Gill endosymbiont respiration and nutritional strategies in the chemosymbiotic bivalve family Lucinidae

The metabolism and physiological role of D-alanine was investigated in the lucinid species Lucinoma aequizonata and Codakia orbicularis. D-alanine is the dominant free amino acid in C. orbicularis during all life stages. D-alanine was found at similar concentrations in both aposymbiotic and symbiotic juveniles indicating it is not synthesized by the symbiont. The absence of D-aspartate and D-glutamate, substrates for D-alanine aminotransferase, in the gill and body tissue of C. orbicularis does not support the idea of a stereospecific aminotransferase.; A large fraction of the reduced carbon utilized by the lucinid host is derived from chemoautotrophic symbionts. To learn more about the nutritional role of the symbiont during the development of L. aequizonata, a PCR assay was used to detect the symbiont. Examination of mature gonads, eggs and veligers indicated that the symbiont is acquired from the environment at a later developmental stage. Field and laboratory studies demonstrated that feeding remains an important source of nutrition in both the larval and adult stage of L. aequizonata.; Like all chemoautotrophic symbioses tested to date, the lucinid C. orbicularis is capable of assimilating ammonia. Ammonia is assimilated via glutamine synthetase in both gill and body tissue, comprising up to 46% of the L-glutamine pool within 1 hour exposure to 15N-ammonium. Three hours later, label is found in both L-glutamate and L-alanine. The turnover rate is rapid; the label is almost completely removed after a 24-hour pulse with unlabeled ammonium. The assimilation of ammonia is most likely an important source of nitrogen since it is acquired rapidly and is present in the habitat of C. orbicularis.; The ecological relevance of nitrate and oxygen respiration by the gill endosymbiont of C. orbicularis was examined by biochemical and enzymatic techniques along with oxygen and sulfide microelectrodes. Nitrate respiration does not appear to be important for the symbiont because of low environmental nitrate concentrations. Oxygen was rapidly consumed by the symbionts indicating that it is the primary oxidant. Under anoxic conditions, the purified symbionts released sulfide into the surrounding medium possibly as a mechanism to maintain redox balance.