Thermal Plasticity and Evolutionary Constraints in Bacillus: Implications for Climate Change Adaptation

Area of Science:

• Microbial ecology
• Evolutionary biology
• Climate change science

Background:

• Rising global temperatures threaten ecosystems and bacterial communities vital for biogeochemical cycles.
• The thermal resilience of wild mesophilic bacteria to projected temperature increases (2-4 °C) is not well understood.
• Understanding bacterial adaptation is crucial for predicting ecosystem responses to climate change.

Purpose of the Study:

• To investigate the thermal adaptation strategies and genetic mechanisms of wild *Bacillus* strains under experimental evolution.
• To compare the thermal plasticity and evolutionary pathways of *Bacillus cereus* and *Bacillus subtilis* lineages.
• To identify genetic factors contributing to bacterial heat tolerance.

Main Methods:

• Experimental evolution of six wild *Bacillus* strains (*B. cereus* and *B. subtilis*) under gradually increasing temperatures.
• Assessment of thermal plasticity and growth at elevated temperatures.
• Analysis of genetic mechanisms, including mutation rates and gene targets, underlying thermal adaptation.

Main Results:

• *Bacillus subtilis* lineages demonstrated improved growth at critical temperatures, with one expanding its thermal niche by 4 °C.
• *Bacillus cereus* strains had higher mutation rates but failed to adapt to increasing temperatures.
• Convergent evolution was observed in five lines, with mutations in genes related to cyclic di-AMP (c-di-AMP) synthesis, a key factor in potassium transport and heat tolerance.
• *B. subtilis* required fewer genetic changes for heat tolerance compared to *B. cereus*, indicating distinct adaptive strategies.

Conclusions:

• Bacterial adaptation to rising temperatures is influenced by lineage-specific genetic backgrounds.
• Cyclic di-AMP synthesis is implicated as a novel mechanism in bacterial heat tolerance.
• Wild bacterial communities may be vulnerable to projected climate change, impacting essential biogeochemical processes.