Incorporation and utilization of bacterial lipids in theSolemya velum symbiosis

We undertook a detailed analysis of the lipid composition ofSolemya velum (Say), a bivalve containing endosymbiotic chemoautotrophic bacteria, in order to determine the presence of lipid biomarkers of endosymbiont activity. The symbiont-free clamMya arenaria (L.) and the sulfur-oxidizing bacteriumThiomicrospira crunogena (Jannasch et al.) were analyzed for comparative purposes. Theδ13C 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 ofS. velum were characterized by large amounts of 18: 1ω7 (cis-vaccenic acid), 16:0, and 16 : 1ω7 fatty acids, and low concentrations of the highly unsaturated plant-derived fatty acids characteristic of most marine bivalves. Cholest-5-en-3β-ol (cholesterol) accounted for greater than 95% of the sterols inS. 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 : 5ω3 and 22 : 6ω3 and a variety of plant-derived sterols. The fatty acids ofT. crunogena were similar to those ofS. velum and were dominated by 18:1ω7, 16:0 and 16:1ω7 fatty acids. Thecis-vaccenic acid found inS. velum is almost certainly symbiontderived and serves as a potential biomarker for symbiontlipid incorporation by the host. The high concentrations ofcis-vaccenic acid (up to 35% of the total fatty acid content) in both symbiont-containing and symbiont-free tissues ofS. velum demonstrate the importance of the endosymbionts in the lipid metabolism of this bivalve. The presence ofcis-vaccenic acid in all the major lipid classes ofS. velum demonstrates both incorporation and utilization of this compound. Theδ13C ratios of the fatty acids and sterols ofS. velum were significantly lighter (−38.4 to −45.3‰) than those ofM. arenaria (−23.8 to − 24.2‰) and were similar to the values found for the fatty acids ofT. crunogena (−45‰); this suggests that the lipids ofS. velum are either derived directly from the endosymbionts or are synthesized using endosymbiontderived carbon.

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