GRAZING-ACTIVATED CHEMICAL DEFENSE IN A UNICELLULAR MARINE ALGA

Marine plankton use a variety of defences against predators, some of w hich affect trophic structure and biogeochemistry(1). We have previous ly shown(2) that, during grazing by the protozoan Oxyrrhis marina on t he alga Emiliania huxleyi, dimethylsulphoniopropionate (DMSP) from the prey is converted to dimethyl sulphide (DMS) when lysis of ingested p rey cells initiates mixing of algal DMSP and the enzyme DMSP lyase. Su ch a mechanism is similar to macrophyte defence reactions(3,4). Here w e show that this reaction deters protozoan herbivores, presumably thro ugh production of highly concentrated acrylate, which has antimicrobia l activity(5). Protozoan predators differ in their ability to ingest a nd survive on prey with high-activity DMSP lyase, but all grazers pref erentially select strains with low enzyme activity when offered prey m ixtures. This defence system involves investment in a chemical precurs or, DMSP, which is not self-toxic and has other useful metabolic funct ions. We believe this is the first report of grazing-activated chemica l defence in unicellular microorganisms.