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.