Predation by a ciliate community mediates temperature and nutrient effects on a peatland prey prokaryotic community.

Temperature significantly impacts microbial communities' composition and function, which subsequently plays a vital role in the global carbon cycle that ultimately fuels climate change. Interactions between different microorganisms might be critical in shaping how these communities react to both temperature changes. Additionally, rising temperatures are occurring in the context of increasingly nutrient-rich ecosystems due to human activity. Nonetheless, we lack a comprehensive understanding of how predation influences microbial communities in future climate scenarios and an increasingly nutrient-rich world. Here, we assess whether predation by key bacterial consumers—ciliates—influences a microbial community's freshwater temperature and nutrient response regarding biomass, diversity, structure, and function. In a three-week microcosm experiment, we exposed mostly prokaryotic microbial communities to a community of ciliate predators at two different temperature scenarios (ambient and +3°C, i.e., a conservative projection of climate change by 2050) and nutrient levels (low and elevated). Nutrients, temperature, and ciliate presence influenced microbial biomass and function separately, but their interaction had the largest explanatory power over the observed changes in microbial community biomass, structure, and function. Our study supports previous findings that temperature and nutrients are essential drivers of microbial community structure and function but also demonstrates that the presence of predators can mediate these effects, indicating that the biotic context is as important as the abiotic context to understand microbial responses to novel climates.