueous ethane, ethene, ethyne and ethanol, corresponding to reactions (13–16) as functions of temperature during mixing of seafloor hydrothermal fluids with sea water at 250 bars.

Figure 7

Calculated affinities for the synthesis of the aqueous organic compounds acetaldehyde, acetic acid, glycolic acid and oxalic acid according to reactions (17–20) as functions of temperature during mixing of seafloor hydrothermal fluids with sea water at 250 bars.

Results for the three hydrocarbons: ethane, ethene (also called ethylene) and ethyne (also called acetylene), together with those for ethanol are plotted in Fig. 6. Comparisons of these plots shows that the affinities to form ethane (upper left‐hand corner) at an activity of 10⁻⁶ are the most positive, and those to form the same activity of ethyne (lower left‐hand corner) are the most negative. Affinity curves for ethene are intermediate between the other two aqueous hydrocarbons, and are slightly less positive than corresponding curves for ethanol. As in the case of formic acid and CO(aq), ethanol can be thought of as a hydrated form of ethene. Likewise, the affinity values for ethanol tend to be somewhat more positive than the values for ethene, and this difference is more pronounced at low temperatures than at high. Comparison of Figs 4 and 6 shows that, at high temperatures, affinities for ethane, based on an activity of 10⁻⁶, are less positive than for methane, based on activities calculated from methane concentration data (see Fig. 3). This makes a direct comparison of the results difficult, but it is evident that affinities for ethane synthesis are large at an ethane activity that is one to five orders of magnitude lower than the initial methane activities of hydrothermal fluids. For reference, ethane concentrations at Lost City are within 20% of the concentrations of methane (Proskurowski et al. 2008) and about 50% of the methane concentration reported for Rainbow (Charlou et al. 2002).

Calculated affinities for 10⁻⁶ activities of acetaldehyde and the three acids: acetic, glycolic and oxalic are plotted against temperature in Fig. 7 for the seven mixing models pursued in this study. Affinities for acetaldehyde and acetic acid reach positive values for all of the calculations except the mixing of TAG hydrothermal fluid with sea water, although the acetaldehyde affinities for the Lau and Endeavor mixing calculations are only barely positive in the 100°–150°C range. The negative affinities calculated for 10⁻⁶ activities of the hydroxy acid (glycolic) and dicarboxylic acid (oxalic) suggest that these more oxidized organic compounds are considerably less stable than the more reduced carboxylic acid (acetic) and point to a dramatic difference in the behaviors of these three groups of organic acids. The positive values of affinity for acetate synthesis suggest that the metabolic process of acetogenesis may support microbes in mixing zones of seafloor hydrothermal systems.

Comparison of Figs 6 and 7 allows us to rank these reactions in terms of affinity. Affinities for ethane are the most positive followed by those for acetic acid, which are slightly more positive than similar values for ethene and ethanol. Acetaldehyde also attains positive affinities, although less so than those for the compounds already mentioned. Glycolic acid, ethyne and oxalic acid all have negative affinities, with those for oxalic acid being most negative. These results, based on a comparison for constant activities of 10⁻⁶, show that, during mixing of hydrothermal fluids and sea water, ethane is the most stable of the C₂ products, followed by acetic acid, ethanol, ethene, acetaldehyde, glycolic acid, ethyne and oxalic acid. This suggests that alkanes, carboxylic acids, alkenes, aldehydes and alcohols are likely products of abiotic organic synthesis in hydrothermal systems.

Carboxylic acids

Results for acetic acid shown in Fig. 7 suggest that other carboxylic acids may be potential products of abiotic synthesis during mixing of hydrothermal fluids and sea water. This is confirmed by the results for four carboxylic acids: propanoic, hexanoic, nonanoic and dodecanoic, shown in Fig. 8. As in Figs 5–7, the activity of each acid was set to 10⁻⁶ to calculate values of Q_r for the reactions

(21)

(22)

(23)

and

(24)

together with values of , and from the seven mixing calculations. Resulting values of Q_r were combined with equilibrium constants for these reactions, calculated with equations, data and parameters from Shock et al. (1989) and Shock (1995) to evaluate the affinities shown in Fig. 8.

