Contrasting oxidative stress response mechanisms in novel strains of Bacillus isolated from the Mars-analog, Mojave Desert

Environmental conditions that limit the presence of life include ionizing radiation, extreme temperatures, and lack of water. These environments are common in our solar system and may contribute to the lack of apparent life. However, analogous environments here on Earth are host to a multitude of thriving microbial life. In order for microbes to survive in dry deserts, they must be must be able to adapt to transient diurnal and seasonal changes in the environment (water, temperature). To uncover response strategies to environmental stress that may prevent cellular damage and ensure adaptation and survival, two distinct, novel strains of Bacillus were isolated from the Mojave Desert (a Mars analog due to its arid conditions and high incidence of ultraviolet light) and classified by their partial 16S RNA gene sequences. These species, despite being closely related, exhibited radically different phenotypes and contrasting strategies for mitigating stress. The two strains had different growth rates, metabolic capacities and sporulation onset times when challenged by crowding and heat-shock. In response to hydrogen peroxide challenge, the intracellular levels of catalase activity, a peroxide-scavenging enzyme, differed for each strain, and were surprisingly lower than that of a non-desert control species of Bacillus. DNA repair mechanisms were more active in one strain than the other, and one isolate responded with an increase in expression of longevity gene orthologs involved in stress response. After multiple rounds of culturing, the peroxide degradation capacity, as well as the growth and sporulation rates remained constant for each strain, which suggests these are permanent features of each strain rather than transient responses. Taken together, these data uncover a diverse arsenal of response mechanisms employed by closely related species to combat stress. These adaptations may provide environmental-niche specificity and the diversity of life even in a scarce environment like the Mojave Desert.