Abstract

Mixotrophs are ubiquitous and integral to microbial food webs, but their impacts on the dynamics and functioning of broader ecosystems are largely unresolved. Here, we show that mixotrophy produces a unique, dynamic type of food web module that exhibits unusual ecological dynamics, with surprising consequences for carbon flux under warming. We find that mixotrophs generate alternative stable carbon states across temperatures---including an autotrophy-dominant carbon sink state, a heterotrophy-dominant carbon source state, and cycling between these two. Moreover, warming always shifts this mixotrophic system from a carbon sink state to a carbon source state, but increasing nutrients erases early warning signals of this transition and expands hysteresis. This suggests that mixotrophs can generate critical carbon tipping points under warming that will be more abrupt and less reversible when combined with increased nutrient levels, having widespread implications for ecosystem functioning in the face of rapid global change.