Figure 8

Calculated affinities for the synthesis of aqueous carboxylic acids: propanoic, hexanoic, nonanoic and dodecanoic according to reactions (21–24) as functions of temperature as seafloor hydrothermal fluids mix with sea water at 250 bars.

Comparison of the plots in Fig. 8 shows that with increasing molecular size the affinity curves steepen and spread apart. Those for more hydrogen‐rich fluids such as Rainbow, Guaymas and Kairei reach increasingly positive affinities for increasingly larger carboxylic acids. Similar, although less dramatic, results are found for fluids from EPR 9°N, Lau and Endeavor, but those for TAG become more negative for increasingly larger compounds. Taking mixing results for the Rainbow fluid with sea water as an example, the maximum in the curve increases from about 40 kcal mol⁻¹ for propanoic to about 190 kcal mol⁻¹ for dodecanoic. This means that energy would be released by the formation of any of these organic acids, and that considerably more energy would be released during synthesis of each mole of larger acids like dodecanoic than for each mole of smaller acids like propanoic. Long‐chain carboxylic acids, larger than dodecanoic, are integral to many microbial membranes, and these results suggest that the synthesis of such compounds would also be accompanied by the release of energy in many seafloor hydrothermal systems.

The trends shown in Fig. 8 demonstrate that the disequilibria established as hydrothermal fluids mix with sea water not only lead to conditions where energy would be released by organic synthesis, but that the potential release of energy increases for larger compounds. One way of examining these results is to consider the stoichiometric coefficient on H₂(aq) in these reactions, which increases from 7 for propanoic acid to 34 for dodecanoic acid. These changes amplify the effects of the H₂(aq) content of these fluids, as well as differences in H₂(aq) contents among the various fluids. Another way to interpret these results is to consider the average oxidation state of carbon in each of these compounds, which can be calculated by assuming each O has a charge of −2 and each H has a charge of 1. The resulting values of the average carbon oxidation state for propoanic, hexanoic, nonanoic and dodecanoic acids are −0.67, −1.33, −1.55 and −1.67 respectively. Reduced conditions that prevail as hydrothermal fluids mix with sea water favor compounds with more reduced average oxidation states for carbon. It follows that more energy would be released per mole of formation of the larger acids.

Affinities for biosynthesis

Conditions that favor abiotic organic synthesis are also energetically favorable for biosynthesis. It seems reasonable to imagine that microbes would take advantage of thermodynamically favorable conditions for making the organic compounds that constitute their cells. If so, then the consequences of fluid mixing relate directly to the thermodynamics of biosynthesis by thermophiles and hyperthermophiles. The following discussion begins with amino acid syntheses and continues with carbohydrate, purine and pyrimidine syntheses. Amino acids are polymerized into proteins, carbohydrates serve as energy sources and as building blocks of DNA with purines, pyrimidines and other monomers.

Amino acids

Affinities determined for 10⁻⁶ activities of glycine, alanine, valine and leucine are plotted in Fig. 9 for the seven mixing calculations conducted in this study. Values of Q_r were obtained with the values of , , , and pH for each mixture as shown in Figs 2 and 3, and values of K_r were calculated with equations and parameters from Shock et al. (1989, 1997) and Dick et al. (2006) for the reactions

(25)

(26)

(27)

and

(28)

As in the case of the carboxylic acids, the affinity curves are steeper and more splayed apart as the size of the amino acid increases from C₂ glycine to C₆ leucine. One consequence of these changes is that affinities for alanine, valine and leucine become positive in the EPR 9°N mixture between about 50° and 150°C, while those for glycine do not reach positive values at any temperature. Furthermore, the energy that would be released at 50°C upon the formation of these acids in the Rainbow mixture increases from 5 kcal mol⁻¹ for glycine to 20 kcal mol⁻¹ for alanine to 50 kcal mol⁻¹ for valine and about 70 kcal mol⁻¹ for leucine. Rather than costing energy, as amino acid synthesis does in low‐temperature, oxidized, surface environments, all that is needed to trigger amino acid synthesis in seafloor hydrothermal ecosystems is catalysis.

