Conserved upper thermal limits and small safety margins in soil copiotrophic bacteria.
One of the key uncertainties in climate change models is how microbes will adapt to rising temperatures. Large-scale comparisons of bacterial thermal performances show a clear boundary between mesophiles and thermophiles. Here, we investigated whether phylogenetic constraints limit the adaptive potential of bacteria to warming soils. Focusing on copiotrophs within Gammaproteobacteria, we found that both thermal optima and upper thermal limits are constrained; variation in these traits decreases above 42{degrees}C across the phylogeny, with minimal influence of present-day bioclimatic variables. This, along with the reduced thermal safety margins found in fluctuating hot climates, suggests that many isolates may already be maladapted to local temperature variability. Our findings indicate that these constraints interact with the geometry of thermal performance curves, imposing a trade-off between high-temperature performance and the risk of substantial fitness losses above Topt. Overall, this work underscores potential limits to thermal adaptation and their implications for bacterial fitness.