SEARCH

SEARCH BY CITATION

Keywords:

  • origin of life;
  • habitats for life in ice;
  • microbial metabolism;
  • methanogens;
  • iron-reducing bacteria;
  • methane on Mars

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Liquid veins in glacial ice as a habitat for microbial life at low temperatures
  5. Unfrozen water at subfreezing temperatures on mineral surfaces
  6. Microbial life in silty ice at the bottom of the Greenland (GISP2) glacial ice
  7. Microbial life in clear ice at GISP2
  8. Metabolic rates in veins and on clay grains and the question of immortality
  9. Methanogens on Mars
  10. Cold origin of life
  11. Summary
  12. Acknowledgements
  13. References

Application of physical and chemical concepts, complemented by studies of prokaryotes in ice cores and permafrost, has led to the present understanding of how microorganisms can metabolize at subfreezing temperatures on Earth and possibly on Mars and other cold planetary bodies. The habitats for life at subfreezing temperatures benefit from two unusual properties of ice. First, almost all ionic impurities are insoluble in the crystal structure of ice, which leads to a network of micron-diameter veins in which microorganisms may utilize ions for metabolism. Second, ice in contact with mineral surfaces develops a nanometre-thick film of unfrozen water that provides a second habitat that may allow microorganisms to extract energy from redox reactions with ions in the water film or ions in the mineral structure. On the early Earth and on icy planets, prebiotic molecules in veins in ice may have polymerized to RNA and polypeptides by virtue of the low water activity and high rate of encounter with each other in nearly one-dimensional trajectories in the veins. Prebiotic molecules may also have utilized grain surfaces to increase the rate of encounter and to exploit other physicochemical features of the surfaces.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Liquid veins in glacial ice as a habitat for microbial life at low temperatures
  5. Unfrozen water at subfreezing temperatures on mineral surfaces
  6. Microbial life in silty ice at the bottom of the Greenland (GISP2) glacial ice
  7. Microbial life in clear ice at GISP2
  8. Metabolic rates in veins and on clay grains and the question of immortality
  9. Methanogens on Mars
  10. Cold origin of life
  11. Summary
  12. Acknowledgements
  13. References

In their recent review of Earth's icy biosphere, Priscu & Christner (2004) covered the relevant literature up to 2003, much of which will not be repeated here. The present review discusses the metabolism of microorganisms in two distinct icy habitats. The first habitat – a eutectic phase of pure polycrystalline ice and aqueous ionic solution – provides liquid water, nutrients and biomolecules such as C, N and S for microorganisms small enough to fit into a network of veins with diameters as small as a few microns. The second habitat consists of surfaces of ice-entrained mineral grains – clay minerals in particular. It provides a thin film of unfrozen water within which microorganisms can attach to surfaces from which they can extract nutrients and release waste products. Recent studies of microorganisms in the Greenland ice core (GISP2), including methanogens and Fe-reducing anaerobes, are summarized. Measurements of the rates of microbial metabolism as a function of temperature provide insight into the question of microbial longevity and may allow one to place constraints on the ongoing production of methane on Mars. Analogously to the two habitats for microbial life in ice, experiments on the polymerization of amino acids and nucleotides in aqueous veins in ice and on clay mineral surfaces are suggested as important steps toward life on icy planetary bodies.

Liquid veins in glacial ice as a habitat for microbial life at low temperatures

  1. Top of page
  2. Abstract
  3. Introduction
  4. Liquid veins in glacial ice as a habitat for microbial life at low temperatures
  5. Unfrozen water at subfreezing temperatures on mineral surfaces
  6. Microbial life in silty ice at the bottom of the Greenland (GISP2) glacial ice
  7. Microbial life in clear ice at GISP2
  8. Metabolic rates in veins and on clay grains and the question of immortality
  9. Methanogens on Mars
  10. Cold origin of life
  11. Summary
  12. Acknowledgements
  13. References

Due to the entropy of mixing, the equilibrium freezing point of an aqueous solution is lower than that of pure water. Almost all solutes are excluded from the ice structure and become more concentrated in the remaining unfrozen fraction. In polycrystalline ice the solute ions concentrate at the triple junctions, leading to a vein structure. To maintain thermodynamic equilibrium, the size of the veins shrinks with decreasing temperature, adjusting the ionic concentration in order for the liquid to remain at the freezing point. Price (2000) calculated both the molarity and the vein diameter as a function of freezing point depression for Antarctic glacial ice with the typical concentration of various solute species (dominantly sulfuric acid) measured in glacial ice from the Vostok ice core. Equation 1 gives the relationship among vein diameter (dv), grain diameter (D), bulk concentration of the ions (Cb), and concentration in the veins (Cv):

  • image(1)

Typical vein diameters range from ∼1 μm at −50°C to ∼10 μm at −2°C. As the temperature decreases, the veins become narrower and the ion concentration increases enough to maintain the liquid phase. See Price (2000) for a discussion of the thermodynamics and for estimates of vein size as a function of temperature and impurity composition of glacial ice. In order to metabolize in the veins at deep subfreezing temperatures, microorganisms must adapt to an extremely harsh chemical environment, which in Antarctic glacial ice favors acidophiles and in sea ice favors halophiles. Nutrients are available via redox reactions with ions in the veins.

Two groups have used SEM and X-ray microanalysis to study the composition of veins at several depths in glacial ice from both Greenland and Antarctica (Baker et al., 2003; Barnes & Wolff, 2004). Barnes & Wolff (2004) concluded that connected veins occurred at depths where the concentration of soluble ions such as sulfate was high enough to produce at least a monolayer on two-grain boundaries and for the excess to go into three-grain boundaries, i.e. veins. Thus, the proportion of ions distributed in veins depends on both the total concentration of ions in the ice and the grain size.

From direct observations, Junge and coworkers showed that microorganisms take advantage of the vein habitat in ice. In work carried out in a cold room at temperatures −2 to −20°C, they stained cells in sea ice (Junge et al., 2004a) and lake ice (Junge et al., 2004b) and showed by epifluorescence microscopy that most cells were excluded from the ice phase into brine channels and veins.

Terrestrial glacial ice is in the low-pressure phase denoted Ih; at depths greater than ∼103 m the air bubbles in ice transform into a cubic phase that is more stable: air clathrate hydrate. By contrast, using microscopy to view ice grown in a diamond-anvil chamber, Sharma et al. (2002) studied the physiological and metabolic activity of Escherichia coli and Shewanella oneidensis (a piezophile) added to the ice. At pressures of from 1.2 to 1.6 GPa and at 25°C, the bacteria were found to reside in liquid veins in crystals of a high-pressure phase (ice-VI), to oxidize formate, and to move in the veins. The main point of their experiment was to show that bacteria could metabolize in veins at pressures far greater than present in natural ice anywhere on Earth.

Recently Mader et al. (2006) showed experimentally that either bacteria or fluorescent beads added to water used to make ice were rejected from the solid phase and incorporated into liquid veins, provided they were small enough to fit. For veins only a few microns in diameter they found that beads of diameter ≥5 μm were frozen into solid ice instead of fitting into veins. An important implication of their work is that micron-size bacteria and archaea with physiological adaptations to the concentrated vein environment may remain metabolically active in glacial ice, whereas eukarya, which are typically larger than 5 μm and would therefore be trapped in the solid phase, will not.

The exclusion of micron-size microorganisms from crystal interiors into veins has an interesting application to the study of the accretion ice that grows at the interface between Lake Vostok and the overlying glacier, forming crystals with grain size ∼20 cm or larger (Jouzel et al., 1999). Microbial concentrations at various depths in the accretion ice have shown wide dispersion: 98–430 cells mL−1 in a 490 g sample of type I accretion ice and 77–89 cells mL−1 in an 810 g sample of type II accretion ice (10 depths analyzed) by Christner et al. (2006), 0.1–10 cells mL−1 by Scott Rogers (private communication) and 0–10 cells mL−1 in a sample of unknown mass by S.A. Bulat (private communication). It would be tempting to attribute the higher values to contamination. However, both groups followed strict protocols for avoiding contamination. A more likely explanation is a consequence of the localization of microorganisms in veins. Out of a few ice samples taken at random from such coarse-grained accretion ice, some should contain no microbial cells, simply because the probability of a sample intersecting a vein is small if the grain size is large. In contrast, if the cells were not concentrated into veins, or if grain size was very heterogeneously distributed down to small sizes, all samples would contain cells with small variance.