Figure 9

Calculated affinities at 250 bars for the synthesis of aqueous glycine, alanine, valine and leucine, based on reactions (25–28) as functions of temperature during mixing of seafloor hydrothermal fluids with sea water.

The positive enhancement of affinities as the size of amino acid molecules increases is also demonstrated by the plots in Fig. 10 for the C₄ compounds aspartic acid and asparagine, and the C₅ compounds glutamic acid and glutamine. Affinities were evaluated using activities of H₂(aq), H₂O, CO₂(aq), and H⁺ from the seven mixing calculations, as well as the constraint that the amino acid activities are each 10⁻⁶, together with equilibrium constants calculated as for the amino acids in Fig. 9 for the reactions

(29)

(30)

(31)

and

(32)

Note that affinities for the C₅ acids (lower plots) are more positive for most of the mixing calculations than for their C₄ counterparts (upper plots). These trends are analogous to those for the increasingly positive affinities documented for the successively larger carboxylic acids in Fig. 9 and amino acids in Fig. 10.

Figure 10

Calculated affinities for the synthesis of aqueous aspartic acid, asparagine, glutamic acid and glutamine based on reactions (29–32) as functions of temperature as seafloor hydrothermal fluids mix with sea water at 250 bars.

Glutamic acid shown in Fig. 11 and valine in Fig. 10 are both C₅ amino acids. The major difference between them is that glutamic is a dicarboxylic acid, while valine contains just one carboxyl group. Comparison of these figures shows that affinities for a 10⁻⁶ activity of glutamic acid are less positive than those for the same activity of valine. Less positive affinities for a dicarboxylic acid relative to monocarboxylic acids are also evident in Fig. 7 where results for oxalic acid can be compared with those for glycolic acid or acetic acid. Apparently the trend in submarine hydrothermal systems toward lower stability for more oxygen‐rich compounds observed for C₂ compounds continues when considering amino acids. This theme returns in the discussion of results for carbohydrates below.

Figure 11

Calculated chemical affinities at 250 bars for the synthesis of aqueous serine, proline, phenylalanine and tryptophan based on reactions (33–36) as functions of temperature during mixing of submarine hydrothermal fluids with sea water.

The examination of amino acid synthesis can be extended to include several other compounds, and examples are shown in Fig. 11 for serine, proline, phenylalanine and tryptophan where the overall synthesis reactions are given by

(33)

(34)

(35)

and

(36)

Affinities were calculated following the same procedures for quantifying Q_r and K_r as described above for other amino acids. Again, the larger amino acids tend to yield more positive affinities for mixing calculations involving the most H₂‐rich fluids. As an example, for the Rainbow mixing calculation, affinities for phenylalanine reach values that are more positive than those for leucine shown in Fig. 9. At the same time, larger amino acids have less positive affinities in calculations for H₂‐poor fluids such as TAG, Lau and Endeavor (compare, for example, the results for proline with those for tryptophan). Hydroxyl groups on amino acids seem to have an effect similar to their effect on carboxylic acids. Comparison of results for serine in Fig. 11 with those for alanine in Fig. 9 shows that affinities are less positive for the hydroxyl‐bearing serine, analogous to the comparison of glycolic and acetic acids in Fig. 7.

Carbohydrates, purines and pyrimidines

Unlike carboxylic and amino acids, which have relatively high ratios of carbon to oxygen, calculations for carbohydrates yield less positive values of affinity. This is illustrated by the results shown in Fig. 12 for ribose, ribulose, deoxyribose and glucose. Equilibrium constants were calculated using data and parameters for ribulose and glucose from Amend & Plyasunov (2001) and ribose and deoxyribose from LaRowe & Helgeson (2006a) for the reactions

(37)

(38)

(39)

and

(40)

