Predation influences the temperature response of microbial communities

Microbial community structure and function are strongly driven by temperature, and in turn influence the climate through effects on global carbon cycling. Recent studies suggest that biotic interactions amongst microbes play an important role in determining their temperature responses, but little is known as to how microbial predation will regulate microbial communities in future climates. Here we assess whether predation by one of the most important bacterial consumers globally –protists– influences the temperature response of microbial community structure and function. To do so, we factorially exposed freshwater microbial communities to two cosmopolitan species of protists at two different temperatures, in a month-long microcosm experiment. We found increased microbial biomass, respiration, and shifts in microbial community structure with elevated temperature that changed in systematic ways over time. Protists influenced microbial biomass and function through effects on community structure. Interestingly, predation reduced microbial respiration rate at elevated temperature. Indicator species and TITAN analyses showed that protist identity, density, and cell size alter microbial community structure. Our study supports previous findings that temperature is an important driver of microbial communities, but also demonstrates new ways in which predation can mediate these responses to warming, with important consequences for the global carbon cycle and future warming.