Microbial ecological perspectives of space-exposed microbes: A genetic approach

A radiation resistant strain of Bacillus pumilus isolated from a spacecraft assembly environment is being exposed to several conditions associated with the space environment (Feb, 2008). The radiation exposures are being carried out aboard ISS using the European Technology Exposure Platform and Experiment Facility (EXPOSE) and include: (i) space vacuum, (ii) solar extraterrestrial UV radiation including vacuum-UV, (iii) simulated Martian UV radiation regime, and (iv) galactic cosmic radiation. The viability of exposed and unexposed microorganisms as well as the integrity of the cell wall or spore envelop (membrane, wall and coat layers) and damage to the DNA will be assessed when the exposed spores return to Earth. In addition to these standard methods for assessing cellular damage, the global response elicited in spores by space exposure will be probed in germinated spore survivors using transcription microarrays. During ground simulation experiments the desiccated spores survived full Martian UV (200 - 400 nm) for 5 min (30 W m-2) and were only slightly affected by Martian atmospheric conditions in the absence of UV. Although prolonged UV irradiation (>5 min to 12 hours; 30 W m-2) killed substantial portions of the spore microcosms (~5 to 6 logs reduction under Martian UV), dramatic spore survival was apparent when spores were shield by dust (~2 logs reduction). It is presumed that the mitigation of UV damage (200-400 nm) to dust covered, desiccated spores and their survival on spacecraft-grade aluminum is strain specific. The more pronounced UV resistances observed in wild-type strains, as compared to laboratory strains suggested that discussions and conclusions regarding the survival of microbes in extraterrestrial environments should not be generalized based solely on laboratory strain responses to simulated conditions. The data generated is important for the assessment of the probability and mechanisms of microbial survival, microbial contaminants of risk to forward contamination, in situ life detection, and sample return missions. It may be possible to use these results (together with future experimental results of space exposed samples) to calculate the rates of inactivation of microbial species caused by the low pressure and high desiccation in simulation experiments as well as actual space environmental conditions. Such empirical data sets will give the best insight on the ability of terrestrial microorganisms to survive in the space environment.