Growth of Bacillus subtilis into chlorinated oxyanions seemingly Mars´ brines conditions

Abstract

Liquid water is one of the essential requirements for life as we know it together with a source of chemical elements (including carbon) to build biomolecules useful for structure and maintenance, as well as a source of energy that promotes a redox metabolism. If a planetary object fulfills at least one of these requirements, it is identified as a habitable place.

Nowadays, the astrobiological interest on Mars is rising as the number of robotic and future manned missions can testify. Mars is the fourth planet in the Solar System, smaller than Earth (15% of its mass) with a gravity of only 38% of the terrestrial value. Its reddish color is due to a soil rich in iron oxide (Fe2O3), but high concentrations of sodium (Na+), calcium (Ca2+), and magnesium (Mg2+) have been reported (1). Additionally, Martian polar caps contain icy water, and small amounts of steam water have been observed in its atmosphere (2). Recent observations evidence the existence of sulfate (SO42-), chlorates (ClO3-), and perchlorates (ClO4-) brines that could be responsible for partial surface hydration (3).

With such an scenery, we are evaluating the growth of Bacillus subtilis, a mesophilic Gram-positive bacterium, in culture media modified with KClO3, NaClO3, NaClO4 and Mg(ClO4)2 at concentrations similar to those reported for the surface of Mars. B. subtilis is a good astrobiological model because it has been demonstrated that its spores resist the outer space conditions (4) and can grow when exposed to different NaCl or MgCl2 concentrations (5). We propose that this halotolerant bacterium can also grow in the presence of chlorinated oxyanions due to the expression of the HemHYQ complex, homologous to the chlorite dismutases described by other biological groups (6).