Geobiology of Microbial Ecosystems in Terrestrial Serpentinizing Environments

The geobiology of terrestrial serpentinizing fluid springs was investigated in the Zambales ophiolite range (Luzon, the Philippines) and the Tekirova ophiolite complex (southwestern Turkey). Assessment of taxonomic biodiversity and metabolic capacity was contextualized with regards to the fluid and solid geochemistry of these ecosystems. Field observations of pH, temperature, and dissolved oxygen indicated that spring fluids were highly alkaline and ranged from anoxic to hypoxic/oxic. Fluids collected from the springs for dissolved organic and inorganic carbon yielded very low concentrations. Amplicon sequencing of the 16S rRNA gene indicated that springs were dominated by heterotrophic and lithotrophic microorganisms. In both environments, a large percentage of taxa were associated with the Betaproteobacteria Hydrogenophaga, which has been identified in many different serpentinizing spring environments. Other prominent taxa included the archaeal methanogens in the class Methanobacteria, methanotrophic Gammaproteobacteria, and anaerobic fermenting Clostridia. Shotgun metagenomic analysis of genes related to methanogenesis, methanotrophy, hydrogen oxidation, sulfate reduction, and nitrogen cycling were detected in both the Philippines and Turkey. Methanogenesis pathways dominated in springs originating from the Zambales ophiolite, while methanotrophic pathways were more prevalent in the Tekirova ophiolite. This difference may be causally related to the abundance of hydrogen gas generated in the Zambales ophiolite pore/fracture fluids and the presence of methane gas in the Tekirova ophiolite pore/fracture fluids. Disentangling the thermogenic/abiotic nature of these gases from biotic production will require more sophisticated analytical techniques, namely clumped isotopologue analysis of gases.