Adaptation of Bacillus subtilis cells to Archean-like UV climate: relevant hints of microbial evolution to remarkably increased radiation resistance.

In a precursory study for the space experiment ADAPT ("Molecular adaptation strategies of microorganisms to different space and planetary UV climate conditions"), cells of Bacillus subtilis 168 were continuously cultured for 700 generations under periodic polychromatic UV irradiation (200-400 nm) to model the suggested UV radiation environment on early Earth at the origin of the first microbial ecosystem during the Archean eon when Earth lacked a significant ozone layer. Populations that evolved under UV stress were about 3-fold more resistant than the ancestral and non-UV-evolved populations. UV-evolved cells were 7-fold more resistant to ionizing radiation than their non-UV-exposed evolved relatives and ancestor. In addition to the acquired increased UV resistance, further changes in microbial stress response to hydrogen peroxide, increased salinity, and desiccation were observed in UV-evolved cells. This indicates that UV-sensitive ancestral cells are capable of adapting to periodically applied UV stress via the evolution of cells with an increased UV resistance level and further enhanced responses to other environmental stressors, which thereby allows them to survive and reproduce under extreme UV radiation as a selection pressure.