Heritable variation drives rapid evolution of thermal performance curves in the protist Tetrahymena thermophila

Microbial respiration is a key biotic driver of climate change. Warming boosts microbial population growth, which increases biomass and respiration, potentially leading to more warming. This feedback might be disrupted by adaptation in thermal performance curves (TPCs) -whose shape describes how temperature drives growth. In this study, we uncover substantial genetic variation (G) in the intrinsic population growth rates (r) of the protist Tetrahymena thermophila, demonstrate a causal link between heritable variation in r and heritable variation in TPC shape, and show how this variation constrains predicted r-TPC shape evolution along specific evolutionary paths across temperatures. We also uncover Gene-by-Environment (G × E) variation in r, which results in specific signatures in TPC shape and predictable temperature-dependent TPC evolution that can erode heritable variation, thus reducing future evolutionary potential. Overall, we show how temperature-dependent evolution in microbial TPC shape-a linchpin of global ecosystem function-is determined by a combination of heritable and non-heritable variation in intrinsic growth rates.