Stable isotope partitioning in seep and vent organisms: chemical and ecological significance

Several hundred stable isotopic ratios (C, N and S) acquired over seven years of investigations at both seep and vent locations are compiled and interpreted. The stable isotopic compositions of tissues derived from the chemosynthetic fixation of carbon reflect a complex interaction between chemical and biological processes. The stable isotopic composition of bivalves that utilize reduced sulfur suggests that seawater-and/or vent water-dissolved inorganic carbon (DIC) is their primary source of carbon during chemosynthesis. All thiotrophic bivalves studied appear to possess a similar sulfide oxidation metabolism. The δ¹³C-values of tissues from methanotrophic mussels are close to the δ¹³C of the methane utilized. Apparently, little of the kinetic isotope fractionation associated with methanotrophy is expressed in the host's tissue. Vestimentiferan carbon isotopic composition reflects both carbon limitation and the isotopic composition of the substrate utilized. The δ¹³C-values of vent vestimentiferans tend to be affected by carbon limitation, whereas those of seep vestimentiferans reflect the variable isotopic composition of pore-water DIC. Stable nitrogen isotopic compositions are consistent with nitrogen (N₂) fixation, but the presence of the enzyme responsible for nitrogen fixation, nitrogenase, has not been conclusively demonstrated. A variety of nitrogen sources [N₂, NH₄⁺, PON (particulate organic nitrogen), DON (dissolved organic nitrogen) and NO₄⁻] may be utilized by vent and seep organisms. However, the δ¹⁵N data suggest that the mechanism of nitrogen metabolism is not a function of the species or the symbiont type. Sulfur is a key element in vent and seep environments and thiotrophy is the major chemosynthetic activity. The sources of sulfur are highly variable in quantity and isotopic composition but are almost always linked to bacterial activity,l either free-living and/or symbiont. Nitrogen and sulfur nutritional requirements appear to be derived from a wide variety of sources. The relative importance of nutrition derived from heterotrophy and chemoautotrophy depends on the chemical environment and animal physiology. Stable isotope compositions provide insight into these diverse metabolic strategies; however, a complete inventory of the concentration and isotopic composition of inorganic and organic substrates, as well as supporting biochemical, enzymatic and observational studies, are needed to resolve fundamental ecological questions.