Competition between phytoplankton and mixotrophs leads to metabolic character displacement

While plankton are often characterized by metabolism as either phototrophic primary producers or heterotrophic consumers, many actually combine photosynthesis and phagotrophy within a single cell. These “mixotrophic” plankton, which play an important biogeochemical role in marine food webs, exhibit diverse metabolic strategies with varied contributions from photosynthesis and phagotrophy. Mixotrophs co-exist with specialist phototrophs and heterotrophs, competing for shared resources; yet we do not know how this competition alters a mixotroph’s metabolic strategy or impacts biogeochemistry. We constructed a mathematical model to simulate the dynamics of a planktonic community which consists of mixotrophs and their specialist phototrophic competitor, phytoplankton. Our simulation demonstrates how the presence of competing phytoplankton causes metabolic character displacement, shifting mixotrophs to a more heterotrophic niche. We find that the displacement is affected by various environmental and physiological factors. For example, this displacement effect is temperature dependent, suggesting a link between community-level competitive mechanisms and global climate change. The proposed model therefore may be used to develop a more comprehensive analysis of the competition between constitutive mixotrophs and specialist phototrophs or heterotrophs. Our model also provides a mathematical framework for predicting constitutive mixotroph survival in the context of global warming.