Bacterial Growth at the High Concentrations of Magnesium Sulfate Found in Martian Soils

Abstract

The martian surface environment exhibits extremes of salinity, temperature, desiccation, and radiation that would make it difficult for terrestrial microbes to survive. Recent evidence suggests that martian soils contain high concentrations of MgSO 4 minerals. Through warming of the soils, meltwater derived from subterranean ice-rich regolith may exist for an extended period of time and thus allow the propagation of terrestrial microbes and create significant bioburden at the near surface of Mars. The current report demonstrates that halotolerant bacteria from the Great Salt Plains (GSP) of Oklahoma are capable of growing at high concentrations of MgSO 4 in the form of 2 M solutions of epsomite. The epsotolerance of isolates in the GSP bacterial collection was determined, with 35% growing at 2 M MgSO 4 . There was a complex physiological response to mixtures of MgSO 4 and NaCl coupled with other environmental stressors. Growth also was measured at 1 M concentrations of other magnesium and sulfate salts. The complex responses may be partially explained by the pattern of chaotropicity observed for high-salt solutions as measured by agar gelation temperature. Select isolates could grow at the high salt concentrations and low temperatures found on Mars. Survival during repetitive freezethaw or drying-rewetting cycles was used as other measures of potential success on the martian surface. Our results indicate that terrestrial microbes might survive under the high-salt, low-temperature, anaerobic conditions on Mars and present significant potential for forward contamination. Stringent planetary protection requirements are needed for future life-detection missions to Mars.

Identification and Characterization of Early Mission Phase Microorganisms Residing on the Mars Science Laboratory and Assessment of Their Potential to Survive Mars-like Conditions

“…Two of the isolates grew using sulfate as a terminal electron acceptor. Sulfate has been shown to be a major constituent of martian soils, and because sulfate systems exhibit high water activity, areas with sulfate salts are potentially more favorable to life (Crisler et al, 2012). Although neither selenium nor arsenic has been detected on Mars, various forms of arsenic and selenium are somewhat common on Earth.…”

“…Studies have shown that salt tolerance would be required by microorganisms to survive and grow on Mars due to the high salt concentrations found in martian soils (Crisler et al, 2012). The majority of the isolates collected from MSL were able to grow in media containing elevated NaCl (93% of the isolates grew at ‡10% NaCl).…”

Are We Alone? The Search for Life on Mars and Other Planetary Bodies

“…Martian soils were expected to be acidic, but the Phoenix Lander showed that the soils at its landing site were mildly basic with a pH of 7.7 ± 0.5 [17]. Salt tolerance would be required for life to survive and grow on Mars due to the high salt concentrations found in Martian soils in the form of NaCl, MgSO 4 , CaSO 4 , FeSO 4 , MgCl and CaCl 2 [17].…”

Growth of Bacillus pumilus and Halomonas halodurans in sulfates: prospects for life on Europa

“…However, the stress imposed by salts different from sodium chloride is not necessarily the same. In an experimental study on epsotolerance, Crisler et al (2012) report the growth of bacterial isolates in culture media with MgSO 4 ·7H 2 O, and argue a favorable implication for Mars habitability, because sulfates are also present in this planet (Crisler et al, 2012). The adaptive strategies for tolerance at high MgSO 4 concentrations were not explored, but the authors noticed some unexplained differences in the growth rate and stationary-phase maximum densities when their isolates were exposed to different sulfate salts.…”