INHIBITION OF COPEPOD GRAZING BY DIATOM EXUDATES - A FACTOR IN THE DEVELOPMENT OF MUCUS AGGREGATES

Dissolved polymers, especially polysaccharides, are thought to play an important role in formation of larger organic aggregates in the sea. Phytoplankton may exude considerable quantities of polymers, although the particular conditions conducive to release of these substances are not known. Such processes are implicated in the formation of large-scale gelatinous mucus aggregations in the Adriatic Sea. We investigated extracellular carbohydrate release in the diatoms Coscinodiscus granii, Stephanopyxis palmeriana, Skeletonema costatum, Nitzschia delicatissima, Thalassiosira weissflogii, and 2 species of Chaetoceros, under different culture conditions, and examined whether these exudates interfered with copepod grazing. The copepod species studied are typical of Adriatic coastal enviroments. Our results indicate that during the exponential growth phase the contribution of the high-molecular-weight carbohydrate (HMWCHO) fraction to extracellular release was minor, except in phosphorus limited medium. A shift towards an increased significance of HMWCHO was observed in the stationary phase for all species tested. Grazing experiments with Temora longicornis and Pseudocalanus elongatus demonstrated that copepod feeding was inhibited by HMWCHO diatom exudate and also stationary phase culture filtrate. Feeding rates were also adversely affected by the presence of pure HMW dextran used as a model substance. Copepod feeding was more strongly inhibited by diatom HMWCHO exudates than an equivalent concentration of pure dextran, suggesting an additional response to the chemical stimuli of algal exudates.