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. Geological, geochemical, and geomorphological observations have provided definitive evidence for large, ancient fluvial systems that might have been conducive to life (8-13), but the timing, temperature, and composition of surface water have been controversial (14-17). Although unequivocal evidence for contemporary liquid water reservoirs has not yet been obtained, experimental evidence suggests plausible conditions under which brines might form (18), and observations from the Mars Reconnaissance Orbiter suggest that recurring slope lineae (RSL) at latitudes between 32°S and 48°S might develop in association with seasonally moderate conditions during late spring-summer (19-21). McEwen et al. (19) have proposed that these features can be best explained by near-surface liquid water brines that form between temperatures of about −10°C and 25°C during late spring-summer.However, even if liquid water was at one time or is now sufficient to support microbial life, energy sources that could sustain metabolism in near-surface regolith have not been identified. Although the 1976 Viking lander results appeared to exclude surface organic matter (e.g., 22, 23), recent evidence suggests that low organic matter levels might indeed occur in some deposits (e.g., 12). Even so, it is uncertain whether this material exists in a form or concentrations suitable for microbial use.The Martian atmosphere has largely been ign...