INCORPORATION AND UTILIZATION OF BACTERIAL LIPIDS IN THE SOLEMYA-VELUM SYMBIOSIS

We undertook a detailed analysis of the lipid composition of Solemya velum (Say), a bivalve containing endosymbiotic chemoautotrophic bacteria, in order to determine the presence of lipid biomarkers of endosymbiont activity. The symbiont-free clam Mya arenaria (L.) and the sulfur-oxidizing bacterium Thiomicrospira crunogena (Jannasch et al.) were analyzed for comparative purposes. The delta-C-13 ratios of the fatty acids and sterols were also measured to elucidate potential carbon sources for the lipids of each bivalve species. Both fatty acid and sterol composition differed markedly between the two bivalves. The lipids of S. velum were characterized by large amounts of 18:1-omega-7 (cis-vaccenic acid), 16:0, and 16:1-omega-7 fatty acids, and low concentrations of the highly unsaturated plant-derived fatty acids characteristic of most marine bivalves. Cholest-5-en-3-beta-ol (cholesterol) accounted for greater than 95% of the sterols in S. velum. In contrast, M. arenaria had fatty acid and sterol compositions similar to typical marine bivalves and was characterized by large amounts of the highly unsaturated fatty acids 20:5-omega-3 and 22:6-omega-3 and a variety of plant-derived sterols. The fatty acids of T. crunogena were similar to those of S. velum and were dominated by 18:1-omega-7, 16:0 and 16:1-omega-7 fatty acids. The cis-vaccenic acid found in S. velum is almost certainly symbiont-derived and serves as a potential biomarker for symbiont-lipid incorporation by the host. The high concentrations of cis-vaccenic acid (up to 35% of the total fatty acid content) in both symbiont-containing and symbiont-free tissues of S. velum demonstrate the importance of the endosymbionts in the lipid metabolism of this bivalve. The presence of cis-vaccenic acid in all the major lipid classes of S. velum demonstrates both incorporation and utilization of this compound. The delta-C-13 ratios of the fatty acids and sterols of S. velum were significantly lighter (-38.4 to -45.3 parts-per-thousand) than those of M. arenaria (-23.8 to -24.2 parts-per-thousand) and were similar to the values found for the fatty acids of T. crunogena (-45 parts-per-thousand); this suggests that the lipids of S. velum are either derived directly from the endosymbionts or are synthesized using endosymbiont-derived carbon.