Date Awarded


Document Type


Degree Name

Master of Science (M.Sc.)


Virginia Institute of Marine Science


Nicole C Millette

Committee Member

Kimberly S Reece

Committee Member

Mark J Brush

Committee Member

Bongkeun Song

Committee Member

Rebecca J Gast


Mixotrophic protists, which combine the use of photosynthesis and prey ingestion to obtain nutrients for growth, comprise a substantial portion of the plankton community. However, there is a major gap in our understanding of how mixotroph prevalence varies spatially and temporally and under what conditions they dominate. I utilized a recently developed molecular technique to experimentally identify active mixotrophs (taxa identified to be grazing when samples were collected) and combined this with microscopy data to estimate active mixotroph abundance and proportion at two locations in a temperate estuary over a year. Active mixotroph abundance was compared to potential mixotroph (taxa that have demonstrated mixotrophic capability in previous peer-reviewed studies) abundance to assess potential overestimations of mixotrophs when not accounting for which taxa are actively ingesting. Measurements of potential mixotrophs demonstrated overestimations of mixotroph abundance. More importantly, analyses demonstrated that presence of taxa with mixotrophic capability does not necessarily mean they are engaging in mixotrophic activity. Constraining the identification of mixotrophs to known active mixotrophs present in the same environment combined with environmental conditions conducive to mixotrophy provides the most accurate estimation of the abundance and proportion of mixotrophs. The abundance of active mixotrophs at both locations was correlated to the abundance of major taxonomic groups. At one location, dinoflagellates were the dominant mixotrophic ASVs throughout the whole year, and the environmental patterns associated with a high abundance of active mixotrophs were similar with the patterns associated with a high abundance of dinoflagellates. At the other location, dinoflagellates and cryptophytes were the dominant mixotrophic ASVs depending upon the time of year, and the environmental patterns associated with a high abundance of mixotrophs were similar to patterns associated with a high abundance of cryptophytes. The results obtained suggest that in situ mixotroph abundance might not be only regulated by environmental conditions favorable to mixotrophy but, instead, environmental conditions favorable to each specific taxon’s utilization of phagotrophy. These findings substantially increase our understanding of how in situ mixotrophic abundance and proportion are influenced by the planktonic community composition in combination with environmental factors.



© The Author