Hydrothermal systems integrate geologic processes at a planet’s surface with those from the deep subsurface, yielding an incredible array of geochemical compositions. Studies of extremophiles inhabiting such environments have provided key insights that have substantially informed our understanding of the habitability of Earth and other planetary bodies. While considerable evidence exists that describes the diversity and distribution of microorganisms in surface accessible hydrothermal environments, little is so far known of the extent and nature of highly reduced, subsurface hydrothermal biospheres.

Several important questions regarding the deep continental biosphere remain largely unanswered, including: 1) What factors control the lowest depth limit of life? 2) What are the sources of carbon for deep life? and 3) What processes regulate the energy flux for deep life? Opportunities to address these important questions are limited by the cost of drilling to great depth.

Despite 40 years of research, knowledge of in situ metabolism of microbes carrying out carbon fixation in marine hydrothermal systems is still very limited. Particularly lacking are studies identifying the chemosynthetically active microbes and measuring rates of CO2 fixation in situ. Here, we present data from innovative incubation approaches for assessing chemoautotrophic production in deep-sea vent systems. We quantified chemosynthetic primary productivity of natural microbial communities sampled from Crab Spa (9oN East Pacific Rise) during incubations that maintained seafloor pressure and temperature.