WHAT CAN SURFACE SEEPS TELL US ABOUT TERRESTRIAL SUBSURFACE ECOSYSTEMS IN OPHIOLITE-HOSTED SERPENTINIZING FLUIDS?

Subsurface serpentinization in ophiolite-hosted regimes produces highly reduced, high pH fluids that provide microbial habitats. Typically, these subsurface fluids contain copious H2 and CH4 gas, little/no inorganic carbon, and limited electron acceptors. As deeply-sourced fluids reach the oxygenated surface environment, microbial biomes shift and organisms capable of metabolizing O2 thrive (Woycheese et al., 2015). It is not clear how microbial communities are related, connected, or communicated between surface expressions (such as fluid seeps) and the subsurface biosphere.

Our work in the Zambales and Palawan ophiolites (Philippines) defines surface habitats with geochemistry, targeted culturing efforts, and community analysis (Cardace et al., 2015; Woycheese et al., 2015). Spring sources range from pH 9-11.5, and contain 0.06-2 ppm DO, 0-3.7 ppm sulfide, 30-800 ppm silica, H₂ and CH₄ > 10uM, CO₂ > 1 mM. Outflow channels extend from the source pools. These surface data encourage prediction of the subsurface metabolic landscape.

To understand how the subsurface and surface environments might be connected [physically and ecologically] we focus on community analysis and culturing, and the geochemical context of the ecosystem. Shotgun metagenomic analyses indicate carbon cycling at these sites is reliant on methanogenesis, acetogenesis, sulfate reduction, and H2 and CH4 oxidation. The metabolic capacity for methanogenesis, suggests microbial origins for some CH4. Methyl coenzyme M reductase, and formylmethanofuran dehydrogenase were detected, and relative abundance increased near the near-anoxic spring source. In this tropical climate, cellulose is also a likely carbon source, possibly even in the subsurface. Enrichment cultures [up to pH 12] and isolated strains [pH 8-10] from Zambales springs show degradation of cellulose and production of cellulose. In addition, strains resistant to elevated concentrations of trace elements (Ni, Co, Cu, Cr) and capable of reducing amorphous iron have been obtained from the Zambales spring locations. These results indicate a metabolically flexible community at the surface where serpentinizing fluids are expressed.

References:

Cardace et al., 2015. doi: 10.3389/fmicb.2015.00010

Woycheese et al., 2015 doi: 10.3389/fmicb.2015.00044