Low-Temperature Ionizing Radiation Resistance ofDeinococcus radioduransand Antarctic Dry Valley Bacteria

Abstract

The high flux of cosmic rays onto the unshielded surface of Mars poses a significant hazard to the survival of martian microbial life. Here, we determined the survival responses of several bacterial strains to ionizing radiation exposure while frozen at a low temperature characteristic of the martian near-subsurface. Novel psychrotolerant bacterial strains were isolated from the Antarctic Dry Valleys, an environmental analogue of the martian surface, and identified by 16S rRNA gene phylogeny as representatives of Brevundimonas, Rhodococcus, and Pseudomonas genera. These isolates, in addition to the known radioresistant extremophile Deinococcus radiodurans, were exposed to gamma rays while frozen on dry ice (-79°C). We found D. radiodurans to exhibit far greater radiation resistance when irradiated at -79°C than was observed in similar studies performed at higher temperatures. This greater radiation resistance has important implications for the estimation of potential survival times of microorganisms near the martian surface. Furthermore, the most radiation resistant of these Dry Valley isolates, Brevundimonas sp. MV.7, was found to show 99% 16S rRNA gene similarity to contaminant bacteria discovered in clean rooms at both Kennedy and Johnson Space Centers and so is of prime concern to efforts in the planetary protection of Mars from our lander probes. Results from this experimental irradiation, combined with previous radiation modeling, indicate that Brevundimonas sp. MV.7 emplaced only 30 cm deep in martian dust could survive the cosmic radiation for up to 100,000 years before suffering 10⁶ population reduction.

Oxidative Stress Resistance inDeinococcus radiodurans

“…In contrast to data from previous reports showing that frozen conditions do not offer much protection against radiation damage in D. radiodurans (166), more recent data show that D. radiodurans cells irradiated on dry ice (Ϫ70°C) have a higher gamma ray survival rate than do cells irradiated on ice or at room temperature (128,130,519). The increase in radiation resistance under frozen irradiation conditions may be due to the diminished oxygen effects on proteins (120,122).…”

Bio-signature of Ultraviolet-Radiation-Resistant Extremophiles from Elevated Land

“…Microorganism Brevundimonas sp. is known to be a resistant to UV-C, gamma radiations and hydrogen peroxide [36,37]. B. pumilus has been well known for its UVR resistance [28,32,38].…”

Ionizing Radiation and Life

“…More specifically in the interests of astrobiology, several modeling studies have involved calculation of the radiation dose profile generated by CR through the martian regolith to predict the likely survival times of spore-forming or radiation-resistant bacterial strains such as D. radiodurans in the subsurface (Mileikowsky et al, 2000;Pavlov et al, 2002;Kminek et al, 2003;Dartnell et al, 2007aDartnell et al, , 2007bDartnell et al, , 2010. While there are uncertainties in both the physics of high-energy particle transport and the biological response to irradiation, these studies broadly agree and have found that the top 20 cm of the martian surface is sterilized of even the most radiation-resistant microorganisms (based on terrestrial models such as D. radiodurans) within a million years or so, with greater survival found at increasing depths due to shielding, especially in ice deposits (Dartnell et al, 2007a).…”