Unfrozen water at subfreezing temperatures on mineral surfaces

  1. Top of page
  2. Abstract
  3. Introduction
  4. Liquid veins in glacial ice as a habitat for microbial life at low temperatures
  5. Unfrozen water at subfreezing temperatures on mineral surfaces
  6. Microbial life in silty ice at the bottom of the Greenland (GISP2) glacial ice
  7. Microbial life in clear ice at GISP2
  8. Metabolic rates in veins and on clay grains and the question of immortality
  9. Methanogens on Mars
  10. Cold origin of life
  11. Summary
  12. Acknowledgements
  13. References

Macromolecules, membranes and other hydrophilic components such as mineral surfaces in equilibrium in an aqueous solution depress the freezing point within a distance called the ‘hydration distance’. At temperatures below the normal melting point, Tm, several effects lower the chemical potential of the unfrozen water to equal that of ice. Wettlaufer (1999) developed the theory, which accounts for a host of environmental phenomena such as the ability of ice skaters to glide at temperatures far below freezing. His equation for equilibrium undercooling, Tm–T, as a function of thickness d of the unfrozen water contains four terms – an expression for depression of the freezing point of an ionic solution; the short-range attractive van der Waals contribution within a few tenths of a nm of a surface; the Coulombic (Debye-Hückel) contribution due to charges on the mineral surface; and the Gibbs-Thomson contribution, which depends on curvature of the walls of any small pores present.

In the case of clay, the thickness of the unfrozen layer decreases from several tens of nm at temperatures near 0°C to ∼0.3 nm or about the thickness of one water molecule at a temperature of −80°C. Experiments by Hoekstra & Miller (1967) and by Pearson & Derbyshire (1974) showed that the unfrozen water still has considerable mobility at such a low temperature. Figure 1 shows examples of the weight fraction of unfrozen water in contact with various types of clay and with a basalt, measured by differential thermal analysis (Anderson, 1967; Anderson & Tice, 1973). Due to their small grain size and enormous specific surface area (700–800 m2 g−1), swellable clays of the montmorillonite type comprise a group at the top of the plot; nonswellable clays of the kaolinite type, with specific surface area 25–200 m2 g−1, comprise an intermediate group; and basalt, at the bottom of the plot, is typical of minerals with very little unfrozen water relative to their mass.

image

Figure 1.  Data for fraction of water that remains unfrozen as a function of ΔT for minerals labeled B (kaolinite), E (Li-montmorillonite), F (Na-montmorillonite), G (K-montmorillonite), H (Ca-montmorillonite), J (basalt), and K (bentonite).

Download figure to PowerPoint

Until recently, experiments on unfrozen water in soils did not take into account the presence of soluble ions, even though Brown (1969) and others had shown that permafrost soils contained soluble salts up to 20 milliequivalents per 100 g of soil. Recently, however, using an NMR technique, Watanabe & Mizoguchi (2002) compared the Wettlaufer theory (1999) with measurements of the unfrozen water fraction in silty soil and bentonite powder containing a range of concentrations of dissolved NaCl. The general trends were reproduced, but with large deviations for large NaCl concentrations. For undercooling greater than ∼20 K, they found, in accord with theory, that the weight fraction and thickness of the unfrozen layer decreased as (Tm–T)−1/3. Extrapolating this expression to very low temperatures led to the conclusion that a monolayer of unfrozen water should remain at temperatures as low as 170 K. Figure 1 shows measurements of the fraction of unfrozen water as a function of temperature below 0°C, taken from a variety of sources. Because of the lack of knowledge of specific surface area and of ionic concentration, it is not possible to account in detail for the data in Fig. 1 on the basis of the Wettlaufer theory.

Israelachvili & Wennerström (1996) have reviewed experiments that led to the current view that a layer of structured water molecules, extending several molecular thicknesses from each surface, provides a protective force barrier against the approach of two hydrophilic surfaces or groups. Pollack (2001) has given a more general discussion of unfrozen water around micromolecules and inside cells.

Microbial life in silty ice at the bottom of the Greenland (GISP2) glacial ice

  1. Top of page
  2. Abstract
  3. Introduction
  4. Liquid veins in glacial ice as a habitat for microbial life at low temperatures
  5. Unfrozen water at subfreezing temperatures on mineral surfaces
  6. Microbial life in silty ice at the bottom of the Greenland (GISP2) glacial ice
  7. Microbial life in clear ice at GISP2
  8. Metabolic rates in veins and on clay grains and the question of immortality
  9. Methanogens on Mars
  10. Cold origin of life
  11. Summary
  12. Acknowledgements
  13. References

Some years ago Tison et al. (1998) found huge concentrations of CO2, CH4, clay grains, organic and inorganic ions in silty ice at the base of GRIP, a deep borehole at Summit, Greenland, 28 km east of the GISP2 borehole. The age vs. depth and temperature vs. depth in GRIP tracked those in GISP2 fairly closely. The temperature slowly increased from −32°C in the top few hundred meters to −9°C in the silty ice at the base (Gow & Meese, 1996). Tison et al. (1998) interpreted the excess of gases and organic ions in GRIP basal ice as due to local degradation of biological residues. Gow & Meese (1996) measured concentrations of silt grains at several depths in the GISP2 basal ice. Researchers at Pennsylvania State University (Sheridan et al., 2003; Miteva et al., 2004) and at Berkeley (Tung et al., 2005, 2006) independently conjectured that the excess CO2 and CH4 are due to ongoing in situ production by microorganisms that originated in a wetland area, rich in organic matter, and that survived the glaciation of Greenland some 400 000 year ago. Using flow cytometry and direct counting of stained cells from an ice sample at a depth of 3043 m, Sheridan et al. (2003) found 6.1 × 107 and 9.1 × 107 cells mL−1 in two duplicate samples, measured a live : dead ratio of ∼5 : 1, and determined that some of the cells were culturable anaerobes. Their analysis of 24 of the bacterial sequences obtained showed a diverse community of phylogenetic groups.

In a follow-up paper Miteva et al. (2004) isolated nearly 800 aerobic organisms and found that they belonged to the high-G+C gram-positives, low-G+C gram-positives, Proteobacteria, and Cytophaga–Flavobacterium–Bacteroides groups. Next Miteva & Brenchley (2005) discovered that ultra-small microorganisms, with volume <0.1 μm3, some of which were able to pass through 0.1-μm filters, dominated the population. They identified some of the isolates and found that they maintained their small cell sizes after recultivation.

The Berkeley group took an approach complementary to that of the Penn State group: they used SEM and direct counts to determine microbial habitats and concentrations at seven depths in the 13 m of silty ice between 3041 and 3054 m without carrying out molecular phylogenetic analyses. Their direct counts of DAPI-stained microorganisms showed that cell concentration correlated with concentration of silt grains and that between 90% and 95% of the cells were attached to thin clay platelets of typical diameter 1–2 μm and thickness 0.1–0.3 μm (Tung et al., 2006). With epifluorescence microscopy they were not able to resolve cell sizes, but they concluded that many of them were significantly smaller than 1 μm in size in order for as many as seven separately resolved fluorescing cells to be attached to silt grains only ∼3 μm in size.

Using F420 autofluorescence, Tung et al. (2006) found that ∼2.4% of the cells on silt grains were methanogens, which is roughly consistent with the ratio of CH4 to CH4+CO2 measured by Tison et al. (1998) in the nearby GRIP ice core. Another published study that relied on F420 autofluorescence to enumerate methanogens as a function of depth was that of Kotelnikova & Pedersen (1997), who found concentrations of ∼105 mL−1 of methanogens at all depths from 68 to 446 m below sea level in granitic rock groundwater from the Äspö Hard Rock Laboratory (Sweden). The methanogens they observed were cocci of diameter up to 4 μm, whereas the ones Tung et al. (2006) observed in the GISP2 ice were <1 μm in size.

Table 1 compares observations at GISP2 with those at GRIP as well as features of the overlying glacial ice with those of the silty ice at the bottom of the cores. The last column shows that the silty ice in GISP2 has far higher concentrations of microorganisms, mineral grains, gases, and organic and inorganic ions than does the glacial ice.

