Award: OCE-1136727

Deep-sea hydrothermal vents, first discovered in 1977, are exemplary ecosystems where microbial chemosynthesis rather than photosynthesis is the primary source of organic carbon. Chemosynthetic microorganisms use the energy generated by oxidizing reduced inorganic chemicals contained in the vent fluids, such as hydrogen sulfide (H2S) or hydrogen (H2), to convert carbon dioxide (CO2) into cell material. By doing so, they effectively transfer the energy from a geothermal source to higher trophic levels, in the process supporting the unique and fascinating ecosystems that are characterized by high productivity - oases in an otherwise barren landscape. While this general view of the functioning of these ecosystems is well established, there are still major gaps in our understanding of the microbiology and biogeochemistry of these systems. The integration of microbiological and molecular biological approaches, analytical chemistry, thermodynamics, and isotopic analysis at a single deep-sea vent site (Crab Spa at 9ºN on the East Pacific Rise) is unique, and has provided novel insights into the functioning of deep-sea vent ecosystems, and the constraints regulating the interactions between the microbes and their abiotic and biotic environment. Up to this point, process-oriented studies investigating activities of vent microbes and the rates of the reactions they catalyze were largely missing. To this end, we performed shipboard incubation experiments under simulated seafloor conditions to generate data that are necessary to assess the larger role of these ecosystems in global biogeochemical cycles. Isobaric gas-tight fluids samplers were developed as a tool to conduct incubation experiments at sea under in situ pressures, representing a new capability for the field of experimental microbiology. The growth of microbes under conditions that closely mimic natural environments allows quantitative investigation of factors that regulate metabolic strategies within the oceanic crust. Using this experimental set up we were able, for the first time, to derive empirical data allowing us to determine not only the amount of primary production at a deep-sea vent site, but also to constrain the extent and turnover of the subseafloor biosphere. The microbial communities are further characterized by functional redundancy, meaning that different taxa appear to perform similar functions using homologous pathways, but being optimally adapted to the thermal and redox gradients existing in the subseafloor. The incubations performed under simulated in situ conditions point to differences in oxygen sensitivity among some of the identified Epsilonproteobacteria as one possible factor contributing to niche formation. Furthermore, we have monitored Crab Spa from early 2007, one year after a volcanic eruption, up to late 2014. Surprisingly, the chemistry and the microbial community composition have remained remarkably stable over this time, in contrast to the succession of faunal communities observed above the seafloor. Overall, our quantitative and integrative approach has generated data that enable us to place these deep-sea hydrothermal vents in a quantitative framework and thus a larger global context. Broader Impacts. This project involved the participation of undergraduate and graduate students, resulting in one PhD thesis. For the cruise, a website was set up (http://www.divediscover.whoi.edu/expedition15/) to educate and inform a broad audience about our research and related activities. A big part of the success of the website was the participation of science writer David Levin. During the cruise, we had close to 93,000 visits to the website from around the world. Visitor statistics showed that visitors stayed on the site for a significant amount of time and that it was heavily used by educators. We also had Jennifer Barone, a science writer from Scholastic magazine, on board, who contributed to the website, as well as wrote small blogs and articles that were subsequently published in Science World (for grades 6-10, circulation of half a million), Math magazine (grades 6-9) and Science Spin (grades 3-6). Barone further wrote an article for Nautilus magazine on the origin of life that was inspired by her participation in the cruise (http://nautil.us/issue/17/big-bangs/in-search-of-lifes-smoking-gun). Project personnel also contributed K-12 and community outreach related to marine microbes and deep-sea hydrothermal vents by acting as judges of high school science projects, presenting at a marine educators conference, giving public presentations, e.g. at the Cape Cod Museum of Natural History in Brewster and the New Bedford Ocean Explorium, and making class room visits at local and regional schools. Last Modified: 01/13/2017 Submitted by: Stefan M Sievert