Bacterial growth tolerance to concentrations of chlorate and perchlorate salts relevant to Mars

Abstract

The Phoenix lander at Mars polar cap found appreciable levels of (per)chlorate salts, a mixture of perchlorate and chlorate salts of Ca, Fe, Mg and Na at levels of ~0.6% in regolith. These salts are highly hygroscopic and can form saturated brines through deliquescence, likely producing aqueous solutions with very low freezing points on Mars. To support planetary protection efforts, we have measured bacterial growth tolerance to (per)chlorate salts. Existing bacterial isolates from the Great Salt Plains of Oklahoma (NaCl-rich) and Hot Lake in Washington (MgSO4-rich) were tested in high concentrations of Mg, K and Na salts of chlorate and perchlorate. Strong growth was observed with nearly all of these salinotolerant isolates at 1% (~0.1 M) (per)chlorate salts, similar to concentrations observed in bulk soils on Mars. Growth in perchlorate salts was observed at concentrations of at least 10% (~1.0 M). Greater tolerance was observed for chlorate salts, where growth was observed to 2.75 M (>25%). Tolerance to K salts was greatest, followed by Mg salts and then Na salts. Tolerances varied among isolates, even among those within the same phylogenetic clade. Tolerant bacteria included genera that also are found in spacecraft assembly facilities. Substantial microbial tolerance to (per)chlorate salts is a concern for planetary protection since tolerant microbes contaminating spacecraft would have a greater chance for survival and proliferation, despite the harsh chemical conditions found near the surface of Mars.

Perchlorate-Coupled Carbon Monoxide (CO) Oxidation: Evidence for a Plausible Microbe-Mediated Reaction in Martian Brines

“…For example, Oren et al (2014) found no effect of 0.2 M perchlorate on aerobic growth by several archaeal extreme halophiles, but noted partial to substantial inhibition at higher concentrations (0.4–0.6 M). Limited tolerance of 1 M perchlorate has been also reported for several bacterial isolates during aerobic growth assays ( Al Soudi et al, 2017 ), but those assays were conducted at much higher water potentials (-1.4 to -5.6 MPa, e.g., moderate salt concentrations and less physiological water stress) than used in this study (-19 MPa). Thus, the ability to oxidize CO during substantial water stress (i.e., lower water potentials) and simultaneously high concentrations of a potent chaotrope [perchlorate ( Cray et al, 2013 )] is unprecedented.…”

Radiolytic H2Production in Martian Environments

“…Hygroscopic sulfate salts have also been detected in martian soils (Smith et al , 2014 ). Through deliquescence, these salts can produce saturated brines with very low freezing points (Davila et al , 2010 ; Al Soudi et al , 2017 ). If the hygroscopic sulfate salts contain radionuclides, this may produce another habitable microenvironment for microorganisms that obtain energy from sulfate reduction by H 2 .…”

Survivability of Soil and Permafrost Microbial Communities after Irradiation with Accelerated Electrons under Simulated Martian and Open Space Conditions

“…Moreover, it is shown that econiches with liquid water and positive temperatures can occur on the current Mars [88,[90][91][92][93]. Terrestrial microorganisms can grow under combined low pressure, temperature, and anoxic atmosphere conditions [94], it can withstand the impact of Martian salts and strong oxidizers [95][96][97][98], and can survive in subzero brines [99,100]. If even slow repairing of cells' damages occurs, it can be of great effect during geological time.…”