Project: Collaborative Research: Investigating the relationship between size and the balance between carbon acquisition modes in mixotrophic protists

NSF Award Abstract:
Microplankton in aquatic systems have traditionally been categorized as phytoplankton or zooplankton. However, a growing body of research has documented that a substantial portion of plankton belongs to a third group, mixotrophs, that can use both photosynthesis and prey ingestion to obtain resources necessary to maintain growth. Biological oceanographers have a poor understanding of mixotrophs’ activity and their role in the aquatic food web due to the methodological challenges of studying them. In this project, the investigators are assessing how mixotrophs’ cell size relates to their dependence on photosynthesis and prey ingestion for nutrient acquisition. Among phytoplankton, size is considered a “master trait” because it impacts metabolic processes, growth rates, cellular composition, and nutrient uptake kinetics. While size has been used to constrain mixotrophic activity in some numerical models, the functional relationship between cell size and the balance of nutrient acquisition modes has not yet been empirically established. As part of broader impacts, all project investigators are participating in an Ocean Carbon and Biogeochemistry (OCB) mixotrophy working group. The project team includes early career investigators and is providing training to a postdoctoral scientist, graduate student, and multiple undergraduates through an established research experience for undergraduates (REU) program. Finally, educational outreach includes collaboration with a professional development program for science teachers, providing experiential science curricula and equipment kits for teaching ocean science in the classroom and outdoors.

Mixotrophs obtain carbon from photosynthesis and ingestion simultaneously but the contribution from each nutrient mode to an individual organism is rarely measured simultaneously. Less is known about how the contribution of each nutrient mode differs between species and or with changes in growth-limiting factors. The investigators are conducting incubation experiments with eight species of mixotrophs to measure the proportion of carbon obtained from both nutrient acquisition modes across a range of environmental conditions. Using cutting-edge isotopic labelling methods, the investigators are assessing the relationship between cell size and trophic strategy among mixotrophs, with the potential for new, generalized understanding of carbon transfer and transformation in marine ecosystems. A relationship between mixotroph size and their two nutrient modes could transform biological oceanographers’ understanding of mixotrophs at the cellular and community levels, improve the accuracy of ocean ecosystem models, and enable more accurate predictions of how plankton communities and biogeochemical cycling will be altered with changing ocean conditions.

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.