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Carbon monoxide as a metabolic energy source for extremely halophilic microbes: Implications for microbial activity in Mars regolith

Carbon monoxide occurs at relatively high concentrations (≥800 parts per million) in Mars’ atmosphere, where it represents a potentially significant energy source that could fuel metabolism by a localized putative surface or near-surface microbiota. However, the plausibility of CO oxidation under co...

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Published in:Proceedings of the National Academy of Sciences - PNAS 2015-04, Vol.112 (14), p.4465-4470
Main Author: King, Gary M.
Format: Article
Language:English
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Summary:Carbon monoxide occurs at relatively high concentrations (≥800 parts per million) in Mars’ atmosphere, where it represents a potentially significant energy source that could fuel metabolism by a localized putative surface or near-surface microbiota. However, the plausibility of CO oxidation under conditions relevant for Mars in its past or at present has not been evaluated. Results from diverse terrestrial brines and saline soils provide the first documentation, to our knowledge, of active CO uptake at water potentials (−41 MPa to −117 MPa) that might occur in putative brines at recurrent slope lineae (RSL) on Mars. Results from two extremely halophilic isolates complement the field observations. Halorubrum str. BV1, isolated from the Bonneville Salt Flats, Utah (to our knowledge, the first documented extremely halophilic CO-oxidizing member of the Euryarchaeota), consumed CO in a salt-saturated medium with a water potential of −39.6 MPa; activity was reduced by only 28% relative to activity at its optimum water potential of −11 MPa. A proteobacterial isolate from hypersaline Mono Lake, California, Alkalilimnicola ehrlichii MLHE-1, also oxidized CO at low water potentials (−19 MPa), at temperatures within ranges reported for RSL, and under oxic, suboxic (0.2% oxygen), and anoxic conditions (oxygen-free with nitrate). MLHE-1 was unaffected by magnesium perchlorate or low atmospheric pressure (10 mbar). These results collectively establish the potential for microbial CO oxidation under conditions that might obtain at local scales (e.g., RSL) on contemporary Mars and at larger spatial scales earlier in Mars’ history. Significance Increasing evidence indicates that Mars might have been habitable early in its history, and that it might harbor liquid water at present in the form of brines associated with features known as recurrent slope lineae (RSL). However, even if brines do occur, it is unclear what substrates any relict microbes could metabolize. Results presented here show that carbon monoxide, which is abundant in Mars’ atmosphere, could be used at local scales under conditions that occur at RSL, including moderate temperatures, low pressure, high CO ₂, low oxygen concentrations, and extreme water potentials. Halophilic CO-oxidizing Proteobacteria, and recently discovered extremely halophilic CO-oxidizing Euryarchaeota described in this study, represent ideal models for understanding the capacity of Mars’ atmosphere to support microbial communities.
ISSN:0027-8424
1091-6490
DOI:10.1073/pnas.1424989112