Table 1.   Microbes, gases, ions and mineral grains in silty and glacial ice at GISP2 and GRIP
 GISP2 silty ice at 3051 mGISP2 glacial ice at 3040 mGRIP silty ice at 3028 mGRIP glacial ice (range of values)Ratio in GISP2 silty to glacial ice
Cells (mL−1 meltwater)4 × 107–1 × 10101 × 104–2 × 105Not measuredNot measured103–106
Org. carbon (humics)19 250 μmoL L1*Not measuredNot measuredNot measuredNA
CO2>120 000 ppmv∼250 ppmv120 000 ppmv200–300 ppmv>500
Methane12 000 ppmv350–700 ppbv6000 ppmv§350–700 ppbv∼3 × 104
Ammonia8930 ng g1*7.9 ng g1370 ng g1§0.5–40 ng g1**1 × 103
Nitrate1165 ng g1*66 ng g1Not measured7.5–135 ng g1**18
Chloride7420 ng g1*44 ng g1Not measured2.5–45 ng g1**170
Sulfate20 680 ng g1*114 ng g1Not measured10–180 ng g1**180
Formate3100 ng g1*Not measured400 ng g1§1–16 ng g1**NA
Acetate3000 ng g1*Not measured400 ng g1§1.2–22 ng g1**NA
Oxalate530 ng g1*Not measured1800 ng g1§0.25–4 ng g1**NA
Phenol380 ng g1*Not measuredNot measuredNot measuredNA
Mass conc. of grains10−3–10−2 g g−1 wetland silt*5 × 10−8–5 × 10−6 g g−1 aeolian dust ††3 × 10−4–7 × 10−3 g g−1 silt‡‡5 × 10−8–5 × 10−6 g g−1 aeolian dust††200–2 × 105
Mineral grainsAll minerals:* 72% smectite 14% chlorite 14% otherClay minerals:§§ ∼0% smectite 51–62% illite 10–21% kaolinite 22–30% chloriteNot measuredAll minerals:¶¶ 0% smectite 1.25% kaolinite 6.25% illite 6.25% chlorite 21.25% micas 30% quartz 35% otherHigh smectite to illite ratio
Fe(III)/ΣFe in smectites∼92%∥∥Not measuredNot measuredNot measured 

Common types of anaerobes in anoxic freshwater sediments and bog waters are iron-reducers, denitrifiers, methanogens and possibly sulfate-reducers. Tung et al. (2006) obtained evidence for the presence of two of those groups living on clay grains in glacial ice. Figure 2 shows their unexpected finding that the number of microorganisms attached to clay grains in the silty ice is accurately proportional not to grain surface area but to the average diameter (and more important, the perimeter) measured in the plane of the plate.

image

Figure 2.  Measured values of number of cells as a function of diameter of platelike clay grains for DAPI-stained cells and for methanogens counted by their F420 autofluorescence, from Tung et al. (2006).

Download figure to PowerPoint

The linear relationship provided the clue that enabled them to solve the long-standing problem of how Fe-reducing bacteria attached to smectite (also known as montmorillonite) grains could gain access to the interior of the grains and reduce essentially all of the Fe3+ to Fe2+. Figure 3 shows the electron-shuttle mechanism by which Fe-reducers attached to clay grains carry out anaerobic metabolism. At the ambient temperature −9°C, both clay grains and microorganisms attached to their surfaces are surrounded by basal ice. Microbes, organic and inorganic solute ions, and gaseous metabolic products including H2, CO2, CH4 and probably also N2 and N2O are concentrated in a layer of unfrozen water of thickness ∼0.6 nm enclosing each clay grain (see Fig. 1). Solute ions and gases can move but microorganisms cannot. The concentrations of humic and phenol molecules in the silty ice (Table 1) are high enough that either of them can serve to shuttle electrons from Fe-reducers on the clay surfaces to edges of the grains (Lovley et al., 1996), from which they attach to Fe3+ ions in octahedral layers. The Fe-reducers accept electrons from formate and other organic ions in the unfrozen water. Reduction of Fe3+ ions takes place by a mechanism analogous to that involved in direct-current conductivity of Fe-rich phyllosilicates. The only difference is that in d.c. conductivity the electrons are driven along octahedral planes by an electrical potential difference, whereas in Fe3+ reduction initiated by shuttle molecules the electrons diffuse along octahedral planes, driven by a chemical potential difference (Rosso & Ilton, 2003). As shown in Fig. 4 (adapted from Rüscher & Gall, 1995), the d.c. conductivity of iron-rich mica increases roughly exponentially with iron content, depends on absolute temperature as exp(−U/RT), and is four to five orders of magnitude greater along basal planes than perpendicular to them. One can confirm this anisotropy by applying a d.c. potential difference first parallel and then perpendicular to the basal planes of a crystal of Fe-rich biotite. One finds that, whereas the current perpendicular to the basal planes is undetectably small, it is detectable in the basal planes. Tung et al. (2006) showed that electron shuttling works equally well in nonexpandable clays such as kaolinite as in expandable clays such as smectite.

image

Figure 3.  Electron shuttling by phenol or humic molecule from Fe-reducer to Fe3+ ions inside a clay grain. Dark blue background indicates solid ice; light blue indicates unfrozen water layer. A methanogen may make syntrophic use of metabolic products of an Fe-reducer.

Download figure to PowerPoint

image

Figure 4.  Extremely anisotropic direct current conductivity of Fe-rich phyllosilicates (data from Rüscher & Gall, 1995).

Download figure to PowerPoint

Other common types of anaerobes trapped in the unfrozen water on clay surfaces do not need access to the interior of the clay: denitrifiers utilize nitrate ions, and sulfate-reducers utilize sulfate ions, all of which are mobile in the unfrozen water surrounding the clay and microorganisms (see Table 1). Methanogenesis is an interesting case: at low temperatures metabolism can utilize either acetate or H2+CO2. As Fig. 3 suggests, the source of hydrogen in the unfrozen water around clay grains may be via a syntrophic relationship between Fe-reducers and methanogens. See Cord-Ruwisch et al. (1998) for a discussion of such a syntrophy under milder conditions.

Microbial life in clear ice at GISP2

  1. Top of page
  2. Abstract
  3. Introduction
  4. Liquid veins in glacial ice as a habitat for microbial life at low temperatures
  5. Unfrozen water at subfreezing temperatures on mineral surfaces
  6. Microbial life in silty ice at the bottom of the Greenland (GISP2) glacial ice
  7. Microbial life in clear ice at GISP2
  8. Metabolic rates in veins and on clay grains and the question of immortality
  9. Methanogens on Mars
  10. Cold origin of life
  11. Summary
  12. Acknowledgements
  13. References

Microbes in the glacial ice above the silty region are usually assumed to have been transported from desert regions (mainly the Gobi Desert in the case of GISP2 ice) and deposited onto the growing ice surface from above. Some aerobes and facultative anaerobes, following the several-day trip through the atmosphere, will end up inside micron-size aqueous veins, where some species may survive the high concentration of acidic or alkaline ions therein. Whether methanogens, which are strict anaerobes, can function after having been exposed for several days in air is problematic. Although the average mole fraction of oxygen in the ice is only ∼1.6 × 10−5, it would be much higher if concentrated in veins. I will return to this question at the end of this section.

Figure 5 shows the surprising results of the study by Tung et al. (2005) of microorganisms in the 3040 m of GISP2 glacial ice, which led them to propose a scenario in which the microorganisms were exhumed from the 14-m thickness of silty ice underneath. The top two panels, from measurements of trapped methane sampled every few meters of depth (Brook, unpublished results), shows concentrations that range from ∼350 to ∼750 ppbv, in strong correlation with climate, with the exception of anomalously high values at three depths: 2954, 3018 and 3036 m. To see whether the large excesses might be due to microbial metabolism, Tung et al. (2005) enumerated cells stained with Syto-23, observed with epifluorescence microscopy and shown in the middle panels, and methanogens, detected via the blue-green autofluorescence of their F420 coenzyme. They found excesses of both methanogens and Syto-23-stained cells at those three depths. At 3000 m there were also high concentrations of stained cells and of methanogens, but Brook did not measure methane at that depth. At 2238 m they saw an excess of stained cells; again, at that depth Brook did not measure methane. A visual inspection of the section of core from 2953 to 2954.1 m showed a dramatic change in appearance in a narrow region at ∼2953.7 m corresponding to the region with high cell concentrations: inclined, wavy, distorted layering contrasted sharply with the appearance of normal polycrystalline ice at 2953–2953.5 m. In Fig. 5, at all three depths, the regions with excess cells and methane were confined to narrow depth intervals of only ∼0.3 m. The interpretation of those regions by Tung et al. (2005) was that jerky ‘stick-slip’ glacial flow of ice across a frozen wetland repeatedly scraped thin layers of microbe-rich ice off of the silty region, intermixing it with glacial ice at distances up to nearly 100 m above the interface between silt and ice. Since Brook measured methane only at intervals of several meters, additional layers in the lower 100 m or so of glacial ice may also exist but not yet have been detected. It should be emphasized that the excesses at those three depths came from a microbial community that had been living in an anaerobic wetland before being frozen and incorporated into the 14-m thick silty ice at the bottom and into several layers of ice above the silty region. Implications for astrobiology of the in situ production of methane by methanogens in ice will be discussed in a later section.

image

Figure 5.  Upper two panels: methane concentration vs. depth in GISP2 ice core (E. Brook, unpublished results); middle two panels: concentration of cells stained with Syto-23 vs. depth; lower two panels: concentration of methanogens determined by counting F420 autofluorescence (adapted from Tung et al., 2005).