Activity products were evaluated from the results of mixing calculations shown in Fig. 3, together with the constraint that the activities of the carbohydrates are set to 10⁻⁶. Given this constraint, the affinities for ribose, ribulose and glucose never attain positive affinities. By contrast, those for deoxyribose reach positive values for mixing calculations involving the fluids from Rainbow, Kairei and Guaymas. In fact, comparison of the plots in Fig. 12 shows that all of the affinity curves for deoxyribose are more positive than for the other carbohydrates in this study. As the only difference between ribose and deoxyribose is a single oxygen, the comparison between the plots for these two carbohydrates helps to focus on the effect of changing the carbon‐to‐oxygen ratio in carbohydrates. It is evident that the more reduced carbohydrate, deoxyribose, is more stable than its oxidized counterpart, ribose. It follows that synthesis of deoxyribose is less costly than the same activity of ribose at conditions throughout submarine hydrothermal ecosystems.

Figure 12

Calculated affinities for the synthesis of aqueous ribose, ribulose, deoxyribose and glucose based on reactions (37–40) as functions of temperature during mixing of submarine hydrothermal fluids with sea water at 250 bars.

Deoxyribose is linked with purines (adenine and guanine) and pyrimidines (thymine and cytosine) to form deoxynucleosides that ultimately form parts of the structure of DNA. Data and parameters for purines and pyrimidines from LaRowe & Helgeson (2006a) make it possible to calculate equilibrium constants for synthesis reactions given by

(41)

(42)

(43)

and

(44)

which were combined with activity products based on the mixing calculations conducted in this study to yield the affinities shown in Fig. 13 after setting activities of the purines and pyrimidines to 10⁻⁶. As in the cases of the carbohydrates shown in Fig. 12, most calculated affinities are negative for 10⁻⁶ activities of purines and pyrimidines at the conditions of the mixing calculations. However, those for thymine become positive in the low‐temperature ranges for Guaymas, Endeavour and Kairei. The order and arrangement of curves in Fig. 13 differ from those in other figures, owing to the presence of in the reactions, and its stoichiometic reaction coefficients which are generally larger than most corresponding values for amino acids. As a consequence, calculations for the more ammonia‐rich fluids, especially Guaymas, attain enhanced affinities. Likewise, calculated affinities for adenine and guanine in the Endeavour mixing calculations reach values that are more positive than those for the EPR 9°N results, reflecting the two order of magnitude greater abundance of ammonia at Endeavour (see Table 1).

Figure 13

Calculated affinities for the synthesis of aqueous adenine, guanine, tyrosine and cytosine based on reactions (41–44) as functions of temperature during mixing of submarine hydrothermal fluids with sea water at 250 bars.

Discussion

The results summarized above are meant to provide a framework for thinking about the potential for organic synthesis and biosynthesis in submarine hydrothermal ecosystems. Reducing the entire variability of submarine hydrothermal systems to seven representative fluids from around the world is inherently incomplete but meant to capture much of the global diversity. In addition, selecting or assembling a fluid composition for an area of hydrothermal venting cannot reflect the variability within an individual location. It is hoped that local and global variations can be referenced to the set of calculations conducted in this study, at least in a comparative mode. The following discussion includes efforts to examine the global variability in the potential for abiotic organic synthesis and biosynthesis in submarine hydrothermal systems, to consider the energetic consequences for the overall synthesis processes and to summarize the types of variations that would lead to results that would go beyond the scope of the present study.

