Sulfide Oxidation Products Support Microbial Metabolism at Interface Environments in a Marine-Like Serpentinizing Spring in Northern California.

Interface environments between extreme and neutrophilic conditions are often hotspots of metabolic activity and taxonomic diversity. In serpentinizing systems, the mixing of high pH fluids with meteoric water, and/or the exposure of these fluids to the atmosphere can create interface environments with distinct but related metabolic activities and species. Investigating these systems can provide insights into the factors that stimulate microbial growth, and/or what attributes may be limiting microbial physiologies in native serpentinized fluids. To this aim, changes in geochemistry and microbial communities were investigated for different interface environments at Ney Springs-a marine-like terrestrial serpentinization system where the main serpentinized fluids have been well characterized geochemically and microbially. We found that reduced sulfur species from Ney Springs had large impacts on the community changes observed at interface environments. Oxygen availability at outflow environments resulted in a relative increase in the taxa observed that were capable of sulfur oxidation, and in some cases light-driven sulfur oxidation. A combination of cultivation work and metagenomics suggests these groups seem to predominantly target sulfur intermediates like polysulfide, elemental sulfur, and thiosulfate as electron donors, which are present and abundant to various degrees throughout the Ney system. Fluid mixing with meteoric water results in more neutral pH systems which in turn select for different sulfur-oxidizing taxa. Specifically, we see blooms of taxa that are not typically observed in the primary Ney fluids, such as Halothiobacillus in zones where fluids mix underground with meteoric water (~pH 10) or the introduction of Thiothrix into the nearby creek as fluids enter at the surface (~pH 8). This work points to the potential importance of oxidants for stimulating microbial respiration at Ney Springs, and the observation that these serpentinized fluids act as an important source of reduced sulfur, supporting diverse taxa around the Ney Springs system.