Evidence for Microbial Sulfur Cycling in an Extremely High Ph Marine-Like Terrestrial Serpentinizing System: Ney Springs

Ney Springs, a continental serpentinizing spring in northern California, has among the highest reported pH (12.4) of naturally a occurring water source. With high conductivity fluids, it is geochemically similar to marine systems when compared to other terrestrial locations. Our geochemical analyses also revealed high concentrations of methane (83% volume gas content), sulfide (544 mg/L), and dissolved organic carbon (22 mg/L) that are orders of magnitude above what is reported in other serpentinizing systems. Thermodynamic calculations support the potential for anaerobic metabolisms including sulfate reduction, anaerobic methane oxidation, denitrification, and anaerobic sulfide oxidation. Interestingly, microbial evidence for metabolic reactions involving methane, even those with high energy density (i.e., anaerobic methane oxidation) was not observed, nor were sequences from Archaea. Assessment of the microbial community via 16S rRNA taxonomic gene surveys and metagenome sequencing revealed a community composition dominated by poorly characterized members of the Izemoplasmatales and Clostridiales . The genomes of these dominant taxa point to a fermentative lifestyle, though other highly complete (>90%) metagenome assembled genomes support the potential for organisms to perform sulfate reduction, sulfur disproportionation and/or sulfur oxidation (aerobic and anaerobic). Two isolates of bacteria identified in the metagenome, a Halomonas sp . and a Rhodobacteraceae sp., were shown to oxidize thiosulfate and were capable of growth in conditions up to pH 12.4. While there is strong evidence for active sulfur metabolisms at Ney Spring, the lack of Archaea and the abundance of methane call into question whether there are inherent limitations on certain physiologies in this high pH environment.