CRUI - Collaborative Research: Investigation of a phytoplankton DMSP-based chemical defense system active against protist grazers

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Chemical defenses ? chemicals used by individuals to reduce or eliminate agents of mortality are diverse, widespread, and important for species interactions. Less studied in planktonic organisms than in terrestrial and marine benthic species, chemical defenses are nonetheless thought to play a role in structuring planktonic communities. They may be particularly important in promoting the formation and persistence of algal blooms. In recent NSF?supported research, we showed that the coccolithophorid Emiliania huxleyi is chemically defended against most tested protist grazer species. At least two aspects of E. huxleyi sulfur chemistry were shown to reduce rates of protist grazing: high DMSP lyase activity (DLA), and exogenous DMSP (dimethylsulfoniopropionate) itself. As both high phytoplankton DLA and dissolved DMSP are present in the sea, we propose that this defense system is important in allowing E. huxleyi blooms to form and persist. Furthermore, as many bloom?forming dinoflagellate and prymnesiophyte species have high intracellular DMSP concentrations and at least some have high DLA, this chemical defense system may be of widespread importance for phytoplankton bloom dynamics. Numerous experiments on this DMSP "defense system" indicated that DMSP is not toxic to protists. Rather, this sublethal defense system might be based on intercellular chemical communication among grazer and prey species. The present proposal constitutes an investigation into the source, mechanism(s) of action, and significance of this defense system.To accomplish this goal, a multi?level investigation will be conducted. Addition of high lyase E. huxleyi cells and defense chemicals to natural water samples and mixed laboratory assemblages will allow testing of community?level defense effects, including possible reductions in microzooplankton grazing, transfer of grazing to other prey types, and allelopathic effects on other phytoplankton species. Population?level studies will reveal whether protist sensitivity to defenses is related to protist taxon, feeding mode, or size. On the individual and molecular levels, a spectrum of possibilities for the mechanism of DMSP?induced feeding inhibition is presented. The effects of chemical cues (DMSP cleavage and oxidation products, algal osmolytes) on protist feeding rates and individual behavior will be studied as a means of testing these possibilities.BROADERIMPACTSThe proposed study will include numerous graduate student and undergraduate participants, including students in Shannon Point Marine Center's Minorities in Marine Science Undergraduate Program. Other outreach activities will include incorporation of concepts, techniques and findings into teaching activities, and preparation of web and print materials aimed at a general audience.INTELLECTUAL MERITDeveloping an increased understanding of sublethal chemical defenses as they involve cell signaling and, ultimately, phytoplankton bloom formation is an exciting area for oceanography, biological sciences, and education. By contributing to our ability to compare chemical defense theory among terrestrial, marine benthic, and planktonic ecosystems, such understanding could yield insights into ecological and evolutionary principles that govern biological interactions across these seemingly diverse ecosystems.