Mixotrophs generate alternative stable carbon states under warming

Background/Question/Methods

Microbial organisms play a critical role in ecosystem carbon and nutrient cycling that is likely to change with rapidly changing global conditions. Understanding the net impacts of global change on ecosystem flux requires untangling the roles of a diverse assortment of ecological strategies within the microbiome. Mixotrophy—use of multiple energy acquisition (or trophic) modes (e.g., autotrophy and heterotrophy)—is a ubiquitous ecological strategy in microbial food webs and an integral component of many ecosystems. Despite its prevalence, mixotrophy is rarely considered in ecological theory and its consequences for the dynamics and functioning of broader ecosystems are largely unresolved. Here we develop and analyze a mixotrophic model to evaluate the impacts of environmental change on ecological dynamics, stability, and carbon flux. Our model specifically focuses on mixotrophs that combine phototrophy and phagotrophy. We address three main questions: 1) Does variation in temperature and nutrient availability alter ecological dynamics and stability in mixotrophic systems? 2) Do these shifts lead to transitions between alternative carbon flux states (i.e., carbon sink vs. carbon source)? 3) Are there early warning signals for tipping points between alternative carbon states?

Results/Conclusions

We find that mixotrophic systems generate a variety of alternative stable states, including a form of tri-stability where either autotrophy dominates (net carbon sink), heterotrophy dominates (net carbon source), or the system cycles between these two states. Increasing temperature shifts the system from a carbon sink state to a carbon source state, but this transition includes a period of fluctuations that serve as an early warning signal for a sudden state shift. Moreover, varying temperature and nutrient concentrations together creates a rich equilibrium landscape where increasing nutrients erases early warning signals, cementing an abrupt tipping point and increasing hysteresis. These results expose fundamental features of this prevalent, but poorly understood, microbial food web component that could have widespread consequences for ecosystem nutrient flux and carbon sequestration in the face of rapid global change.