DIVERSITY, TROPHIC STRUCTURE, AND METABOLIC FUNCTION OF OPEN OCEAN HETEROTROPHIC PROTIST COMMUNITIES

Author

• Erin L. Jones, University of Rhode IslandFollow

Date of Award

2026

Degree Type

Dissertation

Degree Name

Doctor of Philosophy in Oceanography

Specialization

Biological Oceanography

Department

Oceanography

First Advisor

Tatiana A. Rynearson

Abstract

Heterotrophic protists, encompassing microzooplankton grazers and parasitic taxa, play essential yet underappreciated roles in mediating marine carbon export. Through diverse feeding strategies, including phagotrophy, mixotrophy, and parasitism, these organisms influence the fate of primary production by altering phytoplankton community structure, driving nutrient recycling, and reshaping energy flow within marine food webs. While microzooplankton grazing is widely recognized as a major pathway of carbon transformation, the taxonomic and functional diversity of heterotrophic protists and the metabolic signals of grazing remain poorly resolved in field observations. Similarly, despite evidence that parasitic mortality can rival or exceed grazing losses, parasite-driven carbon fluxes are largely unmeasured in global biogeochemical models. This knowledge gap limits our understanding of how heterotrophic protist communities shape the biological pump and regional differences in carbon export. This dissertation addresses these gaps by investigating the diversity, activity, and export-related impacts of heterotrophic protist communities across two contrasting ocean basins within the NASA EXPORTS program.

First, to better interpret the ecological roles of heterotrophic protists, we developed the Trophic Modes Database (TMD) in Chapter 2, a curated reference linking taxonomy with trophic capabilities for dinoflagellate and ciliate taxa. This synthesis combined an extensive literature review with validation of an existing mixotrophy database to assign trophic classifications and sub-classifications at the lowest possible taxonomic resolution. In addition to trophic mode, the database compiles ecological traits including feeding strategies, prey types, and size ranges. The resulting dataset (TMD v1.1) includes 378 taxa across Dinoflagellata and Ciliophora and serves as a standardized reference for interpreting protist community composition. By consolidating decades of observations, this work provides a resource for linking protist biodiversity with trophic function and improving assessments of their role in marine carbon cycling.

Heterotrophic protists are taxonomically and functionally diverse communities capable of shaping phytoplankton abundance and carbon transport through food web interactions. The taxonomic composition, feeding strategies, and vertical distribution of dinoflagellates and ciliates were investigated in Chapter 3 using samples from the Northeast Pacific during the 2018 EXPORTS campaign. DNA sequencing revealed a vertically structured community dominated by parasitic Syndiniales and frequent carnivorous ciliate grazers, suggesting complex trophic interactions and an underappreciated role for parasitism in carbon cycling.

Grazing by microzooplankton is the largest loss factor for phytoplankton biomass and influences carbon available for export. In Chapter 4, the transcriptional activity of grazing communities was measured using dilution experiments and in situ sampling in the North Atlantic during the 2021 EXPORTS campaign. Heterotrophic grazers exhibited metabolic shifts when grazing was detectable, including increased expression of transcripts related to translation, signal transduction, and intracellular trafficking. Transcripts associated with cilia and flagella motility were significantly upregulated under detectable grazing and downregulated when grazing was absent, with several showing positive correlations with grazing rates. These findings identify potential mRNA indicators of grazing activity in mixed field communities.

Non-traditional heterotrophic activity, such as parasitism, can disrupt carbon flow by targeting hosts across trophic levels. In Chapter 5, we characterized Syndiniales community composition in surface waters and exported material from sediment traps and examined relationships with host communities and export production in the North Atlantic. Syndiniales diversity was widespread across seawater and sinking particles, with individual particles containing a large cumulative pool of parasite diversity. Community composition differed between seawater and exported material, and richness in sediment traps increased with particulate organic carbon flux. Network analyses indicated extensive associations with potential hosts, suggesting sinking particles as sites for parasite-host interactions and pathways for export to the mesopelagic ocean.

Altogether, this dissertation advances understanding of the diversity, metabolic capabilities, and export mechanisms associated with heterotrophic protist communities and highlights the importance of incorporating protistan activity into future models of carbon export.

Creative Commons License

This work is licensed under a Creative Commons Attribution 4.0 License.

Recommended Citation

Jones, Erin L., "DIVERSITY, TROPHIC STRUCTURE, AND METABOLIC FUNCTION OF OPEN OCEAN HETEROTROPHIC PROTIST COMMUNITIES" (2026). Open Access Dissertations. Paper 4572.
https://digitalcommons.uri.edu/oa_diss/4572