The subseafloor biosphere associated with deep-sea hydrothermal vents sustains a diverse range of chemical energy sources capable of driving chemolithoautotrophic metabolism. Based upon studies of microbial isolates, there are at least six known pathways of carbon fixation, each with a unique phylogenetic distribution, and specific requirements for energy, metal cofactors, and reducing power. All of the newest pathways have been elucidated in thermophilic and hyperthermophilic microorganisms, particularly Archaea. As these studies require the enrichment and isolation of pure cultures, which can be challenging even in temperate environments, the overall diversity of carbon fixation pathways, how, and why they vary under different environmental conditions is unknown. We propose that studying microbial carbon fixation in anaerobic, thermophilic microcosm experiments by tracing 13-C labeled DIC into DNA and subsequently sequencing its meta-genome, will elucidate both who is fixing carbon at high temperatures and how it is being fixed. This work will complement phylogenomic and biogeochemical studies associated with CORK observatories installed on the eastern flank of the Juan de Fuca Ridge. Results of this work will provide critical data to integrate with rate measurements of biogeochemical activities and with cultivation independent genomic data derived from the subseafloor biosphere.