Figure 10 Average stable isotopic signatures (513C and 515N) ± 1 SD of macroinfauna from seeps and background sediments. Data are from Levin & Michener (2002), Van Dover et al. (2003) and Levin unpublished. Note there is a positive linear relationship between 513C and 515N, but that the Pacific (and one Atlantic) sites fall along a different line than the Gulf of Mexico sites.

2002) and 15-40% for sipunculans from the Blake Ridge (Van Dover et al. 2003). The fraction of this methane that is derived from fossil sources, rather than from recent biogenic generation, is unknown. Paull et al. (1985) showed, by 14C analysis of mussel tissues from the Florida Escarpment seeps, that very little (<25%) of the carbon is of fossil origin, despite exceptionally light 813C signatures.

In other regions seep macrofauna have much heavier 813C signatures. This is observed in clam beds off Oregon and California, and microbial mat sediments on the Eel River margin or in Atwater Valley, Gulf of Mexico (Levin & Michener 2002, Levin et al. unpublished data) (Figure 10), reflecting incorporation of largely thiotrophically-derived C or even photosynthetically fixed organic matter.

Possible evidence for the influence of seepage intensity on animal nutrition comes from the observation of distinctive isotope signatures in different local habitat patches. Microbial mat infauna have lighter 813C signatures than those in nearby clam beds on the Oregon margin, but not on the California margin (Levin & Michener 2002). Average infaunal 813C signatures varied by nearly 5%e (-24.6 ± 1.1%e vs. -19.9 ± 0.3%e, P < 0.001) in two nearby Calyptogena pacifica patches, suggesting different seepage activities, but fluxes or sulphide concentrations were not measured (Levin et al. 2000). Even greater differences were observed for macrofauna inhabiting nearby black (average 813C = -67.0%e) and white sediment patches (average 813C = -36.9%e) at seeps on the Florida Escarpment in the Gulf of Mexico (Levin et al. unpublished data).

It is apparent from the nitrogen isotope signatures of Pacific seep macrofauna, including crustaceans, polychaetes, molluscs, sipunculans, echinoderms and cnidarians, that the majority of individuals derive their nutrition heterotrophically rather than from symbionts. Well-known exceptions are thyasirid, lucinid, solemyid, vesicomyid and bathymodiolid bivalves, and all pogonophorans and vestimentiferans. Several other seep polychaetes, including polynoids and ampharetids from Kodiak seeps in the Gulf of Alaska (Levin & Michener 2002) and the dorvilleid Parougia sp. from the Eel River margin, northern California (Levin unpublished data), have exceptionally light 815N signatures

(near or below 0) characteristic of taxa known to possess symbionts and should also be examined for possible endosymbioses.

Notably, within a single seep site, 813C signatures may vary among species by 70%e (Levin & Michener 2002), reflecting a mixture of nutritional sources including carbon from archaeal-derived lipids (which can have 813C values of -69 to -111%e in seep sediments; Werne et al. 2002, Zhang et al. 2003), methanotrophy, thiotrophy and photosynthesis. Several lines of evidence indicate that closely related infauna can specialize on different food sources in seeps sediments. Three co-occurring dorvilleid polychaete species have highly distinctive isotopic signatures in Eel River seep sediments, suggesting strong diet partitioning (L. Levin unpublished data). Isotopic signatures much lighter than that of methane are usually associated with lipids derived from anaerobic methane oxidation (Hinrichs et al. 2000). The observation of extremely negative 813C values (<-70%e) in some annelids from seeps in the Pacific and Gulf of Mexico (Levin & Michener 2002) indicates that these organisms either selectively ingest archaeal-derived carbon or live in sediment horizons where this is the primary carbon source available.

Stable isotopes provide only indistinct clues about the diets of seep infauna. Terrestrial organic matter with isotope signatures near -25%c can mask the contributions of chemosynthetically fixed carbon (Levin et al. 2000). A suite of more complex techniques should be employed to examine the relative contributions of food items to animal diets and of different carbon-fixation pathways to the animal carbon pool. These methods might include fatty acid analysis of tissues (including stable isotopic signatures of fatty acids; Pond et al. 1997, 2000), molecular sequencing of gut contents (Duplessis et al. 2004), use of inorganic isotopic tracers such as 14CO2, 13CO2, and 35SO4, as well as more traditional feeding experiments and gut content observations. MacAvoy et al. (2002b) demonstrated that chemosynthetically produced essential and precursor fatty acids isolated from host tissues (mussels and tubeworms) will retain the 813C signature of the bacteria that made them, allowing specific identification of the carbon sources used by symbionts for fatty acid synthesis, and ultimately the tracking of these through the food web.

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