Download figure to PowerPoint

Whereas most of the cells in the silty ice were attached to clay grains, the cells in the three regions with excess methane were not. Although biofilm and stream-bed studies have shown that microorganisms remain attached to particles in turbulent aqueous flow, ‘stick-slip’ flow of ice over frozen sediment on a microscale (Knight, 2002; Jansen & Hergarten, 2006) may account for stripping and separation of microorganisms from clay grains.

Tung et al. (2006) estimated that those microbes survived in aqueous veins in the clear ice for ∼180 000±80 000 year. At depths shallower than 2954 m they found no methanogens (upper limit 100 cells mL−1), consistent with their view that cells that had been deposited onto the growing glacial ice from the air did not survive the long exposure to oxygen. At depths below ∼1400 m, all air bubbles are known to have transformed into the high-pressure clathrate phase, locking up O2, N2 and other atmospheric gases that had initially been present in the bubbles (Kipfstuhl et al., 2001). Thus, methanogens exhumed from the anaerobic wetland region to a depth >1400 m below the surface would probably not encounter oxygen along the way and would likely survive in anaerobic veins.

The work of Tung et al. (2006) does not exclude the possibility that some species of methanogens can metabolize even in oxygen-rich veins in GISP2 ice. The null results for F420 fluorescence in the five samples scanned at depths shallower than 2400 m (Fig. 5, lower left) do not rule out methanogens at concentrations lower than ∼100 mL−1. Nor do Brook's methane concentrations that are consistent with an atmospheric source rule out methanogenic metabolism at shallow depths, as the concentration of methane produced by methanogens is proportional not only to methanogen concentration but also to age (young for shallow ice) and to the Arrhenius factor exp (−U/RT), which is very small for temperatures characteristic of shallow depths.

There is considerable variability in the sensitivity of methanogens to oxygen (Kiener & Leisinger). Some species in cultures have been shown to survive exposure to oxygen for years if stored at a temperature below their minimum growth temperature (Reysenbach & Shock, 2002), and viable methanogens have been detected in nominally aerobic environments such as desert soil (Peters & Conrad, 1995), or where anaerobic microenvironments or transient aerobic anaerobic conditions occur (Skidmore et al., 2000). In addition, some species of methanogens have been found to contain catalase and superoxide dismutase, which serve as antioxidants (Brioukhanov et al., 2002). Furthermore, Campen et al. (2003) concluded (without microscopic examination) that excess methane in ice of meteoric origin in the Sajama (Bolivia) glacier was due to in situ methanogenesis, which supports the view that some methanogens originated from the atmosphere and metabolized in the ice.

Metabolic rates in veins and on clay grains and the question of immortality

  1. Top of page
  2. Abstract
  3. Introduction
  4. Liquid veins in glacial ice as a habitat for microbial life at low temperatures
  5. Unfrozen water at subfreezing temperatures on mineral surfaces
  6. Microbial life in silty ice at the bottom of the Greenland (GISP2) glacial ice
  7. Microbial life in clear ice at GISP2
  8. Metabolic rates in veins and on clay grains and the question of immortality
  9. Methanogens on Mars
  10. Cold origin of life
  11. Summary
  12. Acknowledgements
  13. References

Ions such as acetate, ammonia, sulfate, formate, acetate and bicarbonate (the preferred form of CO2 in aqueous solution at pH<8) are present in the vein structures and are available to microorganisms in the veins for metabolism, even at great depths (Tung et al., 2006).

Recently Price & Sowers (2004) showed that communities of microorganisms that are free to move but in maintenance mode, competing for nutrients, metabolize at rates three orders of magnitude higher than rates for confined microorganisms, and microorganisms that are not only free to move but are in an exponential growth phase with unlimited nutrients metabolize at rates some six orders of magnitude higher. They found that, within experimental error, the activation energy is the same for all three kinds of metabolism.

Figure 6, adapted from Tung et al. (2005), shows the dependence of metabolic rate on temperature for communities of microorganisms that are trapped in ice, rock, and deep subsurface sediment and are thus unable to grow. The rate is proportional to exp(−U/RT), where T is in kelvin and U is the activation energy in kJ mole−1, given by the slope of the straight line drawn through the data. An interesting feature of those data for imprisoned microorganisms is that the metabolic rate is remarkably similar to the rate, extrapolated from laboratory measurements at higher temperature (Brinton et al., 2002), of spontaneous racemization of amino acids in microorganisms from Siberian permafrost (Price & Sowers, 2004). Furthermore, extrapolated data for the rate of depurination of DNA (Lindahl & Nyberg, 1972) fall not far below the line for the metabolic rate of imprisoned microorganisms. The approximate agreement of the data on rates of racemization and depurination with metabolic rates for survival of immobilized, starved microorganisms suggests that microbial communities imprisoned in environments such as veins or clay grains in ice use the limited source of nutrients entirely to repair macromolecular damage, enabling them to stay alive for millions of years, even if their habitat is too confining for them to grow or move toward sources of nutrient. They survive because gaseous and dissolved nutrients diffuse toward them and waste products diffuse away. Clarke (2003) has shown that almost all cells contain a methyltransferase that can repair aging proteins, and he suggests that additional repair enzymes may also exist.

image

Figure 6.  Arrhenius plot of metabolic rates of microbial communities imprisoned in ice, rock, and deep subsurface ocean sediment (adapted from Tung et al., 2005). Methanogens in silty ice, ○; methanogens in glacial ice, ♦; stained cells in glacial ice, ▾; indicated locations, ▪; arrows show average temperatures in the floor of a Mars crater and on the Mars surface.

Download figure to PowerPoint

Methanogens on Mars

  1. Top of page
  2. Abstract
  3. Introduction
  4. Liquid veins in glacial ice as a habitat for microbial life at low temperatures
  5. Unfrozen water at subfreezing temperatures on mineral surfaces
  6. Microbial life in silty ice at the bottom of the Greenland (GISP2) glacial ice
  7. Microbial life in clear ice at GISP2
  8. Metabolic rates in veins and on clay grains and the question of immortality
  9. Methanogens on Mars
  10. Cold origin of life
  11. Summary
  12. Acknowledgements
  13. References

The recent discovery of ∼10 ppbv methane in the martian atmosphere (Formisano et al., 2004; Krasnopolsky et al., 2004) has important implications for life on that planet. The lifetime for destruction of methane in the martian atmosphere by solar UV photolysis is only ∼300 year, which requires that ∼270 tons of methane per year must be generated within Mars in order to offset the loss. Oze & Sharma (2005) and Lyons et al. (2005) proposed that methane could be produced abiogenically below the surface by alteration of olivine-rich rocks by CO2-bearing hydrothermal fluid, in reactions such as the following:

  • image(2)

or

  • image(3)

The requirements are an inventory of 1017 kg olivine in basalt at a moderate temperature such as ∼300 K, the presence of CO2 under permafrost and the absence of SO2. If SO2 were present in the martian interior, H2S would be produced instead of methane, by

  • image(4)

The usual way to distinguish whether a gas such as CO2 or CH4 is biogenic or abiogenic is to measure the ratio of two stable isotopes in the gas. A mass spectrometer able to measure the composition of the stable carbon isotopes in martian methane with high sensitivity could determine whether it is biogenic or abiogenic. Figure 7 shows the range of values of δ13C in various methane sources, using the conventional definition for δ13C as the permil deviation of the isotopic ratio of a sample relative to the PDB carbonate standard that defines zero permil on the δ scale: δ13C=[(13C/12C)sample/(13C/12C)PDB−1]103 (‰). Reading down, one sees that methane in carbonate rocks, in the mantle and in carbonaceous chondrites is abiogenic, and methane emitted by cattle, by termites, in sedimentary organics, in the Earth's atmosphere, from plants (recently discovered), in natural gas, in rice paddies, in peat deposits, from methanogens, in sea-floor methane hydrates and in Greenland silty ice (Souchez, Jouzel & Landais et al., manuscript in preperation) is biogenic. Methane on the moon was found by MS of lunar soil to be abiogenic (Chang et al., 1974), and methane in the lower atmosphere of Titan was found by the Gas Chromatograph/Mass Spectrometer (GC/MS) on the orbiting Huygens probe to be abiogenic (Niemann et al., 2005).

image

Figure 7.  Range of values of δ13C of methane for biogenic and abiogenic sources relative to PDB standard carbonate rock [adapted from Schidlowski (1987) and Whiticar (1999), with data added for Titan (Niemann et al., 2005) and Earth's moon (Chang et al., 1974)].

