RAPID DE NOVO EVOLUTION OF TOLERANCE TO DESICCATION AND ULTRAVIOLET-C RADIATION IN A NON-EXTREMOPHILE : IMPLICATIONS FOR POSSIBLE LIFE ON MARS AND VENUS

Mars was once Earth-like, with habitable environments ranging from benign to extreme. Venus might also have been Earth-like, perhaps having an ocean for two billion years. But today, the surface of Mars is a dry-ice-cold, radiation-soaked, hyper-arid desert with an oxidizing regolith under a hypobaric atmosphere, while that of Venus is a lead-meltingly hot, crushingly hyperbaric desert. Understandably, most astrobiologists consider the surfaces of both Mars and Venus to be uninhabitable. However, if life ever existed on the surface of those worlds, could it have adapted to their drastic environmental change and evolved mechanisms to persist today in Mars’s shallow subsurface and Venus’s middle to lower clouds? If so, desiccation and, to a lesser extent, ultraviolet-C (UV-C) radiation would exert tremendous selective pressures on any such Martian and Venusian life. While the mechanisms of tolerance to those stressors in bacteria are fairly well understood, the evolutionary dynamics that can produce those tolerances have been largely unexplored. Therefore, I performed an evolution experiment in which replicated populations of a desiccation- and UV-C radiation-sensitive strain of Escherichia coli were exposed to daily pulses of either desiccation only, UV-C radiation only, or both stressors combined. Tolerance to those stressors, both separately and combined, evolved within a mere 500 generations. I also hypothesized that cross-tolerance would evolve, i.e., treatment with one stressor would result in a correlated gain of tolerance to the non-treatment stressor, but this hypothesis was rejected. Thus, the evolution of co-tolerance required selection with both stressors combined. My results show that a non-extremophile can readily and rapidly adapt to two Mars- and Venus-relevant stressors. The implications of my findings for our neighboring planets are that life might persist today in Mars’s shallow subsurface and in Venus’s clouds, provided that the drastic environmental changes that occurred on those worlds allowed adaptation to one or two stressors at a time.