Geobiology of the Zambales Ophiolite, Philippines and Coast Range Ophiolite, California

The deep subsurface is known to harbor a substantial amount of biomass, yet life in the deep subsurface is largely unexplored. Serpentinizination reactions produce hydrogen and methane gas that host chemotrophic mircoorganisms in continental deep subsurface ophiolites. A study of the geobiology of two serpentinizing systems in the Coast Range ophiolite, California and Zambales ophiolite, Philippines was motivated by previous thermodynamic calculations that predicted microbial metabolisms in these subsurface systems. Microbial iron reduction was predicted to be energetically favorable in both systems, and microbial iron oxidation was also predicted to be energetically favorable in the Zambales system. Microcosms simulated surface and subsurface serpentinizing systems using samples collected from each study region, and were sampled over a period of six weeks to document microbe-mineral interactions. Iron redox in the two systems was investigated through batch culturing techniques employing ferric hydroxide as a growth substrate and sediments collected from the serpentinizing study sites as inoculum. Microcosm experiments indicated microbial interaction with mineral surfaces, shown by visual observations of the solids in the microcosms and scanning electron micrographs of rock chips in the experiments. Cell morphologies and taxonomic assemblages of communities were identified via fluorescence microscopy and DNA sequencing, which indicated the presence of iron metabolizing microbes. Culture experiments indicated that microbes were engaging in iron redox reactions as predicted by thermodynamic modeling. Indications included visual observations of color change and magnetic attraction of precipitates, as well as scanning electron micrographs of the precipitates and oxidation states of the precipitates measured through X-ray photoelectron spectroscopy. DNA sequencing indicated the presence of iron metabolizing microbes in these experiments. A comparison of microbial communities in this study to communities in other studies from global serpentinizing systems indicated the persistence of two taxa across all systems: Firmicutes and Proteobacteria.