The role of endosymbiotic bacteria in the nutrition of Solemya velum: Evidence from a stable isotope analysis of endosymbionts and host

C, N, and S stable isotope compositions were investigated in tissues of the protobranch bivalve Solemya velum. This species is known to harbor endosymbiotic chemoautotrophic bacteria. C and N isotope values were also examined in an enriched bacterial fraction, separated from intact gills by differential centrifugation. The bacterial fraction and host tissues had similar δ13C values, suggesting that endosymbiotic bacteria provide as much as 98% of the carbon requirements of S. velum. The δ15N values of both S. velum and the endosymbionts were comparable and considerably lower than those of bivalve controls that do not harbor endosymbionts. Uptake of pore‐water ammonium by the endosymbionts and subsequent translocation of nitrogenous compounds to the host may account for the low δ15N of S. velum. In this context, the symbionts may provide 100% of the host nitrogen. The δ34S values of S. velum suggest the use of biogenically produced H2S as a source of S. The stable isotope ratios of sulfur and nitrogen are among the lowest reported for animal tissues. This study attempts to estimate the nutritional significance of bacterial endosymbionts by analyzing the stable isotope ratios of both bacteria and host tissue.