Consequences of global variability

As a preface, it should be emphasized that the results presented here are dependent on several assumptions. First, it should be kept in mind that dissolved hydrogen and oxygen are allowed to equilibrate with each other and with H₂O throughout these calculations. This is not strictly realistic for fluid mixing, as there are many conditions at which this reaction departs from equilibrium. Evidence for this departure comes from the culturing of microbes capable of gaining their energy from this reaction. Nevertheless, using this constraint means that the resulting affinities for other reactions that are not allowed to equilibrate are minimum values. In a natural system resulting from fluid mixing, there can always be more energy than what was calculated in this study and depicted in the figures shown above. It should be kept in mind that assuming this equilibrium dramatically affects some of the low‐temperature calculations as the abundance of dissolved oxygen in sea water overwhelms the amount of dissolved hydrogen in the hydrothermal fluid. Curves that plunge steeply may diverge from what could be measured in a natural system where hydrogen oxidation is kinetically inhibited. Another assumption about the results shown above is that none of the other oxidation–reduction reactions are allowed to proceed at all in these calculations. This means that the energy represented by affinities for each reaction depend on none of the other reactions having been initiated. A more realistic model would incorporate rates of all of the reactions considered so that the most likely reactions could be predicted. As those reactions would then be allowed to proceed, reactants and products would be consumed and produced, resulting in changes to the activity products and affinities for all of the other reactions. The absence of such kinetic data represents one of the largest gaps in the understanding of geochemistry. With these thoughts in mind, we can nevertheless make some generalizations.

The results summarized above demonstrate that the potential for abiotic organic synthesis in submarine hydrothermal systems is the greatest for those fluids that contain the highest concentrations of H₂(aq). The composition selected to represent the Rainbow field contains the most H₂(aq), and that for TAG contains the least; mixing calculations for Rainbow typically yield the most positive affinities, while those for TAG are typically the most negative. Plots for all compounds in the C–H–O chemical system (Figs 4–8, and 12) have Rainbow and TAG at the extremes of behavior except for oxalic acid in Fig. 7. It appears that the higher concentrations of dissolved inorganic carbon at Guaymas and Lau allow the corresponding oxalic acid affinities to exceed those for Rainbow. This is the first indication that H₂(aq) concentration alone is not sufficient to predict accurately all of the relative locations of curves in the affinity plots shown above.

Moving to the C–H–O–N system (Figs 9–11 and 13) reveals that the interplay of calculated affinities becomes considerably more nuanced than simply reflecting the relative magnitudes of H₂(aq) contents. The synthesis reactions for amino acids involve and H⁺, which, together with CO₂(aq), H₂(aq) and H₂O means that there are five variable activities affecting the results. As the number of variables increases, predictability based on any one variable is less robust. Nevertheless, it is possible to gain some insights concerning the trends linking fluid composition and affinities for synthesis reactions. Take, for example, the position of the calculated affinity curves for the Guaymas fluid in the plots for glycine in Fig. 9, asparagine and glutamine in Fig. 10 and all of the plots in Fig. 13. As shown in Table 1, the total concentration of ammonia in the Guaymas fluid exceeds most other fluids by more than an order of magnitude (the exception being the Lau fluid, which the Guaymas ammonia concentration exceeds by a factor of three). As the number of N atoms in the organic molecule increases, matched by the stoichiometric reaction coefficient of , the effects of differing total ammonia concentrations are observable. These effects are the greatest for those compounds with the greatest N:C ratios. Thus, the effect is apparent for glycine, with N:C = 0.5 but not for the other amino acids in Fig. 9 (with N:C ratios of 0.33, 0.2 and 0.16). Likewise, the Guaymas mixing calculations shown in Fig. 10 yield affinities for glutamine and asparagine that exceed at many temperatures those for the Rainbow calculations because of the push supplied by the higher ammonia concentration at Guaymas for these compounds with comparatively higher N/C ratios than their dicarboxylic counterparts glutamic and aspartic acid. This trend continues and is magnified in the plots in Fig. 13.

