Microbial communities regulate ecosystem responses to climate change. However, predicting these responses is challenging because of complex interactions among processes at multiple levels of organization. Organismal traits that determine individual performance and ecological interactions are essential for scaling up environmental responses from individuals to ecosystems. We combine protist microcosm experiments and mathematical models to show that key traits—cell size, shape, and contents—each explain different aspects of species’ demographic responses to changes in temperature. These differences in species’ temperature responses have complex cascading effects across levels of organization—causing nonlinear shifts in total community respiration rates across temperatures via coordinated changes in community composition, equilibrium densities, and community–mean species mass in experimental protist communities that tightly match theoretical predictions. Our results suggest that traits explain variation in population growth, and together, these two factors scale up to influence community- and ecosystem-level processes across temperatures. Connecting the multilevel microbial processes that ultimately influence climate in this way will help refine predictions about complex ecosystem–climate feedbacks and the pace of climate change itself.

Microbial communities regulate ecosystem responses to climate change. But predicting these responses is challenging due to complex interactions among processes at multiple ecological scales. Organismal traits that determine individual performance and ecological interactions are essential for scaling up predictions of environmental responses from individuals to ecosystems. We combine experiments and mathematical models to show that key microbial traits—cell size, shape, and cell contents—independently drive shifts in demographic rates across temperatures, having cascading effects on community structure, dynamics, and ecosystem function. Moreover, intra- and interspecific trait variation play distinct, trait-specific roles in temperature responses. These species-level responses scale up to cause predictable, nonlinear shifts in microbial community composition and respiration rates, with direct implications for the effects of warming on the global carbon cycle. Mechanistically linking microbes with climate using traits will help refine predictions about complex ecosystem-climate feedbacks and the pace of climate change.