PHYTOPLANKTON CHEMICAL SIGNALS INFLUENCE HERBIVORY BY PROTIST GRAZERS

Abstract

Strom, S. L.1 & Wolfe, G. V.2 1Shannon Point Marine Center, Western Washington University, Anacortes WA 98221 USA; 2Department of Biological Sciences, California State University Chico, Chico CA 95929 USADeterrence of herbivores through chemical signaling ‐ plant chemical defense ‐ is known to be widespread throughout terrestrial and marine benthic ecosystems. The occurrence and significance of chemical defenses produced by unicellular marine phytoplankton has not, however, been systematically explored. We are investigating the role of the phytoplankton‐produced compounds dimethylsulfo‐niopropionate (DMSP) and its cleavage products dimethylsulfide (DMS) and acrylate in reducing herbivory by protist grazers. DMSP is produced in high intracellular concentrations by numerous phytoplankton species, especially notorious bloom‐forming dinoflagellates and prymnesiophytes, and is cleaved by the enzyme DMSP lyase during grazing, physiological stress or cell lysis. Using four different strains of Emiliania huxleyi, a coccolithophorid (prymnesiophyte), we have shown that strains with high DMSP lyase activities experience consistently reduced levels of herbivory in comparison with low lyase strains. Curiously, the products of the cleavage reaction (DMS and acrylate) do not affect rates of herbivory, indicating that this is not an activated chemical defense. Feeding rates are, however, proportionally and substantially reduced by the reaction precursor, DMSP. We also have preliminary evidence that high lyase strains release more DMSP to seawater than do low lyase strains. DMSP, while not evidently toxic to protist grazers, appears to act as a chemical signal that reduces grazing. We hypothesize that these unicellular herbivores may have evolved to recognize and respond to a compound (DMSP) that is a precursor of the potentially deleterious cleavage product acrylate. This and related chemical signaling processes may be important in promoting formation of phytoplankton blooms in the sea.