The functioning of plant communities depends crucially on the soil microbial communities with which they interact. Soil microbes are, therefore, expected to play an important role in shaping how plant communities respond to anthropogenic climate change. However, we know very little about climate-driven evolutionary change in soil microbes and how this will alter plant–soil interactions and associated ecosystem functions. Here, we ask (i) how experimental drought treatment in the lab influences rhizobacterial evolution and (ii) how evolutionary responses shape microbial phenotypes and contribution to ecosystem processes. Our study was based at the Buxton Climate Change Impacts Lab, UK (BCCIL), where experimental climate treatments have been applied to an intact grassland for more than 25 years. We isolated a collection of two plant-growth-promoting rhizobacteria, Bacillus licheniformis and Pseudomonas koreensis, from the rhizosphere of Festuca ovina plants growing at BCCIL. Twelve clonal isolates of each of these microbes were subjected to experimental evolution in a microcosm experiment to investigate evolutionary responses to drought. The bacterial isolates were inoculated into 100 g of autoclaved soil collected from BCCIL and subjected either to cyclical drought treatment or well-watered control conditions in a factorial design, which was replicated for each microbial species. Microbial population sizes were monitored by RT-qPCR. Respiration phenotypes for pre- and post-evolution microbial isolates were determined colourimetrically using MicroRespTM plates. Genome sequences of pre- and post-evolution isolates were compared with each other and with sequences from a wider collection of isolates derived from the drought and control plots at BCCIL to characterise genomic responses to drought.
Results/Conclusions
Living microbial population size decreased strongly in response to experimental drought treatment in soil microcosms, in both P. koreensis and B. licheniformis (p < 0.001). Pseudomonas isolates exhibited greater variation in their response to experimental drought than Bacillus isolates, indicating that the strength of selection on gram-negative Pseudomonas may have been greater than for gram-positive Bacillus. We discuss the evidence for climate-driven microbial evolution in the context of (i) genomic responses to experimental drought within soil microcosms and in the field at BCCIL, and (ii) the impacts of evolutionary change on microbial respiration rates. Our results reveal that soil rhizobacteria are able to evolve rapidly in response to drought conditions, and suggest that microbial evolution may be a significant driver of community-level adaptive processes observed in previous field-based climate change experiments.