Relationships between species
The most striking result of the PCA analyses was the observation of two different groups. Despite the fact that no species presented a specific PLFA profile, organism associations were found. The two associations: mussel–commensal worm–whelk and shrimps–crab have different PLFA indicative of different bacteria in their diets. The mussel–commensal worm–whelk association relies more on S-oxidizing bacteria, while the crab–shrimp association has more SRB anaerobic biomarkers. This indicates that the latter group of animals apparently lives in conditions closer to anoxia than the other, which is congruent with the microdistribution of these organisms (Desbruyères et al. 2001). The large central group represents omnivorous species that feed on bacteria in addition to other types of food (e.g. C. chacei). The individual FAs do not differentiate groups but the percentage and composition of PUFA are responsible for the observed distribution.
These two associations (mussel–commensal worm–whelk and shrimps–crab) were also demonstrated by stable isotope analysis, with the first association presenting very depleted δ13C (around −30‰), and the second presenting less depleted δ13C (around −11‰) values (Colaço et al. 2002).
The individuals at the center of the PCA axes have a more diversified bacterial diet, with several different biomarkers from S-oxidizing and S-reducing bacteria. The association of mussels and Phymorhynchus sp. was also observed through stable isotope analysis (Fisher et al. 1994; Colaço et al. 2002) as was the mussel–commensal worm coupling (Colaço et al. 2002).
The FA composition of M. fortunata fell midway between that of the other shrimp species (R. exoculata, C. chacei, A. markensis). Rimicaris exoculata is considered to be bactivorous, and C. chacei and A. markensis to be carnivorous or scavengers. The PLFA profile of M. fortunata indicates that this species is omnivorous, feeding not only on other animals (carnivorous) but also on S-oxidizing bacteria (bactivorous), as indicated by the presence of branched FAs, monounsaturated ω7 FAs and more NMID (cf. Table 2). However, according to our definition, this species might also be a secondary consumer, eating animals that are eating the bacteria, as was indicated by Colaço et al. (2002). The crab S. mesatlantica shows a similar pattern to that of M. fortunata, despite possessing fewer branched FAs. According to Colaço et al. (2002) the crab's trophic position is not that of omnivore, but rather predator, feeding on bactivorous animals like amphipods and other mixotrophs like shrimps. If this is the case, the bacterial FA present in these animals is from the prey food source and not from bacteriophagy.
The polychaete A. lutzi presented a characteristic FA profile, with the higher amounts of branched FAs indicative of a diversified bacterial diet. As a non-selective deposit feeder, it can eat different particles from the hydrothermal vent environment.
Detailed analyses of the FA profiles show an absence of HUFA (C20:5ω3 and C22:6ω3) in all the taxonomic groups studied, suggesting that if the vent fauna relies on photosynthetically produced material, it is as transformed material like nutrients or polysaccharide molecules, rather than lipids. The low amounts of PUFA also support this idea, especially in light of the absence of C20:4ω6.
The presence of phyto-biomarkers at some hydrothermal vent fields (e.g. 13° N) and their absence at the Galapagos field were tentatively related to the mussel condition. The condition parameter was the loss of symbionts (Ben-Mlih et al. 1992). This situation is not surprising, as the mussels did not lose their filter-feeding capacity, and in the absence of symbionts, phytodetritus-derived particles would be one of the available energy sources. Mussel symbionts disappear when the mussel environment loses the symbionts’ energy source (P. Dando, personal communication).
The PUFA present in the PLFA lipids of the studied species are due to two factors, the desaturation possible in all aerobic Δ9 organisms, and the desaturation specific to certain Δ5 marine invertebrates. The presence of high proportions of NMID from C18:1ω7 is a clear indicator that the species rely on chemoautotrophic bacteria, as has already been indicated by several authors (Zhukova 1991; Pond et al. 1997b; Pranal et al. 1997).