Download figure to PowerPoint

Detection of a value of δ13C more negative than about −50‰ would be strong evidence for the presence of living methanogens on Mars. A GC/MS will be part of the instrument package for the rover-based Mars Science Lab, which is scheduled for a launch in 2009 and a Mars landing in 2010. The Planetary Fourier Spectrometer on Mars Express found at least three large areas – Arabia Terra, Elysium Planum and Arcadia-Memnonia – with higher methane concentrations than elsewhere (Allen & Oehler, 2005). It would be interesting to make isotopic measurements in one of those regions. Another justification for landing in Arabia Terra is that it is one of the sites where water-bearing clay minerals have been identified by the OMEGA imaging spectrometer on the orbiting Mars Express (Poulet et al., 2005; Michalski et al., 2006). The clays found include Fe-rich smectites – the same type of clay to which Fe-reducing bacteria in the GISP2 silty ice are attached. Arabia Terra is a dark deposit of fine dust with warmer surface temperature than average for Mars; the summer day–night range is from ∼298 K to ∼170 K, and the highest average annual temperature is ∼225 K. The coexistence of clay, water and methane in the same region suggests that a habitat in which anaerobic microbial life may have arisen in the early history of Mars might survive to the present day.

Despite the fact that the problem is underconstrained, one can use the metabolic rates in Fig. 6 to say something about the number of martian methanogens at various depths that would produce methane at a rate sufficient to account for the presence of 10 ppbv in the atmosphere. Mellon & Phillips (2001) gave temperature gradients for Mars for three materials with different subsurface composition and thermal conductivity. Only the most favorable case they treated will be considered; that of a dry, unconsolidated soil with thermal conductivity 0.045 W m−1 K−1. If a 100-m-thick surface shell were inhabited by methanogens of mass 20 fg cell−1 at a temperature of −43°C where the metabolic rate is 10−8 year−1, the concentration necessary to produce enough methane to offset its loss by photodecomposition would be ∼108 cm−3. At a temperature of −10°C, one can see from Fig. 6 that the metabolic rate would be three orders of magnitude higher. Then, for a uniform coverage of Mars, the concentration of methanogens in a shell 100–200 m deep would be ∼105 cm−3. At a depth of 180 m, where the temperature is even higher, ∼20°C, the metabolic rate would be ∼10−3 year−1 and the concentration in a shell 130–210 m deep might be as low as ∼103 cm−3.

Although we have considered only 100 m shells, methanogens would presumably also be present at other depths, including close to the surface, and near-surface samples containing biomolecules and perhaps microbial cells could be collected and studied by an instrument on a future rover, even though the temperature in the surface soil is too cold for them to make much methane. It would be very exciting to send a probe that could identify and measure the concentration of methanogens near the top of the martian soil. If methanogens were present, Fe-reducers and other anaerobes might also be living in unfrozen water layers on clay grains near the surface, even if their metabolic rates were too low to measure.

Solar UV, solar flares and aggressive oxidants such as superoxides probably destroy organic molecules and all except the hardiest microorganisms (such as Deinococcus radiodurans) in the top 10 cm of the surface (Benner et al., 2000). Gardening of the soil by micrometeorite impacts extends the danger zone down to a depth of a meter or more. Proposals to develop an automated device capable of obtaining and examining a core ∼10 m in vertical extent are thus under consideration for a future Mars lander. Skelley et al. (2005, 2006) have developed a microfabricated capillary electrophoresis instrument that will analyze the composition and chirality of amino-acid biomarkers. Tests in the Panoche Valley, CA, showed that minerals such as jarosite, which are associated with liquid water and which were detected by the Mars rovers, preserve amino acids that were detected by Skelley et al. (2005) at the 70 parts per trillion level. A location inside martian jarosite might provide such acids with protection from superoxides and solar radiation. This instrument, scheduled for launch in 2011 or 2013, will analyze samples to be collected by a rover fitted with a drill. Its sensitivity to amino acids is estimated to be sufficient to detect ∼5000 cells the size of E. coli. For dwarf cells of size 0.1 μm3, the minimum detectable number would be ∼5 × 104 cells.

There is strong evidence for water at mid- and low-latitudes in the martian surface. Boynton et al. (2002), Feldman et al. (2002) and Mitrofanov et al. (2002) used instruments on NASA's Mars Odyssey to detect thermal and epithermal neutrons and 2.2-MeV gamma-rays from cosmic-ray interactions with hydrogen in the top meter or so. Assuming the hydrogen maps water, they produced maps of water in the top meter of Mars. Water in the form of ice in the top meter is lost by sublimation at an average annual temperature of 225 K. The present martian thermal conditions allow only unfrozen water, discussed earlier, to have survived near the surface over geological time scales at mid- and low-latitudes. Hardy microorganisms attached to clay grains near the surface would thus have access to a thin layer of unfrozen water in which to metabolize at the low rate indicated by Fig. 6.

Cold origin of life

  1. Top of page
  2. Abstract
  3. Introduction
  4. Liquid veins in glacial ice as a habitat for microbial life at low temperatures
  5. Unfrozen water at subfreezing temperatures on mineral surfaces
  6. Microbial life in silty ice at the bottom of the Greenland (GISP2) glacial ice
  7. Microbial life in clear ice at GISP2
  8. Metabolic rates in veins and on clay grains and the question of immortality
  9. Methanogens on Mars
  10. Cold origin of life
  11. Summary
  12. Acknowledgements
  13. References

Opinion has been divided over two extreme scenarios for the synthesis of prebiotic molecules leading to the origin of microbial life: a hot origin deep underground or near hydrothermal vents in the ocean vs. a cold origin in veins or on surfaces of mineral grains. Those who favor a cold origin include Stanley Miller, David Deamer, Leslie Orgel, Alex Vlassov, Shin Miyakawa, Antonio Lazcano, Hauke Trinks and Jeffrey Bada.

The arguments that favor a cold origin are attractive. First of all, because of the reduced luminosity of the young Sun, Earth may have been covered with ice during its early history (Bada et al., 1994). Based on their measurements of half-lives for decomposition of nucleotide bases as a function of temperature, Levy & Miller (1998) concluded that if life arose at a temperature of ∼100°C, the bases A, G, U and C could not have accumulated unless synthesis occurred on a time scale of less than a few years. Even at a temperature as low as 0°C, although A, U, T and G would accumulate for up to ∼106 year, cytosine would last only ∼17 000 year, which would have required life to arise quickly after a sterilization event. An argument in support of a hot origin had been that rRNA sequences seemed to show that the first phyla that emerged in the tree of life were hyperthermophilic (for example, see DiGiulio, 2003). However, that conclusion has been questioned. Miller & Lazcano (1995) pointed out that hyperthermophiles may appear at the base of some phylogenetic trees because they outcompeted older mesophiles when they adapted to lower temperatures, rather than having been the sole survivors of an impact event. Using a more reliable analysis of bacterial phylogeny, Brochier & Philippe (2002) found that hyperthermophilic bacteria do not emerge first. Instead, Planctomycetales, a phylum that includes psychrophiles found in several polar regions, could be the Last Uniform Common Ancestor to all life forms (LUCA). Galtier et al. (1999) gave an independent argument: the G+C nucleotide content of rRNA sequences is strongly correlated with the optimal growth temperature of archaea and bacteria, which allows the environmental temperature of LUCA to be inferred from knowledge of the G+C content of its rRNA sequences. The inferred G+C content of the common ancestor to extant life forms appears incompatible with survival at high temperature.

In the current view that the early Earth was dominated by a carbon dioxide atmosphere, the significance of the celebrated experiment of Miller (1953) on synthesis of bioorganic compounds from CH4, NH3, H2 and H2O in a vessel subjected to spark discharges has been discounted. Recently, however, Miller and his colleagues (Miyakawa et al., 2002a) carried out experiments showing that in a more likely atmosphere consisting of CO-CO2-N2-H2O subjected to MeV cosmic ray protons, amino acids and nucleotide monomers could have been synthesized. Furthermore, there is general agreement (Chyba & Sagan, 1992; Botta & Bada, 2002) that the early Earth was probably seeded with complex prebiotic molecules by infall of carbonaceous meteorites (which contain purines, pyrimidines and 17 of the 20 amino acids found in humans) and comets (which contain formaldehyde, hydrogen cyanide and other simple organics).