The details described above emphasize the point that variations in affinities derive from differences in composition. The results of this study allow some generalizations about how those differences affect the potential for abiotic organic synthesis and biosynthesis. Hydrogen content of vent fluid is a major determinant of synthesis potential, and systems hosted in ultramafic rocks will yield greater affinities for organic synthesis reaction than their basaltic, andesitic or rhyolitic counterparts. This is shown by the results for the Rainbow fluid used in this study, and also for the results from Kairei, where fluids seems to be affected by mafic or ultramafic rocks somewhere in the reaction zone. Sedimented systems can also exhibit enhanced hydrogen contents as represented here by the Guaymas fluid. Depending on the content of organic matter in the sediments, such fluids may be enriched in ammonia, which will enhance the potential for synthesis of N‐bearing organic compounds like amino acids, purines and pyrimidines, as well as the protein and nucleic acid polymers made from them. Hydrothermal systems hosted at divergent boundaries are not likely to accumulate sediments unless they occur near the continents, but back‐arc spreading may accumulate continental‐derived sediment far more commonly. The results for the Lau Basin fluid selected in this study may not extrapolate well to other back‐arc basin settings nearer to continental or large island arc sediment sources. Nevertheless, these results show what can happen as fluid compositions are influenced by rocks with more silica than what is typically associated with divergent boundary basalt‐hosted systems. However, even in basalt‐hosted systems there can be some major differences in synthesis potential based on the fluids chosen for this study. Fluids like the EPR 9°N sample that are relatively enriched in H₂ provide greater potential for organic synthesis during mixing with sea water than H₂‐poor samples such as the Endeavour and TAG fluids chosen here. In fact, results for these three fluids can span a considerable range in affinities for given reactions. In many cases the results from EPR 9°N and TAG are at the extremes for basalt‐hosted systems, and those for Endeavour fall about midway between. This framework should make it possible for estimates to be made for other basalt‐hosted systems by comparing compositional data with these three examples.

Energetics of organic synthesis and biosynthesis

Positive affinities for methanogenesis, as determined in this study (Fig. 4), are a necessity if that metabolic strategy supports autotrophic microbes in hydrothermal ecosystems. The geologic system at ridge environments allows mixing of fluids that are dramatically different in composition, especially for oxidation–reduction processes, and the resulting thermodynamic drive is for methane to be formed from dissolved carbon dioxide and hydrogen. Mechanistic difficulties supply a kinetic barrier to this reaction proceeding on its own; so, catalysts in the form of microbes inhabit the system, trigger the reaction and reap some of the energy that is released.

Positive affinities for other organic synthesis reactions might be anticipated owing to how well the geologic system provides for methanogens. Nevertheless, anticipating the magnitude of the thermodynamic drives for abiotic organic synthesis and biosynthesis of various organic compounds may not be intuitive. After all, we live at conditions where organic synthesis in all of its forms is energetically costly. A plant at the surface conducting photosynthesis is working against a 20% O₂ atmosphere to reduce CO₂ to sugar. Copious energy in the form of solar photons allows the plant to fight this uphill battle and prevail. We take advantage of the energy transferred from the sun by the plant either by directly consuming the plant, or another animal that consumed the plant. The trapped solar energy is dissipated by the surface food web in which we reside, after the initial investment is made by the photosynthetic organisms. The situation in submarine hydrothermal ecosystems is radically different.

Positive affinities for the formation of ethane and acetic acid (Figs 6 and 7) suggest that autotrophic ethanogenesis and acetogenesis are metabolic strategies that would release energy and support life in submarine hydrothermal ecosystems. Autotrophic acetogenesis is relatively common (Amend & Shock 2001) and indications of ethanogenesis (and propanogenesis) are reported based on concentrations and isotopic analyses from deep marine sediments (Hinrichs et al. 2006). Based on the results in Fig. 6, seafloor hydrothermal ecosystems derived from fluids that have reacted with peridotites would be excellent environments in which to continue the search for ethanogenesis and the mysterious ethanogens. The same conditions favor the production of methanol, ethanol, acetaldehyde and ethene, suggesting that additional metabolic discoveries may be anticipated.

The potential for amino acid synthesis in seafloor hydrothermal systems was explored earlier using a single composition for a 100°C mixture (Amend & Shock 1998). The results obtained in this study for the dozen amino acids depicted in Figs 9–11 suggest that conditions for amino acid synthesis may regularly be far more favorable than that earlier study showed. As examples, the energies released by synthesis of phenylalanine, tryptophan and leucine from the Kairei, Guaymas and Rainbow mixed fluids at 100°C all exceed values obtained in the earlier study by factors of two to three. Affinities for these three amino acids in the EPR 9°N mixture at temperatures between about 50 and 90°C are also more positive than in the 100°C mixture used by Amend & Shock (1998). However, in the mixtures generated from the Lau, Endeavour and TAG fluids, syntheses of these amino acids are far from being so favorable. As in the earlier study, there are amino acids, including glycine, asparagine and serine, for which synthesis may not be accompanied by a release of energy. It remains to be seen if these amino acids are less common in the proteins of marine thermophiles and hyperthermophiles, but the increasing availability of genome sequences for these microbes facilitates conducting such tests enormously.

