In geologically recent times, diatoms have evolved to become one of the most successful phytoplankton taxa in the sea, responsible for an estimated 40% of marine primary production. The sustained blooms that diatoms often experience are one attribute leading to their high primary production. What allows so many diatom cells to escape the zooplankton grazing pressure that removes most other phytoplankton from the sea on a daily basis? Recently, both chemical and mechanical defenses have been hypothesized to deter grazers that might otherwise consume diatoms. Reactive polyunsaturated aldehydes (PUAs) derived from C20 fatty acids upon diatom wounding have been shown to inhibit cellular processes in numerous marine organisms. When adult copepods are fed a diet rich in PUA-producing diatoms they are unaffected. However, their reproductive potential can be impaired in a number of ways, including decreased egg production, reduced hatching success, and poor survival of larvae. This 'insidious' chemical defense thus appears to function by reducing grazing impacts of subsequent generations of copepods. The diatom frustule has also been hypothesized to reduce predation by mechanically defending the cell - acting as armor - against certain types of consumers. There is preliminary indications that microzooplankton, primarily protists, often graze substantially more diatom production than do crustacean zooplankton, and might provide selective pressure for the evolution of diatom defenses. This research will consist of a series of experiments to test diatom defenses against grazing and reproduction by several taxa of diatom-feeding protists, including heterotrophic dinoflagellates and large ciliates. The production of PUAs by diatoms under different growth conditions will be measured using recently developed GC-MS techniques. Protist grazing, digestion and growth rates will be compared when fed closely related strains of diatoms that produce different levels of PUAs. Fluorescent probes will be used to assess modes of cell damage in affected protist grazers. To test mechanical defenses, we will grow diatoms under different levels of silicic acid availability to generate different frustule thicknesses, and then assess grazing, digestion and growth rates of their protist grazers. This research will contribute to our understanding of the regulation of diatom (and hence total marine) primary production, as well as to fundamental ecological questions about the evolution of plant defense pathways in the marine plankton. Broader Impacts: This project will directly involve undergraduate students in research through three programs. At OSU, chemistry-oriented students from the Honor College will be recruited to participate in the organic chemistry aspects of this research. At SPMC, undergraduate participants in SPMC's summer REU program and ethnic minority students from the Minorities in Marine Science Undergraduate Program (MIMSUP) will participate in phyto- and microzooplankton experimental aspects of the project. Funds are included to support the research of one or more WWU graduate students. Scientific findings will be disseminated broadly though participation in national meetings and publication in peer-reviewed journals.