In regard to the model of a cold origin, a common misconception is that polymerization of amino acids and nucleotides into proteins, RNA and DNA would have proceeded too slowly at low temperatures. Two independent experimental and theoretical research strategies have succeeded in compensating for a low temperature at which prebiotic molecules might form macromolecules: liquid veins in ice and the surfaces of clay grains. The only difference in the sites proposed for polymerization of biomolecules and in the habitats for microbial life is that the studies of polymerization on clay have been carried out at ∼25 to ∼100°C, whereas the clay grains discussed in this minireview as habitats for microbial life in ice and permafrost were at subfreezing temperatures. In view of the short lifetime of cytosine at 0°C, it would be interesting to run polymerization experiments on clay grains at subfreezing temperatures.

It is necessary to concentrate the correct prebiotic molecules and organize them into the desired structure, and at the same time to avoid their tendency to hydrolyze, not polymerize, in water. Some key molecules including proteins and carbohydrates form from water-soluble units – amino acids and nucleotides. To polymerize them from an initially dilute solution, they must be concentrated in veins in ice or on a mineral surface that provides a scaffolding.

Freezing provides a unique environment, reducing RNA degradation while also concentrating solvents and reducing water activity, which can catalyze a number of diverse reactions (Vlassov, 2005). Three of the many recent experiments that used liquid veins as the concentrating medium are particularly noteworthy. Miyakawa et al. (2002b) froze a solution of NH4CN at −78°C and held it continuously in a sealed vessel for 27 year. They detected monomers of 11 different purines and pyrimidines including A, G and U, and concluded that a cold earth is more favorable for chemical evolution than a warm earth.

Polymerization of nucleotides in aqueous solution is an uphill reaction and does not occur spontaneously to a significant extent (Orgel, 2004). Thus, attempts to polymerize nucleotides from aqueous solution must make use of external activating agents. Kanavarioti et al. (2001) showed that in the presence of Mg2+ and Pb2+ ions, frozen samples of phosphoimidazolide-activated uridine (ImpU) react within days at −18°C to form oligouridylates at least 11 bases long. The same method led to oligomers of ImpC, ImpA and ImpG up to five bases long. To show that the reactions took place in liquid veins, they introduced acridine orange into the initial solution and photographed the fluorescence of the solution in the veins, as shown in Fig. 8 (lower left).

image

Figure 8.  Top: Gel electrophoretic separation of 5′-labeled oligonucleotides [poly(A)], from Trinks et al. (2005). Lower left: Epifluorescence micrograph of frozen ice containing oligouridylate products of synthesis at −18°C as described in Kanavarioti et al. (2001). Imaging was done by adding acridine orange to the solution that segregated into the veins. Lower right: Synthesis of polypeptides via amino acids adsorbed on clay surfaces (from Hazen, 2001).

Download figure to PowerPoint

Recently Trinks et al. (2005) prepared sea ice by freezing sterile solutions of sea water salts devoid of organic material, introduced poly(U) and 5′-adenyl (2-methyl)-imidazolide, and cycled the ice between −7°C and −24°C for a year. As shown in Fig. 8 (top), they found poly(A) chains of RNA up to at least 420 monomers in length, which they suggested was due to poly(U)-instructed synthesis.

Other researchers have used mineral surfaces, albeit at warm or room temperatures, to bring adsorbed prebiotic monomers together. Clays are especially useful, as was recognized by Bernal (1951) and Cairns-Smith & Hartman (1986). Being very fine-grained and with flat surfaces, they have a huge surface to mass ratio; in addition, they have a surface electrostatic charge that helps to adsorb organic molecules and prevent them from hydrolyzing. Ferris (2002) has shown that clays can act as scaffolds in the formation of RNA strands of more than 50 nucleotides, provided they start with a solution of RNA nucleotides plus imidazole, which activates bonding between nucleotides. Liu and Orgel (1998) have succeeded in forming protein-like chains of amino acids of similar lengths. Clays can also serve as scaffolds for polymerization of amino acids into polypeptides (Fig. 8, lower right).

Clay has additional advantages. DNA molecules change from the B- to the A-form when adsorbed on clay, increasing their resistance to degradation by solar UV radiation by a factor of ∼200 (Scappini et al., 2004). DNA molecules remain biologically active when adsorbed on clay, in contrast to their short lifetime when not attached (Demaneche et al., 2001). Furthermore, after the emergence of cellular life, horizontal gene transfer to cells attached to clay surfaces is promoted via cell-to-cell contact, via phage transduction from host to recipient cells and via adsorbed DNA to a host (Gallori et al., 1994).

Summary

  1. Top of page
  2. Abstract
  3. Introduction
  4. Liquid veins in glacial ice as a habitat for microbial life at low temperatures
  5. Unfrozen water at subfreezing temperatures on mineral surfaces
  6. Microbial life in silty ice at the bottom of the Greenland (GISP2) glacial ice
  7. Microbial life in clear ice at GISP2
  8. Metabolic rates in veins and on clay grains and the question of immortality
  9. Methanogens on Mars
  10. Cold origin of life
  11. Summary
  12. Acknowledgements
  13. References

The stability of unfrozen water at temperatures far below the melting point in triple junctions of grain boundaries in ice containing ionic impurities and in thin films on solid surfaces in ice and permafrost provides two kinds of habitats for microbial life on Earth and, in principle, also on Mars and other cold planetary bodies. Bacteria and archaea that may have originated on other planets during the early history of the solar system, when their surface temperatures were warmer than now, may have survived to the present day in both veins and on clay surfaces, even though their metabolic rates may be extremely low at current subfreezing temperatures.

In their study of microorganisms attached to iron-rich clay grains in the basal ice at GISP2, Tung et al. (2006) inferred from F420 autofluorescence that a few percent of them were methanogens. In addition, they found that the relationship between number of attached microorganisms and clay grain size was linear instead of quadratic, which suggested that the microorganisms used an electron shuttle mechanism to access nutrients via grain edges. In the case of Fe-reducers, the shuttle mechanism accounts for how all of the Fe(III) inside a clay grain can be reduced. Sheridan et al. (2003) and Miteva et al. (2004) used molecular phylogeny to classify the taxa of both anaerobes and aerobes in the same basal ice, and Miteva & Brenchley (2005) showed that dwarf cells of a number of taxa found in the ice could be cultivated.

It would be interesting to make a thorough search in shallow GISP2 glacial ice to see if some species of methanogens are present and can metabolize in oxygen-containing veins.

One could test the hypothesis that microorganisms are localized in liquid veins in solid ice using a compact spectrophotometer to scan along an ice core at the National Ice Core Laboratory and see whether the highly fluorescent amino acid tryptophan is concentrated only at locations where the beam passes through triple junctions (which could be made visible via crossed polarized films).

Communities of bacteria and archaea imprisoned in rock or ice obey an Arrhenius relation from which the metabolic rate can be predicted for similar microorganisms that might have arisen on planetary bodies such as Mars and Europa. The similarity of both the metabolic rate and rate of spontaneous macromolecular damage to living microorganisms led to the conjecture that such immobilized communities might have lifetimes measured in millions of years (Price & Sowers, 2004).

Mars has extensive regions rich in clay minerals. Even at low near-surface temperatures, a thin layer of unfrozen water almost certainly coats mineral grains (Möhlmann, 2003), accounting for the survival of water detected within a meter of the Mars surface despite the high vapor pressure of solid water.

By means of a mass spectrometer planned for a 2010 landing, a measurement of the δ13C of martian methane in a region with higher than average methane concentration may be able to determine if it was produced by methanogens, thus potentially resolving the puzzle of whether the martian methane is predominantly biogenic or abiogenic.

There is growing evidence in favor of a cold origin of terrestrial life. Furthermore, there is a striking coincidence between the two habitats for microbial life in ice and two sites suggested for polymerization of biomolecules, namely, veins at grain boundaries in ice and unfrozen water films on surfaces of mineral grains.

