Abstract
The presence of hydrated salts on Mars indicates that some regions of its surface might be habitable if suitable metabolizable substrates are available. However, several lines of evidence have shown that Mars’ regolith contains only trace levels of the organic matter needed to support heterotrophic microbes. Due to the scarcity of organic carbon, carbon monoxide (CO) at a concentration of about 700 parts per million (about 0.4 Pa) might be the single most abundant readily available substrate that could support near-surface bacterial activity. Although a variety of electron acceptors can be coupled to CO oxidation, perchlorate is likely the most abundant potential oxidant in Mars’ brines. Whether perchlorate, a potent chaotrope, can support microbial CO oxidation has not been previously documented. We report here the first evidence for perchlorate-coupled CO oxidation based on assays with two distinct euryarchaeal extreme halophiles. CO oxidation occurred readily in 3.8 M NaCl brines with perchlorate concentrations from 0.01 to 1 M. Both isolates were able to couple CO with perchlorate or chlorate under anaerobic conditions with or without nitrate as an inducer for nitrate reductase, which serves as a perchlorate reductase in extreme halophiles. In the presence of perchlorate, CO concentrations were reduced to levels well below those found in Mars’ atmosphere. This indicates that CO could contribute to the survival of microbial populations in hydrated salt formations or brines if water activities are suitably permissive.
Highlights
The prospects for extraterrestrial life depend on two major factors: liquid water availability and the availability of reduced inorganic and organic substrates that can be used to sustain biochemical reactions (Brack et al, 2010)
To assess the feasibility of carbon monoxide (CO)-coupled perchlorate reduction in brines, denitrifying and nitrate-respiring extremely halophilic enrichments and isolates were obtained from the Bonneville Salt Flats (BSF) and nearby saline soils (UT, United States)
Previous reports have shown that a dissimilatory periplasmic nitrate reductase catalyzes perchlorate, chlorate, and nitrate reduction in the denitrifying extreme halophile, Hfx. mediterranei 33500T, with chlorate reduction rates exceeding those for nitrate (Martinez-Espinosa et al, 2015)
Summary
The prospects for extraterrestrial life depend on two major factors: liquid water availability and the availability of reduced inorganic and organic substrates that can be used to sustain biochemical reactions (Brack et al, 2010). Results presented here provide the first evidence that some denitrifying and nitrate-respiring euryarchaeal extreme halophiles can couple CO oxidation in brines to perchlorate at concentrations up to 1 M.
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