As expected based on previous studies of symbiont metabolism, genes and pathways for sulfur oxidation and carbon fixation were found in all five Ca. Thiodiazotropha endoloripes and in both Ca. Thiosymbion oneisti draft symbiont genomes (Fig. 2). Nematode and bivalve symbionts encoded a complete tricarboxylic acid (TCA) cycle and transporters for uptake of organic compounds and thus have the potential for heterotrophic growth. Both are capable of using oxygen and oxidized nitrogenous compounds such as nitrate as terminal electron acceptors, but only the bivalve symbiont draft genomes encoded genes for uptake hydrogenases, which would allow them to use hydrogen as an energy source. Bivalve and nematode symbionts both produce intracellular elemental sulfur granules21,22. The genomes also revealed the potential to store organic carbon in the form of polyhydroxyalkanoates (PHAs) and phosphorous in the form of polyphosphate granules. A detailed comparative genomics study of the lucinid and stilbonematine symbionts is beyond the scope of this study and will be published elsewhere.