Collaborative Research: Laboratory Investigation of Redox Reactions during Subsurface Mixing in Submarine Hydrothermal Systems

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Submarine hot-springs at oceanic spreading centers form in response to heat associated with volcanic activity responsible for the creation of new oceanic crust. This heating of seawater to temperatures as high as 400°C, as it circulates through the ocean crust, results in extensive chemical reactions that modify the fluid's composition to something that is acidic, metal- and sulfide-rich, and highly reducing. Venting of these fluids at the seafloor and mixing with seawater creates a chemical environment that supports large microbial ecosystems that do not rely on the sun (photosynthesis) as a source of energy. Instead, they are sustained by catalyzing chemical reactions that occur when hot and reduced hydrothermal fluids mix with cold oxidized seawater. The ability of microbes to utilize these reactions as an energy source is highly dependent on the abundance and composition of chemical compounds that participate in these types of oxidation-reduction reactions. This research conducts laboratory experiments to determine the rate of abiotic (i.e., not involving life) chemical reactions that produce or consume chemical compounds used by deep sea microbes. These data improve our understanding of how subsurface microbes and the higher order marine ecosystems, that rely on these microbes as the base of the food web, thrive on the bottom of the seafloor thousands of feet below the sea surface. Broader impacts of the work include a substantial educational component that provides state-of-the-art research experiences for 25 undergraduate science majors at a primarily undergraduate institution (Bridgewater State University) through partnership with a major oceanographic research institution (the Woods Hole Oceanographic Institution). This integration of research and education includes the development and implementation of a research course, offered at Bridgewater State, and intensive summer research experiences at Woods Hole. The goal is to provide undergraduates with opportunities to experience all aspects of the scientific process, including background research, data collection, analysis and interpretation, and presentation of results at a national conference in order to better prepare them for employment and graduate study.This research consists of laboratory experiments that investigate abiotic redox reactions in subsurface mixing zones in seafloor hydrothermal systems. Using a novel laboratory reactor, experiments investigate chemical disequilibria between key redox reactive species in seawater (e.g., H2, H2S, Fe2+, CH4, O2, SO42-, CO2, NO3-), which comprise a major source of chemical energy that supports complex subsurface ecosystems. At present, such reactions are poorly constrained in hydrothermal environments, so this work identifies these reactions and determines their rates at low to moderate temperatures that characterize subsurface mixing zones within hydrothermal systems. The resulting data will allow improvement of models of hydrothermal vent microbial metabolic pathways. Experiments will take place using an open-system flow-through reaction cell that is able to regulate the concentration of H2, O2, H2S, NO3-, and intermediate oxidation state sulfur and nitrogen species as a function of temperature and pH. Data will be used to place fundamental constraints on thermodynamic models used to predict the amount of chemical energy delivered to vent ecosystems, which should lead to an improved understanding of the linkages between chemical environment and biological community composition and function.