Vulnerability of Rosickyite as a Potential Martian Biosignature Due to Microbially Mediated Response to Protracted Environmental Stress
Abstract: The discovery of the presence of a potential microbially mediated biosignature (rosickyite) from Bad Water Basin, California, by Douglas and Yang  invited the proposal that rosickyite may be stable over geological timescales and if found on Mars, indicate past life. Isotopic δ13C and δ15N profiles, and pH pore water analysis from the same 2002 microbial mat as Douglas and Yang (ibid) indicated bulk primary production consistent with nitrogen fixation in the upper canopy driving downcore biogeochemistry. We resampled this same location thirteen (13) years later to test the hypothesis of rosickyite as a robust biosignature over the decadal scale . In March, 2015, sample was again recovered from the same location in Bad Water Basin to determine if rosickyite was still detectable by bulk powder XRD and to again examine both the δ13C and δ15N stable isotope profile and pore water pH. In contrast to findings of 2002, XRD results from 2015 failed to find evidence of rosickyite bulk powder XRD signals. Likewise, 2015 δ13C and δ15N stable isotope results, together with pH indicated a shift in metabolic regimes for this microbial mat towards net denitrification and, sufficient enough, we hypothesized, to induce net dissolution of any previously present signature. Mass mortality and the production of in-situ organic acids was ruled out after determining a total community viability >90%. Sulfur (δ34S) isotope results from 2015 were most similar to mean ocean water composition with the exception of horizons of δ34S excursions associated with buried organic carbon, supporting the hypothesis that at some point between 2002 and 2015, this coupled microbial mat shifted to net denitrification and sulfate reduction, thus driving down pH and inducing diagenesis of a potential biomarker (rosickyite) and creation of a false-negative result. Implications for these results are two-fold. The first implication is that terrestrial Mars analogues are dynamic enough systems to warrant continuous sampling over changing stressors such as periods of protracted drought  and, were ancient microbial communities present on Mars, such assemblages of microbes and minerals would likely have been tightly coupled and conditionally vulnerable to similar diagenesis.
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