Shrimps Rimicaris exoculata
In the deep-sea hydrothermal vent fields at MAR, the three main sources of dietary carbon that are available to the shrimp R. exoculata are: (i) bacteria fixed on the mouthparts of this species; (ii) bacteria associated with the minerals that these organisms ingest from the sulphide chimneys; and (iii) detritus from the oceanic photic zone. The absence of HUFA and the low levels of PUFA with the exception of the NMID, make the third hypothesis rather improbable, consistent with the results of Rieley et al. (1999). The lipid profile of this species is also different from that of deep-sea benthic shrimps (Nematocarcinus gracilis) that rely on phototrophically derived organic matter (Allen 1998). However, according to Pond et al. (1997a), juvenile R. exoculata rely on material from the photic zone of the ocean. Allen et al. (2001) also showed that in this species, adults and juveniles exhibit different partitioning of lipids in tissues, with an increased proportion of bacterial-derived organic matter in adults compared with that of juveniles.
Comparison of mouthparts and digestive gland PLFA profiles revealed a close similarity, with the digestive gland having more NMID. When looking at the MUFAs that might underlie the origin of these NMID, the FA profiles of the mouthpart and the digestive gland are very similar. Earlier studies showed that the bacteria from the mouthparts (epibionts) and bacteria from sulphides are closely related (Rieley et al. 1999). However, by means of a specific compound stable isotope analysis, it was shown that the epibiont bacteria have a δ13C signal similar to that of the shrimp, while the sulphide bacteria are much more 13C depleted. Considering these factors, and from the results presented here, we conclude that this species relies on bacteria present in its mouthparts.
Mussels (Bathymodiolus azoricus, B. puteoserpentis, Bathymodiolus spp.)
Analyses of FAs as biomarkers often allow for an evaluation of host–symbiont energy relationships and carbon sources (Jahnke et al. 1995; Pranal et al. 1997).
Despite the fact that the mussels host S-oxidizing and methanotrophic bacteria (Fiala-Médioni et al. 2002), no methanotrophic biomarker was observed in the mussel PLFA profiles. This result contradicts Pond et al. (1998), in which these biomarkers were found in mussels from Lucky Strike and Menez Gwen. Type I methanotroph biomarkers (C16:1ω6; and C18:1ω6) were observed in other species but in smaller amounts. The presence of the FA C18:1ω13, a methylotrophic bacteria FA, does not allow the determination to which type of methanotrophic bacteria the mussels rely on, as methylotrophs are ubiquitous microorganisms that can use C1 compounds (methanol, methylamine, methane, etc.) for growth and only some specialized methylotrophs, are the methanotrophs. Bowman et al. (1991) characterized several methanotrophic bacteria from PLFA profiles and showed that some bacteria did not present the typical biomarkers of the methanotrophic types. Despite the lack of typical methanotrophic PLFA, the presence of other methanotrophic bacteria cannot be excluded. The presence of NMID C20:2ω6, 15 and C20:2ω8,15 in mussels leads us to believe that these FAs are the results of a specific biosynthetic pathway based on chain elongation and original Δ5 desaturation of the FAs C16:1ω6; C16:1ω8; 18:1ω6 and C18:1ω8 (which are methanotrophic biomarkers). The methanotrophic FA published in Pranal et al. 1997, were not detected in the mussels. As the deep-sea hydrothermal environment is continually changing, the hypothesis that the mussels have lost methanotroph symbionts and began using other symbionts cannot be ruled out. The presence of S-oxidizing bacteria biomarkers (C16:1ω7; C18:1ω7; and C20:11ω7) and NMID derived from FAs (e.g. C18:3ω7,10,13; C20:3ω7,10,15; C20:2ω7,15) and the absence of HUFA are consistent with the results of Pond et al. (1998) and with other studies on animals hosting S-oxidizing endosymbiotic bacteria or feeding on them (Conway & McDowell Capuzzo 1991; Zhukova 1991; Zhukova et al. 1992; Fullarton et al. 1995; Pranal et al. 1997). The presence of PLFA from SRB in mussels proves that they continue to be filter feeders.