MEPS 250:175-181 (2003)  -  doi:10.3354/meps250175

ABSTRACT: Almost half the biogenic sulfur emitted globally originates as dimethylsulfide (DMS) in marine algae. Yet, despite its importance to climate and the global sulfur cycle, the reasons why many diverse algal taxa produce DMS remain unclear. DMS is produced when dimethylsulfoniopropionate (DMSP) is cleaved by the enzyme DMSP lyase into DMS and acrylic acid. Because acrylic acid can deter feeding by some herbivores, the production of DMSP has been postulated to be part of an antiherbivore chemical defense system, with DMS being a by-product of the production of the defensive compound acrylic acid. However, we found that DMS plays a more direct role in herbivore deterrence. Using laboratory bioassays, we demonstrated that DMS functions as a potent feeding deterrent against sea urchins Strongylocentrotus droebachiensis at a wide range of concentrations. The defensive function of DMSP cleavage is further supported by the results of a multiple-choice feeding preference assay in which we offered urchins a choice of 8 algal species. In these assays, algae containing DMSP tended to be consumed at lower rates than species that lacked it. By measuring headspace DMS concentrations during grazing, we showed that DMS is produced when urchins feed on algae containing DMSP, but not when urchins feed on algae without it. DMS production occurred while the urchins were feeding on the algae and for 18 h after the algae were removed, suggesting that post-ingestive processes might be playing a role in activation. These results demonstrate that DMS is produced when urchins feed on algae containing DMSP and that it functions as an herbivore deterrent, causing species that produce DMSP to be avoided by some herbivores.

KEY WORDS: Activated defense · Chemical defense · DMS · DMSP · Herbivory · Macroalgae · Sea urchins