All Mixed Up: How metabolic tradeoffs and synergies shape mixotrophs’ evolutionary responses to climate change

Background/Question/Methods

As Earth’s climate warms, scientists seek to predict how short- (e.g., phenotypically plastic) and long- (e.g., evolutionary) term physiological responses of living organisms will feed back to alter community structure and biogeochemical cycling. One group of organisms for which these predictions are especially complex are mixotrophs, which combine the capacity for photosynthesis and heterotrophy within a single cell. Mixotrophs are increasingly recognized as important components of planktonic ecosystems, but limited and contradictory data exist describing how rates of growth, photosynthesis, and heterotrophy will respond to increasing temperatures. We seek to unify these divergent predictions by quantifying the metabolic tradeoffs that mixotrophs experience using a combination of mathematical models and ecological and evolutionary experiments.

Results/Conclusions

We first developed mathematical models describing how mixotrophs should respond to increases in temperature by adjusting their relative investments in photosynthesis and heterotrophy. These models predict that mixotrophs should evolve to become more heterotrophic at warmer temperatures, a response that we confirmed via a three-year evolution experiment. However, this evolved response is sensitive to assumptions about the tradeoffs mixotrophs experience between these two metabolic modes. Therefore, we conducted a series of laboratory experiments using constitutively mixotrophic nanoflagellate lineages from the genus Ochromonas. We measured rates of photosynthesis and heterotrophy as a function of environmental conditions to quantify tradeoffs between the metabolic rates within and across lineages. To our surprise, these measurements revealed synergies, as well as tradeoffs: In the presence of increased prey, most mixotrophs upregulated photosynthetic machinery. Our findings suggest that standing variation in mixotroph phenotypic plasticity, as well as underlying tradeoffs between photosynthesis and heterotrophy, will shape mixotrophs’ eco-evolutionary responses to warming waters.