ARE BLOOM‐FORMING GREEN ALGAE CHEMICALLY DEFENDED?

Van Alstyne, K. L. Shannon Point Marine Center, Western Washington University, 1900 Shannon Point Road, Anacortes, WA 98221 USA Ulvoid green macroalgae, such as Enteromorpha and Ulva, can form large blooms that have deleterious impacts on the local biota. These algae are often assumed to be very palatable for most invertebrate and vertebrate herbivores because they lack obvious physical defenses, and because there have been few reports of their producing chemical defenses. However, in laboratory feeding preference assays, the ulvoid macroalgae Enteromorpha linza and Ulva fenestrata were low preference foods for green sea urchins, Strongylocentrotus droebachiensis. Both these algae and several other species of green algae produce large quantities of dimethylsulphoniopropionate (DMSP), which is enzymatically converted to dimethyl sulfide (DMS) and acrylic acid when the algae are physically damaged. In laboratory bioassays, both DMS and acrylic acid were potent feeding deterrents towards urchins at concentrations that the urchins would be likely to encounter in the field. The precursor in this system, DMSP, was a feeding attractant. Our data provide evidence that DMSP functions as a precursor in an activated defense system in marine macroalgae and suggests a similar function in phytoplankton. The presence of this activated defense system may contribute to the persistence of macroalgal blooms by making these algae unpalatable to some species of herbivores.