Ecological interactions mediate evolutionary responses to temperature in microbial communities

Microbial populations play a pivotal role in ecosystem-level responses to rising temperatures, and both their ecology and evolution can be directly influenced by warming. However, predicting microbial evolution and its ecological consequences is challenging because different genotypes within a population might respond uniquely to shifts in the abiotic and biotic environment. To understand how these processes unfold, we quantified evolutionary and ecological responses across temperatures in a protist of wide geographical distribution in the presence and absence of other microbial species with which they interact (i.e. heterospecifics). In the absence of heterospecifics, we found that intraspecific interactions and warming selected in favour of a particular genotype, reducing genotypic diversity. In the presence of heterospecifics, genetic diversity was generally further reduced under warming, resulting in stronger temperature-dependent selection. However, the magnitude of this change depended on the sign of the net ecological effect (+, 0, -) the heterospecifics had on the focal species, which was itself temperature-dependent. Together, our results demonstrate that both intra- and interspecific interactions can mediate how temperature shapes rapid evolutionary responses of microbial populations, underscoring the importance of the ecological context in predicting evolutionary outcomes under climate change.