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Liquid veins in glacial ice as a habitat for microbial life at low temperatures
  5. Unfrozen water at subfreezing temperatures on mineral surfaces
  6. Microbial life in silty ice at the bottom of the Greenland (GISP2) glacial ice
  7. Microbial life in clear ice at GISP2
  8. Metabolic rates in veins and on clay grains and the question of immortality
  9. Methanogens on Mars
  10. Cold origin of life
  11. Summary
  12. Acknowledgements
  13. References
  • Allen CC & Oehler DZ (2005) Thinking like a wild-catter–prospecting for methane in Arabia Terra, Mars. Lunar Planet Sci XXXVI paper no. 1398.
  • Anderson DM (1967) Ice nucleation and the substrate-ice interface. Nature 216: 563566.
  • Anderson DM & Tice AR (1973) The unfrozen interfacial phase in frozen soil water systems. Ecological Studies, Vol. 4. pp. 107124. Springer, Berlin.
  • Bada JL, Bigham C & Miller SL (1994) Impact melting of frozen oceans on the early Earth: implications for the origin of life. ProcNatl Acad Sci USA 91: 12481250.
  • Baker I, Cullen D & Iliescu D (2003) The microstructural location of impurities in ice. Can J Phys 81: 19.
  • Barnes PRF & Wolff EW (2004) Distribution of soluble impurities in cold glacial ice. J Glac 50: 311324.
  • Benner SA, Devine KG, Matveeva LN & Powell DH (2000) The missing organic molecules on Mars. Proc Natl Acad Sci USA 97: 24252430.
  • Bernal JD (1951) The Physical Basis of Life. Routledge & Kegan Paul, London, UK.
  • Botta O & Bada JL (2002) Extraterrestrial organic compounds in meteorites. Surv Geophys 23: 411467.
  • Biscaye PE, Grousset FE, Revel M, VanderGaast S, Zielinski GA, Vaars A & Kukla G (1997) Asian provenance of glacial dust (stage2) in the Greenland Ice Sheet Project 2 Ice Core, Summit, Greenland. J Geophys Res 102: 26,76526,781.
  • Blunier T, Chappellaz JA, Schwnder J, Stauffer B & Raynaud D (1995) Variations in atmospheric methane concentration during the Holocene epoch. Nature 374: 4649.
  • Boynton WV, Feldman WC, Squyres SW et al. (2002) Distribution of hydrogen in the near surface of Mars: evidence for subsurface ice deposits. Science 297: 8185.
  • Brinton KLF, Tsapin AI, Gilichinsky D & McDonald GD (2002) Aspartic acid racemization and age-depth relationships for organic carbon in Siberian permafrost. Astrobiology 2: 7782.
  • Brioukhanov AL, Thauer RK & Netrusov AI (2002) Catalase and superoxide dismutase in the cells of strictly anaerobic microorganisms. Microbiology 71: 282285.
  • Brochier C & Philippe H (2002) A non-hyperthermophilic ancestor for bacteria. Nature 417: 244.
  • Brook EJ, Sowers T & Orchardo J (1996) Rapid variations in atmospheric methane concentration during the past 110,000 years. Science 273: 10871091.
  • Brown J (1974) Ionic concentration gradients in permafrost, Barrow, Alaska. Res. Rept. 272, US Army Cold Regions Research and Engineering Laboratory, Hanover, NH, 1969.
  • Bulat SA, Alekhina IA, Blot M et al. (2004) DNA signature of thermophilic bacteria from the aged accretion ice of Lake Vostok, Antarctica: implications for searching for life in extreme icy environments. Int J Astrobiol 3: 112.
  • Cairns-Smith AG & Hartman H (1986) Clay Minerals and the Origin of Life. Cambridge University Press, Cambridge, UK.
  • Campen RK, Sowers T & Alley RB (2003) Evidence of microbial consortia metabolizing within a low-latitude mountain glacier. Geology 31: 231234.
  • Chang S, Lawless J, Romirez M, Kaplan IR, Petrowski C, Sakai H & Smith JW (1974) Carbon, nitrogen and sulfur in lunar fines 15012 and 15013: abundances, distributions and isotopic compositions. Geochim Cosmochim Acta 38: 853872.
  • Christner BC, Royston-Bishop G, Foreman CM, Arnold BR, Tranter M, Welch KA, Lyons WB, Tsapin AI & Priscu JC (2006) Limnological conditions in subglacial Lake Vostok, Antarctica. Limnol Oceanogr, in press.
  • Chyba CF & Sagan C (1992) Endogenous production, exogenous delivery, and impact-shock synthesis of organic molecules: an inventory for the origins of life. Nature 355: 125132.
  • Clarke S (2003) Aging as war between chemical and biochemical processes: protein methylation and the recognition of age-damaged proteins for repair. Ageing Res Rev 2: 263285.
  • Cord-Ruwisch R, Lovley DR & Schink B (1998) Growth of Geobacter sulfurreducens with acetate in syntrophic cooperation with hydrogen-oxidizing anaerobic partners. Appl Environ Microbiol 64: 22322236.
  • Deer WA, Howie RA & Zussman J (1967) Rock-Forming Minerals, Vol.3: Sheet Silicates, pp. 236245. Longmans, Green & Co, London.
  • Demaneche S, Jocteur-Monrozier L, Quiquampoix H & Simonet P (2001) Evaluation of biological and physical protection against nuclease degradation of clay-bound plasmid DNA. Appl Environ Microbiol 67: 293299.
  • DiGiulio M (2003) The universal ancestor and the ancestor of bacteria were hyperthermophiles. J Mol Evol 57: 721730.
  • Feldman WC, Boynton WV, Tokar RL et al. (2002) Global distribution of neutrons from Mars: results from Mars Odyssey. Science 297: 7578.
  • Ferris JP (2002) Montmorillonite catalysis of 30-50 mer oligonucleotides: laboratory demonstration of potential steps in the origin of the RNA world. Orig Life Evol Biosphere 32: 311332.
  • Formisano V, Atreya S, Encrenaz T, Ignatiev N & Giuranna M (2004) Detection of methane in the atmosphere of Mars. Science 306: 17581761.
  • Gallori E, Bazzicalupo M, Dal Canto L, Fani R, Nannipieri P, Vettori C & Stotzky G (1994) Transformation of Bacillus subtilis by DNA bound on clay in non-sterile soil. FEMS Microbiol Ecol 15: 119126.
  • Galtier N, Tourasse N & Gouy M (1999) A nonhyperthermophilic common ancestor to extant life forms. Science 283: 220221.
  • Gow AJ & Meese DA (1996) Nature of basal debris in the GISP2 and Byrd ice cores and its relevance to bed processes. Ann Glac 2: 134140.
  • Hazen RM (2001) Life's rocky start. Sci Am 284: 7785.
  • Hoekstra P & Miller RD (1967) On the mobility of water molecules in the transition layer between ice and a solid surface. J Colloid Interface Sci 25: 166173.
  • Israelachvili J & Wennerström H (1996) Role of hydration and water structure in biological and colloidal interactions. Nature 379: 219225.
  • Jansen F & Hergarten S (2006) Rock glacier dynamics: stick-slip motion coupled to hydrology. Geophys Res Lett 33: L10502.
  • Jouzel J, Petit JR, Souchez R, Barkov NI, Lipenkov VYa, Raynaud D, Stievenard M, Vassiliev NI, Verbeke V & Vimeux F (1999) More than 200 meters of lake ice above subglacial Lake Vostok Antarctica. Science 286: 21382141.
  • Junge K, Eicken H & Deming JW (2004a) Bacterial activity at −2 to −20°C in Arctic wintertime sea ice. Appl Environ Microbiol 70: 550557.
  • Junge K, Deming JW & Eicken H (2004b) A microscopic approach to investigate bacteria under in situ conditions in Arctic lake ice: initial comparisons to sea ice. Bioastronomy 2002: Life Among the Stars, Vol. 213. IAU Symposium pp. 381388.
  • Kanavarioti A, Monnard P-A & Deamer D (2001) Eutectic phases in ice facilitate nonenzymatic nucleic acid synthesis. Astrobiology 1: 271281.
  • Kiener A & Leisinger T (1983) Oxygen sensitivity of methanogenic bacteria. System Appl Microbiol 4: 305312.
  • Kipfstuhl S, Pauer F, Kuhs WF & Shoji H (2001) Air bubbles and clathrate hydrates in the transition zone of the NGRIP deep ice core. Geophys Res Lett 28: 591594.
  • Knight J (2002) Glacial sedimentary evidence supporting stick-slip basal ice flow. Quat Sci Rev 21: 975983.
  • Kotelnikova S & Pedersen K (1997) Evidence for methanogenic Archaea and homoacetogenic Bacteria in deep granitic rock aquifers. FEMS Microbiol Rev 20: 339349.
