Spatial refuges and nutrient acquisition predict the outcome of evolutionary rescue in evolving microbial populations

Microbial populations are often exposed to environmental stressors that impact their survival and evolution. Eco-evolutionary theory suggests microbial populations may be able to survive a stressor through "spatial refuges" - i.e., areas of low or reduced stress such as within biofilms. However, spatial refuges reduce a populations access to nutrients, so may be detrimental depending on the severity of the stressor they are sheltering from. Using predictions from a general mathematical model, and the experimental evolution of the bacterium Pseudomonas fluorescens SBW25 under salinity stress and varying opportunities to form spatial refuges (i.e., agitated or non-agitated culture conditions), we show that spatial refuges can rescue a population from stressors only when nutrient levels are high. In the surviving high-salinity evolved populations (i.e., non-agitated culture conditions and high-nutrients), clones had an increased salinity resistance, indicating that spatial refuges can facilitate evolutionary rescue. Though whole genome resequencing did not reveal a single specific mutation associated with salt resistance, we found that clones evolved under control conditions (lower salt, high-nutrients, and no agitation) acquired mutations in a putative chemotaxis gene and showed increased motility. This indicates that spatial refuges under high salinity may also constrain adaptations to other environmental factors. Together, our combination of theory, laboratory experiment, and genome re-sequencing demonstrate the value and limits of spatial refuges in alleviating environmental stress within microbial populations.