During my tenure as a C-DEBI graduate fellow, my work fell into Research Theme I: Activity in the Deep Subseafloor Biosphere: function & rates of global biogeochemical processes. Few studies have directly measured sulfate reduction at hydrothermal vents, and relatively little is known about how environmental or ecological factors influence rates of sulfate reduction (SR) in vent environments. This work features laboratory studies using radioisotopic 35S-tracer techniques to elucidate rates of microbially mediated sulfate reduction from hydrothermal vents in the Middle Valley and Main Endeavor vent field along the Juan de Fuca Ridge, as well as assessments of bacterial and archaeal diversity, estimates of total biomass and the abundance of functional genes related to sulfate reduction, and in situ geochemistry. The rates of sulfate reduction measured suggest that-within anaerobic niches of hydrothermal deposits-heterotrophic sulfate reduction may be quite common and can contribute substantially to secondary productivity, underscoring the potential role of this process in both sulfur and carbon cycling at vents and the deep subsurface. Direct metabolic rate measurements of sulfate reduction help to facilitate defining key environmental and energetic parameters for microbial community colonization in hydrothermal vents. Better understanding of the metabolic and taxonomic relationship of these endolithic communities in the geochemical context of hydrothermal vents will help to constrain the microbial impact on global biogeochemical cycling.