Date of Award
2025
Degree Type
Dissertation
Degree Name
Doctor of Philosophy in Oceanography
Department
Oceanography
First Advisor
Susanne Menden-Deuer
Abstract
The discovery of mixotrophic plankton has bridged gaps in nutrient cycling budgets and altered the understanding of food webs and energy transfer. Mixotrophic plankton are capable of simultaneous or sequential photosynthesis and heterotrophy, providing multiple avenues of nutrition and thus reducing limitations that affect specialists. These energy dense cells provide increased biomass for higher trophic levels, which may increase organism sizes throughout the food web. Additionally, they may be physically denser, which alters biogeochemical cycling including carbon sequestration. The work presented here uses LysoTracker Green, an acidotropic stain that attaches to phagocytic vacuoles within potentially phagotrophic small (< 20 µm diameter) phytoplankton communities (‘mixoplankton’), to better understand their abundance, relative abundance, and growth rates in relation to the matrix of natural conditions they inhabit. Understanding the prevalence and drivers of mixoplankton is vital to better parameterize biogeochemical cycling and energy transfer in the ocean and for models that predict the fate of future oceans.In the first study, I compared the abundance of mixoplankton in the Northeast U.S. Shelf (NES) and Coastal California Current System (CCS) during two cruises in the NES in summer 2021 and one cruise in the CCS in summer 2022. These two contrasting coastal systems span the breadth of environmental conditions observed in temperate marine ecosystems and thus provided an opportunity to examine indicators of mixoplankton across temperate coastal regions. I discovered higher mixoplankton abundance in the NES than CCS yet similar population sizes of nanoeukaryotic photosynthesizers, resulting in higher relative abundance of mixoplankton in the NES than CCS. Across both systems, I found increased mixoplankton abundance associated with decreasing temperature and combined nitrate + nitrite concentrations (N+N), and increasing light intensity and Synechococcus abundance which is a possible prey choice. For the first time, environmental controls of mixotrophy were demonstrated in complex plankton communities across contrasting shelf environments. This chapter allowed for hypothesis generation of the underlying drivers of mixoplankton in temperate coastal ecosystems, suggesting that an interplay of biotic and abiotic factors together influence mixotrophy.
For the second study, I investigated the direct roles of nutrients and light availability in driving mixoplankton growth and relative abundance in Narragansett Bay, an estuary in Rhode Island. I incubated whole communities of mixoplankton under in situ conditions, increased nutrients, decreased light availability, or a combination of increased nutrients and decreased light availability for 24 hours in the University of Rhode Island Graduate School of Oceanography mesocosm facility during three successive weeks in July 2024. Results show highest mixoplankton growth and relative abundance in controls rather than amended treatments, implying that mixotrophy is a valid strategy when nutrients are scarce but not when light availability is limiting. Thus, I determined that nutrients and light availability drive mixoplankton communities in Narragansett Bay and may be used as proxies for determining the favorability of mixotrophy compared to strict autotrophy.
For the final study I expanded my observations in the NES across all seasons by attending 6 more cruises. This resulted in an analysis of mixoplankton relative abundance during four cruises in the summer, two in the winter, and one each in the spring in fall. By expanding my dataset across seasons and with more depth resolution across the Shelf, I was able to test whether the indicators and drivers I identified in previous chapters upheld over different periods of the year and with increased sampling efforts. Temperature, light availability, and N+N remained indicators of mixotrophy, wherein mixoplankton relative abundance increased with increasing temperature and light availability, and decreased with increasing N+N. Temperature, light availability, and N+N were combined in a multi-factor model to collectively predict mixoplankton relative abundance because of the interplay of these factors with each other in natural systems, and only N+N remained a consistent significant driver of mixoplankton relative abundance.
The research I present here furthers quantification of mixoplankton importance in temperate coastal and estuarine environments. With an underutilized method, I have established indicators of mixoplankton presence across an array of environmental conditions, depths, and community structures. I have also manipulated select environmental conditions to repress mixotrophy within 24 hours, supporting my body of research and providing constraints for mixotrophy response times. These studies were all conducted utilizing natural communities of plankton assemblages, allowing direct linkage to ocean models and hypotheses. Using conclusions gathered from my research, I hypothesize that with the increased warming and stratification that is predicted globally there will be a resulting increase in mixotrophy and thus increases of their roles in biogeochemical cycling and food web energy transfer.
Recommended Citation
Ewton, Erica, "QUANTIFYING MIXOTROPHY POTENTIAL AS A FUNCTION OF ENVIRONMENTAL AND BIOLOGICAL DRIVERS IN COASTAL AND ESTUARINE PLANKTON COMMUNITIES" (2025). Open Access Dissertations. Paper 4497.
https://digitalcommons.uri.edu/oa_diss/4497