Diverse drought responses and evidence for stress priming in glacier-fed stream bacteria

Microorganisms inhabiting dynamic ecosystems are repeatedly exposed to environmental fluctuations and disturbances which require physiological adaptations, often associated with fitness trade-offs. In glacier-fed streams, benthic biofilms, the dominant microbial lifeform, frequently experience environmental fluctuations, including drying and rewetting. However, the diversity of responses of glacier-fed stream biofilm to drought stress is not well understood. For instance, stress priming, where prior exposure to a stressor enhances performance during subsequent events, has not been documented in glacier-fed stream bacteria. Here, we investigated how bacterial strains from glacier-fed stream biofilm respond to recurrent drying and whether they exhibit stress priming. To test this, we exposed 22 bacterial strains to repeated short (6 h) drying-rewetting cycles to assess whether their growth improves after repeated exposures. We further tested stress priming by assessing their survival to longer droughts (24 h and 96 h), quantified their biofilm formation, and explored their genomic traits to evaluate whether these features explain the various drought responses. Most strains survived a single short drought, but often at the cost of prolonged lag phases and reduced growth rates. Repeated drought exposure led to divergent stress responses: several strains exhibited signs of stress priming by improving growth traits and/or gaining the ability to withstand prolonged drought only after prior exposure. These responses were largely independent of taxonomy or genomic potential, suggesting that regulatory mechanisms dominate drought adaptation in glacier-fed stream bacteria. Overall, our findings highlight the response diversity among glacier-fed stream biofilm bacteria and show that adaptation often comes at the cost of altered growth dynamics, a potential strategy to cope with the natural hydrological fluctuations in glacier-fed streams.