Exploring the Metabolic Potential of Microbial Communities in Ultra-basic Reducing Spring at The Cedars, CA: Evidence of Microbial Methanogenesis and Heterotrophic Acetogenesis

Abstract:

The Cedars is a complex of ultra-basic, reducing springs located in the Coastal Range Ophiolite (CA, USA), a site of present day serpentinization. Similar to other serpentinization-associated fluids, the groundwaters discharging at The Cedars contain elevated concentrations of C₁-C₆ alkanes and volatile organic acids (VOAs) which may originate from abiotic or thermogenic processes but can also be produced, consumed, or transformed by microbial activity. In contrast to other continental sites of serpentinization, geochemical indicators (δ¹³C_CH4, δ²H_CH4, CH₄/C₂-C₆ alkanes) are consistent with a partial microbial origin of methane at The Cedars. These indicators, however, can provide only indirect evidence of microbial methanogenesis.

To further explore the metabolic potential of the indigenous microbial communities at The Cedars, we conducted a series of microcosm experiments in which fluids and sediments collected at The Cedars were incubated with ¹³C labeled substrates (formate, acetate, bicarbonate, methanol) under anaerobic conditions. ¹³C from all amended substrates was incorporated into CH₄ demonstrating that these microbial communities can convert both organic and inorganic substrates to CH₄. The apparent fractionation of ¹³C between methane and potential substrates indicated that carbonate reduction was the dominant pathway of methanogenesis, and 16S rDNA based community profiling revealed the presence of an OTU closest related to Methanobacterium sp., an autotrophic (CO₂/H₂) methanogen. Concentrations of C₁-C₄ VOAs increased 5-fold over the course of the experiment indicating the microbial production of VOAs. This acetogenesis occurred heterotrophically as autotrophic acetogenesis can be excluded because (a) δ¹³C values of acetate were similar to those of inorganic carbon (inconsistent with the strong discrimination against ¹³C observed in autotrophic acetogenesis) and (b) no incorporation of ¹³C from labeled bicarbonate was into acetate was observed.