Extrapolating beyond the present study

The range represented by the seven fluids picked for the present study covers much of the known range of submarine hydrothermal fluid compositions. However, it should be anticipated that what is known may be only a small part of the picture. Basalts are common on the seafloor; so, many systems may produce environments that fall somewhere in the range indicated by the results for TAG, Endeavour and EPR 9°N presented here. At the same time, these three fluid compositions do not exhaust the possibilities for basalt‐hosted systems, and barely explore the myriad possibilities that keep petrologists sampling seafloor basalts. Likewise, the full range of fluid compositions that may be produced in ultramafic‐hosted systems cannot be simulated by the sparse set of examples chosen here. Known ultramafic‐hosted systems are in peridotites, but it may be that seafloor hydrothermal systems are hosted in pyroxenites or other varieties of ultramafic rocks, raising the possibility of fluid compositions that could diverge dramatically from what is presented here. Similarly, a single fluid from the Lau back‐arc basin can hardly even do that complex system justice, and is far from representing all of the possibilities that may emerge in back‐arc systems given the potential for fractionation to produce a variety of silica‐rich rocks, and the potential for back‐arc spreading to occur near sources of continental sediments. The results for Guaymas show the dramatic changes that continental sediments can impose on a basalt‐hosted system, and other compositions await exploration.

Results presented here for carboxylic acids, carbohydrates, purines and pyrimidines provide the foundation for exploring the energetics of biosynthesis for many biomolecules. The trend shown by the series of carboxylic acids in Fig. 8 is enticing when extrapolated to lengths representative of molecules found in microbial membranes, but calculations for the appropriate compounds are not yet possible. Likewise, the energetic differences of forming ester and ether bonds, which characterize the differences in archaeal and bacterial membranes, remain to be explored in mixed hydrothermal fluids. Results for ribose, deoxyribose and the purines and pyrimidines suggest that synthesis of genetic material may be costly throughout submarine hydrothermal ecosystems. This can be tested by calculating affinities for nucleotides, nucleosides and their polymerized versions that lead to DNA and RNA. At the same time, it is also possible to evaluate the energy required to make adenosine triphosphate and other molecules involved in the transfer of energy within cells. Taken together, these sorts of results will ultimately make it possible to assess the energy demands of the inhabitants of submarine hydrothermal ecosystems.

Concluding remarks

Conditions established as hydrothermal fluids mix with sea water favor the formation of organic compounds at the expense of inorganic starting materials. The mixtures are far‐from‐equilibrium chemical systems in which the formation of organic compounds would help to dissipate the pent‐up energy held by the disequilibrium states. That this is quite an unfamiliar situation compared with the facts of life at the Earth’s surface is evident. Conditions in hydrothermal ecosystems are not well represented by any familiar surface setting. Abiotic organic synthesis has a compelling thermodynamic drive in these systems. Similarly, the energetics of biosynthesis presents a strong contrast to anything familiar. Autotrophic microbes in submarine hydrothermal ecosystems can make many of the monomers that compose their biomolecules by catalyzing already favorable reactions. The overall thermodynamic costs of many biosynthesis reactions can be negative, and may exceed the positive costs of making other biomolecules, supporting the conclusion that for many autotrophs in hydrothermal ecosystems biosynthesis acts as ‘a free lunch that they are paid to eat’ (Shock et al. 1998).

Acknowledgements

This work was funded in part through NSF grants OCE‐0752541 and OCE‐097406. The authors appreciate helpful discussions with Jeff Dick during the course of this study, and the pioneering efforts of Tom McCollom, Jan Amend and Mitch Schulte who helped initiate this type of theoretical analysis.