Award: OCE-1536702

Seawater passing through mantle rocks in the subseafloor of the ocean causes a series of geochemical reactions that can sustain microbial life and impact the exchange of carbon and energy between the deep Earth and the surface biosphere. Microorganisms living in these environments are exposed to a combination of stressors including elevated pHs, high temperatures, and a scarcity of CO2. The goal of this project was to test the hypotheses that (1) microbial diversity spans a wider range of temperature-pH conditions than currently recognized and (2) the scarcity of CO2 is a key biological limitation to serpentinization-driven ecosystems that can be overcome by the metabolic activity of one or a few foundation species. The Lost City hydrothermal field is an iconic example of a mantle-hosted environment and is located in the middle of the Atlantic Ocean (30N, Mid-Atlantic Ridge). In September/October 2018 we carried out a 20-day expedition with the RV Atlantis and the ROV Jason and collected 86 hydrothermal fluid samples, 20 dissolved gas samples, 25 in situ filters to collect microbial cells, and many samples of chimney minerals and biofilms. As part of this research we developed a new deep-sea water sampler that can be deployed on a submersible and can collect high temperature fluids without contamination. It allowed us to collect larger volumes of fluids than traditional samplers, providing sufficient material for some analytical techniques that would otherwise be precluded due to insufficient material. The primary findings from this project include (1) the identification of potential microbial foundation species that can liberate CO2 for use as a carbon source for other microorganisms in the ecosystem (2) the documentation of extensive hydrogen export from seafloor environments far from the main hydrothermal field and (3) characterization of the carbon carried with fluids that exit the hydrothermal chimneys, allowing us to account for what happens to the organics in deep seawater during circulation through mantle rocks. Broader Impacts: The research cruise and subsequent laboratory analyses provided opportunities for training of two UofSC graduate students and two undergraduate students. For the graduate students, the samples and data collected will constitute a substantial portion of their theses. The video and images captured as part of the expedition have been widely disseminated and are now being used to build 3D images of the chimney structures. A science artist who participated in the expedition is currently constructing an exhibit inspired by the Lost City field. Last Modified: 01/29/2021 Submitted by: Susan Lang