The Case for a Dynamical Subsurface Ecosystem

The introduction and concentration of electron donors and acceptors in the subsurface biosphere is controlled by the mixing of subsurface fluids, but the mechanisms and rates at which microbial communities respond to changes induced by fluid mixing and transport are relatively unknown.

Subsurface microbial ecosystems whose estimated doubling times range from <1 to >3,000 years are often considered to be relatively static. Despite marked changes in geochemistry over a 1-year period, the bacterial community inhabiting a 1339 m below land surface (mbls) fracture (Be326) remained largely unchanged and exhibited PLFA isotopic signatures consistent with the accumulation of 13C-DIC impacted by the microbial oxidation of CH4. These CH4 oxidizing (MO) bacteria and archaea are an essential link between the Be326 subsurface carbon cycle and microbial community and were hypothesized to contain members of the community that are most sensitive to environmental change.

To evaluate this hypothesis, we used a combination of high throughput sequence analysis methods (DNA, RNA, and protein) and geochemical monitoring of Be326's in situ fracture fluids over the course of both longer (2.5 year) and shorter (2-week) timescales and validated our findings through a series of 13C-CH4 laboratory enrichment experiments. We show that Be326's MO organisms responded to changes in electron donor and acceptor availability in their natural subsurface habitat and under laboratory conditions over extended periods of time. These results provide the most definitive evidence to date that, like the marine subsurface, CH4 oxidation occurs and is an integral component of the deep terrestrial subsurface carbon cycle. Further, the responsiveness of this component of the microbial community to changes in geochemistry illustrates a more dynamic subsurface ecosystem than previously understood.

Cara Magnabosco, Peer H.A. Timmers, Maggie C.Y. Lau, Gaetan Borgonie, Borja Linage-Alvarez, Olukayode Kuloyo, Rose Alleva, Thomas L. Kieft, Greg S. Slater, Esta van Heerden, Barbara Sherwood Lollar, Tullis C. Onstott

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