Detecting Endogenous Microbial Metabolism and Differentiating Between Abiotic and Biotic Signals Observed by Bioelectrochemical Systems in Soils.

Unambiguous detection of chemical and physical signatures of microbial life on Mars or other solar system bodies requires differentiation between signals produced by biotic and abiotic processes; instruments aimed at generalized extant life detection would therefore increase the science return of a life-detection mission. Here, we investigate Bioelectrochemical Systems (BES) as a technique to measure microbial metabolism (which produces electrical current and redox changes) and distinguish between potential abiotic and biotic responses in environmental samples. Samples from inhabited niches should contain everything necessary to produce current, that is, catalysts (microorganisms) and fuel (nutrients). BES can also probe for inactive organisms in less energetically rich areas by adding a fuel to drive metabolism. A commercial potting soil and a Mars simulant soil were inoculated in the anodic chamber of microbial fuel cells, and current was monitored over time. Addition of a fuel (electron donor) source was tested for metabolic stimulation of endogenous microbes. Redox reactions between Mars simulant soil and the introduced electron donor (lactate) produced false-positive results, emphasizing the importance of careful interpretation of signals obtained. The addition of lactate to both soils resulted in enhanced biologically produced current, allowing stimulation and detection of dormant microbes. Our results demonstrate that BES provide an approach to detect metabolism in samples without prior knowledge of the organisms present, and that thorough electrochemical analyses and experimental design are necessary to determine if signals are biotic.