Microbially-influenced Fe-Cycling within high pH serpentinizing springs of the Zambales Ophiolite, Philippines

Abstract:

The Zambales ophiolite region in the Philippines contains high pH springs associated with serpentinization. At the surface where calcium-saturated ­fluids mix with air, fluid becomes aerobic and diffusion of CO₂ occurs. At depth, there are low concentrations of dissolved inorganic carbon and O₂, and high concentrations of CH₄ and H₂. Redox potential of iron in the fluids is largely dependent on pH. Fe²⁺ is unstable at a high pH, and spontaneously reacts with atmospheric O₂ to form Fe³⁺, which is then hydrolysed to ferrihydrite. The reaction kinetics may be too rapid for microbes to harness energy for growth, however cells have been documented to act as nucleation sites for ferrihydrite precipitation in natural environments. Precipitates that sink to the subsurface act as substrates for microbes where they may carry out Fe³⁺ reduction in the presence of H₂. Predictions made about Gibbs energy of reaction for iron metabolisms in serpentinizing systems show that Fe³⁺ reduction in the subsurface is energetically favorable (Fig. 1A) (Cardace, et al., 2013).

Spring fluid and rock samples from the Zambales region were collected in September 2013. Time series microcosms including sample rock, spring fluid, and gas simulating the spring surface and subsurface (Fig. 1B) will investigate microbial growth rates and microbial reaction products over one year. Microcosms will undergo cell counts via fluorescence microscopy, SEM, and XRD to examine cell growth rates, microbial action on mineral surfaces, minerals forming around cells, and changes in mineralogy. After one year, microbial community structure and iron metabolizers will be identified via DNA sequencing.­­

Surface microcosms are expected to show abiotic oxidation of Fe²⁺ and formation of Fe³⁺ precipitates preferentially around cells acting as nucleation sites (except in abiotic control microcosms). Subsurface microcosms are expected to show biotic reduction of Fe³⁺ and signs of microbial action on mineral surfaces. This activity will increase directly with increasing cell growth, and will not be evident in abiotic control microcosms. Evidence of reduced iron mineral formation over time will be seen, and DNA sequencing will yield consistent results with microbes capable of metabolizing iron, thus demonstrating microbially-influenced iron cycling in this system.