Successful Adaptation of Bacillus subtilis Cells to Extreme UV Stress Leads Also to an Increased Desiccation Resistance

Abstract

UV radiation has been a ubiquitous stressor since the origin of the first microbial ecosystem during the Achaean Eon, when the Earth lacked a significant ozone layer and was exposed to a full UV radiation spectrum. Any primary colonizers of surface habitats on Earth are vulnerable to the effect of solar UV radiation. Hence, microorganisms that survived at the surface of the early Earth without the protection of an ozone layer would have needed mechanisms to resist UV radiation. 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 periodical polychromatic UV irradiation (200–400 nm), remodeling the suggested UV radiation environment on the early Earth, when the Earth lacked a significant ozone layer. B. subtilis cells evolved under UV-stress were about 3-fold more resistant compared to the ancestral and non-UV evolved populations. In addition, changes in microbial response to desiccation were observed in UV-evolved cells. Vegetative cells of the ancestral population were almost completely inactivated within the first 24 h of desiccation, whereas an increase in the desiccation resistance was observed in the UVevolved cells, implicating that the successful adaptation to the applied UV stress lead to additional benefits e.g. the adaptation to low water availability.