Collaborative Research: Can mixotrophic protists break free from cell size constraints to thrive in the ocean?

Mixotrophic plankton, which have the capacity for both photosynthesis and heterotrophy (consumption of other organisms), play a key role in ocean carbon cycling. This project examines a paradox: ocean warming is expected to favor smaller cell sizes and heterotrophic metabolism, but in mixotrophs, larger cells are more heterotrophic than smaller cells. Using laboratory experiments and mathematical modeling, the team is examining whether under changing ocean conditions mixotrophic plankton can break free of cell size constraints by shifting their metabolism more towards heterotrophy. The results of this study allow for better predictions of carbon cycling and food web dynamics in future oceans. The project also supports interdisciplinary research training for graduate and undergraduate students, public outreach events, workshops for grades 6-12 students, and development of an exhibit on mixotrophs. This project is unraveling how mixotrophy scales with cell size, improving predictions of primary production and carbon cycling under warming and nutrient limited ocean conditions. The team is using physiology, cellular models, flow cytometry, RNA sequencing, and multi-scale modeling approaches to 1) test how mixotrophic plankton cell size varies across a range of temperatures and nitrogen concentrations, 2) uncover the underlying mechanisms and tradeoffs that constrain mixotroph metabolisms, and 3) investigate the ecological niches of mixotrophs across a latitudinal gradient. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.