Dynamics of Protistan Grazers: Diversity, Abundance and Prey Relations

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Some of the most important primary producers and consumers in aquatic ecosystems are protists, or single-celled eukaryotes. It is well established that protistan predation can be a significant source of mortality for bacteria and phytoplankton. Grazing protists in turn serve as prey for zooplankton (copepods), and through the excretion of nitrogen and phosphorus compounds, they play a major role in the release of regenerated nutrients. Despite decades of studies on protistan grazing, knowledge gaps still exist with respect to their abundance, distribution, seasonality, prey selectivity, and co-occurrence patterns. The results from this project will advance the understanding of grazing communities in situ and how they respond to environmental conditions and prey communities. This will be one of very few studies of grazers that is unbiased by artificial prey and containment, and will yield both morphologic and genetic information about the organisms present and the distribution patterns of particular grazer populations. A graduate student will be performing her thesis work as part of this project, and selected undergraduates will be involved in the research. In partnership with the Zephyr Education Foundation's marine science literacy and education program, teachers will develop an educational unit based on the microbial food web that is tailored to grade levels 6-8. This project examines whether the persistence of a group of protistan grazers is determined by its feeding strategy (grazers with specialist feeding strategies are more ephemeral than generalists), and whether certain morphotypes exhibiting generalist feeding strategies have underlying genotypic diversity that maps to specialist feeding strategies. It builds upon an ongoing time series (with hourly resolution since 2006) of automated, high-resolution, measurements of the phytoplankton community by the Imaging FlowCytobot at the Martha's Vineyard Coastal Observatory. These measurements have led to the observation that, in addition to shifts from pico- and small nanoplankton during the summer to larger microplankton in the fall and winter, particular species (especially among the diatoms) exhibit distinct and recurring seasonal patterns. The instrument will be modified to also conduct automated measurements of grazer communities in situ. Links between selected grazer taxa (chosen based on the image time series) and phytoplankton prey will be provided through genetic analyses of individual cells (with their ingested prey). These cells will be obtained by use of a recently developed cell sorter that also captures an image of each sorted cell. In addition to providing predator/prey links, the genetic information will allow the investigators to determine whether a grazer morphotype represents multiple species. A third approach, high throughput sequencing and quantitative PCR analysis of whole water samples, will be applied to investigate abundance patterns of species whose morphology does not reliably map to genotype.