The nutritional role of endosymbiotic bacteria in animal-bacteria symbioses : Solemya velum, a case study

Abstract

The trophie interactions occurring between endosymbiotic bacteria and the host in animal-bacteria symbioses were investigated using the endosymbiont-containing protobranch clam, Solemya velum, as a general model. C, N, and S stable isotope compositions were investigated in tissues of S. velum; 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 which were different to those of organisms utilizing a phytoplanktonbased food chain. The δ15N values of both S. velum and the endosymbiont fraction were comparable and considerably lower than those of bivalve controls that do not harbor endosymbionts. The δ34S values of S. velum were very negative and suggest the use of biogenically produced H2S as a S source. The δ13C and δ15N isotope ratios of S. velum suggest that the endosymbiotic bacteria may provide almost 100% of the hosts C and N budgets. The lipid composition of most organisms is related to dietary intake. Consequently, a detailed analysis of the lipid composition of S. velum was undertaken to verify the nutritional importance of the symbiotic bacteria. The δ13C ratios of the lipids were also measured in order to determine potential carbon sources for the lipids of S. velum. The lipids of S. velum were characterized by large amounts of 18:1w7 (cis-vaccenic acid, a lipid found in many species of bacteria), 1.6:0, 16:1ro7 and low concentrations of the highly unsaturated fatty acids characteristic of most marine bivalves. The predominant sterol found in S. velum was cholesterol. The cisvaccenic acid found in S. velum is almost certainly symbiontderived and could serve as a biomarker for symbiont-lipid incorporation by the animal host. The high concentrations of cisvaccenic acid in the tissues of S. velum suggest an important role for the endosymbionts in the lipid metabolism of this bivalve. In addition, the presence of cis-vaccenic acid in all the major lipid classes of S. velum demonstrates both incorporation and utilization of this fatty acid. The reduced amounts of polyunsaturated fatty acids found in S. velum, along with the absence of sterols of plant origin, provide further evidence to suggest that this symbiosis relies on endosymbiont chemoautotrophy to fulfill the majority of its nutritional requirements. The δ13C ratios of the fatty acids and sterols of S. velum were very negative and similar to the values found for the fatty acids of Thiomicrospira crunogena, a sulfuroxidizing bacterium, suggesting that the lipids of S. velum are either derived directly from the endosymbionts or are synthesized using carbon derived from the endosymbionts. The stable isotope and lipid composition profiles of S. velum were very similar to those of Solemya borealis, a newly discovered symbiosis, and Inanidrilus leukodermatus, a symbiotic annelid. This demonstrates the usefulness of stable isotopes and lipid composition studies in the analysis and characterization of these types of symbioses. In particular, both techniques are valuable as initial screening tools for the presence of symbiotic sulfur-oxidizing bacteria. In order to further characterize differences between S. velum and typical marine bivalves, total and free amino acids were determined for S. velum, and Mya arenaria. Both the relative and absolute amounts of free amino acids differed significantly between the two species. In S. velum, the absolute concentrations of the sulfur amino acid taurine were greater than the total free amino acid concentrations typically found in bivalves. The possible roles for taurine as a sink for oxidized sulfur compounds in the Solemya velum symbiosis was examined. Preliminary experiments suggest that taurine levels increase in the presence of reduced sulfur compounds; this demonstrates a possible like between sulfuroxidation and taurine synthesis in this symbiosis.