  • Krasnopolsky VA, Maillard JP & Owen TC (2004) Detection of methane in the martian atmosphere: evidence for life? Icarus 172: 537547.
  • Legrand MR, De Angelis M & Maupetit F (1993) Field investigation of major and minor ions along the summit (central Greenland) ice cores using ion chromatography. J Cromatogr 640: 251258.
  • Levy M & Miller SL (1998) The stability of the RNA bases: implications for the origin of life. Proc Natl Acad Sci USA 95: 79337938.
  • Lindahl T & Nyberg N (1972) Rate of depurination of native deoxyribonucleic acid. Biochem 11: 36103618.
  • Liu R & Orgel LE (1998) Polymerization on the rocks; β-amino acids and arginine. Orig Life Evol Biosphere 28: 245257.
  • Lovley DR, Coates JD, Blunt-Harris EL, Phillips EJP & Woodward JC (1996) Humic substances as electron acceptors for microbial respiration. Nature 382: 445448.
  • Lyons JR, Manning C & Nimmo F (2005) Formation of methane on Mars by fluid-rock interaction in the crust. Geophys Res Lett 32: L13201.
  • Mader HM, Pettitt ME, Wadham JL, Wolff EW & Parkes RJ (2006) Subsurface ice as a microbial habitat. Geology 34: 169172.
  • Maggi V (1997) Mineralogy of atmospheric microparticles deposited along the Greenland Ice Core Project ice core. J Geophys Res 102: 2672526734.
  • Mayewski PA, Meeker LD, Twickler MS, Whidow SI, Yang Q, Lyons WB & Preatice M (1997) Major features and forcing of high-latitude northern hemisphere atmospheric circulation using a 110,000-year-long glaciochemical series. J Geophys Res 102: 2634526366.
  • Mellon MT & Phillips RJ (2001) Recent gullies on Mars and the source of liquid water. J Geophys Res Planets 106: 2316523179.
  • Michalski JR, Kraft MD, Sharp T, Williams LB & Christensen PR (2006) Emission spectroscopy of clay minerals and evidence for poorly crystalline aluminosilicates on Mars from thermal emission spectrometer data. J Geophys Res 111: E03004.
  • Miller SL (1953) A production of amino acids under possible primitive Earth conditions. Science 117: 528529.
  • Miller SL & Lazcano A (1995) The origin of life – did it occur at high temperatures? J Mol Evol 41: 689692.
  • Miteva VI & Brenchley JE (2005) Detection and isolation of ultrasmall microorganisms from a 120,000-year-old Greenland glacier ice core. Appl Environ Microbiol 71: 78067818.
  • Miteva VI, Sheridan PP & Brenchley JE (2004) Phylogenetic and physiological diversity of microorganisms isolated from a deep Greenland glacier ice core. Appl Environ Microbiol 70: 202213.
  • Mitrofanov I, Anfimov D, Kozyrev A et al. (2002) Maps of subsurface hydrogen from the high energy neutron detector, Mars Odyssey. Science 297: 7881.
  • Miyakawa S, Yamanashi H, Kobayashi K, Cleaves HJ & Miller SL (2002a) Prebiotic synthesis from CO atmospheres: implications for the origin of life. Proc Natl Acad Sci USA 99: 1462814631.
  • Miyakawa S, Cleaves HJ & Miller SL (2002b) The cold origin of life: B. Implications based on pyrimidines and purines produced from frozen ammonium cyanide solutions. Orig Life Evol Biospheres 32: 209218.
  • Möhlmann DTF (2003) Unfrozen subsurface water on Mars: presence and implications. Int J Astrobiol 2: 213216.
  • Niemann HB, Atreya SK, Bauer SJ et al. (2005) The abundances of constituents of Titan's atmosphere from the GCMS instrument on the Huygens probe. Nature 438: 779784.
  • Orgel LE (2004) Prebiotic chemistry and the origin of the RNA world. Crit Revs Biochem Mol Biol 39: 99123.
  • Oze C & Sharma M (2005) Have olivine, will gas: serpentinization and the abiogenic production of methane on Mars. Geophys Res Lett 32: L10203.
  • Pearson RT & Derbyshire W (1974) NMR studies of water adsorbed on a number of silica surfaces. J Colloid Interface Sci 46: 232248.
  • Peters V & Conrad R (1995) Methanogenic and other strictly anaerobic bacteria in desert soil and other oxic soils. Appl Environ Microbiol 61: 16731676.
  • Pollack GH (2001) Cells, Gels, and the Engines of Life. Ebner & Sons, Seattle.
  • Poulet F, Bibring J-P, Mustard JF et al. (2005) Phyllosilicates on Mars and implications for early martian climate. Nature 438: 623627.
  • Price PB (2000) A habitat for psychrophiles in deep Antarctic ice. Proc Natl Acad Sci USA 97: 12471251.
  • Price PB & Sowers T (2004) Temperature dependence of metabolic rates for microbial growth, maintenance, and survival. Proc Natl Acad Sci USA 101: 46314636.
  • Priscu JC & Christner BC (2004) Earth's icy biosphere. Microbial Diversity and Bioprospecting (BullAT, ed), pp. 130145. ASM Press, Washington, DC.
  • Reysenbach AL & Shock E (2002) Merging genomes with geochemistry in hydrothermal ecosystems. Science 296: 10771082.
  • Rosso KM & Ilton ES (2003) Charge transport in micas: the kinetics of FeII/III electron transfer in the octahedral sheet. J Chem Phys 119: 92079218.
  • Rüscher CH & Gall S (1995) On the polaron mechanism in iron-bearing trioctahedral phyllosilicates: an investigation of the electrical and optical properties. Phys Chem Minerals 22: 468478.
  • Scappini F, Casadei F, Zamboni R, Franchi M, Gallori E & Monti S (2004) Protective effect of clay minerals on adsorbed nucleic acid against UV radiation: possible role in the origin of life. Int J Astrobiol 3: 1719.
  • Schidlowski M (1987) Application of stable carbon isotopes to early biochemical evolution of Earth. Ann Rev Earth Planet Sci 15: 4772.
  • Sharma A, Scott JH, Cody GD, Fogel ML, Hazen RM, Hemley RJ & Huntress WT (2002) Microbial activity at gigapascal pressures. Science 295: 15141516.
  • Sheridan PP, Miteva VI & Brenchley JE (2003) Phylogenetic analysis of anaerobic psychrophilic enrichment cultures obtained from a Greenland glacier ice core. Appl Environ Microbiol 69: 21532160.
  • Skelley AM, Scherer JR, Aubrey AD, Grover WH, Ivester RHC, Ehrenfreund P, Grunthaner FJ, Bada JL & Mathies RA (2005) Development and evaluation of a microdevice for amino acid biomarker detection and analysis on Mars. Proc Natl Acad Sci USA 102: 10411046.
  • Skelley AM, Cleaves JH, Jayarajah CN, Bada JL & Mathies RA (2006) Analysis of nucleobase and amine biomarkers using the Mars organic analyzer microchip system. Astrobiology, in press.
  • Skidmore ML, Foght JM & Sharp MJ (2000) Microbial life beneath a high Arctic glacier. Appl Environ Microbiol 66: 32143220.
  • Steffensen JP (1997) The size distribution of microparticles from selected segments of the Greenland Ice Core Project ice core representing different climatic periods. J Geophys Res 102: 2675526763.
  • Tison J-L, Souchez R, Wolff EW, Moore JC, Legrand MR & De Angelis M (1998) Is a periglacial biota responsible for enhanced dielectric response in basal ice from the Greenland ice core project ice core? J Geophys Res 103: 1888518894.
  • Trinks H, Schröder W & Biebricher CK (2005) Ice and the origins of life. Orig Life Evol Biospheres 35: 429445.
  • Tung HC, Bramall NE & Price PB (2005) Microbial origin of excess methane in glacial ice and implications for life on Mars. Proc Natl Acad Sci USA 102: 1829218296.
  • Tung HC, Price PB, Bramall NE & Vrdoljak G (2006) Microorganisms metabolizing on clay grains in 3-km-deep Greenland basal ice. Astrobiology 6: 6986.
  • Vlassov AV (2005) Mini-ribozymes and freezing environment: a new scenario for the early RNA world. Biogeosciences Discussions 2: 17191737.
  • Watanabe K & Mizoguchi M (2002) Amount of unfrozen water in frozen porous media saturated with solution. Cold Regions Sci Technol 34: 103110.
  • Wettlaufer JS (1999) Impurity effects in the premelting of ice. Phys Rev Lett 82: 25162519.
  • Whiticar MJ (1999) Carbon and hydrogen isotope systematics of bacterial formation and oxidation of methane. Chem Geol 161: 291314.