Metagenomic analysis suggests broad metabolic potential in extracellular symbionts of the bivalve Thyasira cf. gouldi

Abstract

Next-generation sequencing has opened new avenues for studying metabolic capabilities of bacteria that cannot be cultured. Here, we provide a metagenomic description of a chemoautotrophic gammaproteobacterial symbiont associated with Thyasira cf. gouldi, a sediment-dwelling bivalve from the family Thyasiridae. Symbionts of thyasirids differ from those of other bivalves by being located outside rather than inside gill epithelial cells, and recent work suggests that they are capable of living freely in the environment. The T. cf. gouldi symbiont genome shows no signs of genomic reduction and contains many genes that would only be useful outside the host, including flagellar and chemotaxis genes. The thyasirid symbiont may be capable of sulfur oxidation via both the sulfur oxidation and dissimilatory sulfate reduction pathways, as observed in other bivalve symbionts. In addition, genes for hydrogen oxidation and dissimilatory nitrate reduction were found, suggesting varied metabolic capabilities under a range of redox conditions. The genes of the tricarboxylic acid cycle are also present, along with membrane bound sugar importer channels, suggesting that the bacteria may be mixotrophic. In this study, we have generated the first thyasirid symbiont genomic resources and lay the groundwork for further research in tracking the changes required for life as a bivalve symbiont.