INFLUENCE OF ZOOPLANKTON GRAZING ON FREE DISSOLVED ENZYMES IN THE SEA

In the Northern Adriatic Sea, extracellular enzymatic activity was measured during a Lagrangian study following a drifting buoy for 40 h. Dissolved free enzymatic activity represented 20 to 70% of total activity depending on the type of enzyme. alpha- and beta-glucosidases exhibited a significantly higher free activity than proteolytic enzymes. In subsequent laboratory experiments we investigated the effect of zooplankton on the free enzyme pool. The 4-step approach included: (1) determination of the enzymatic activities in copepods (mainly Acartia clausi); (2) enzymatic activity in fecal pellets; (3) short- and long-term grazing experiments; and (4) degradability of free glucosidase in seawater. alpha- and beta-glucosidases, leu-aminopeptidase, lipase and chitinase were examined. Experiments in which zooplankton were selectively enriched revealed a significant increase in both particle-bound (due to the increase of bacterial density) and dissolved free enzymatic activity. Incubating water enriched in free enzymes released by zooplankton with natural bacterial consortia, we found that 70% of the original alpha- and beta-glucosidase activity remained after 22 h. The presence of microorganisms did not enhance the degradation of these enzymes as compared to autoclaved controls. We found that a considerable amount of free dissolved enzymes is lost by 0.2 mu m filtration using Nuclepore filters, thereby leading to an underestimation of dissolved enzymes by similar to 30% in our experiments. Based on our results we conclude that mesozooplankton contribute to the free enzymatic activity in natural waters especially during periods of high grazing activity.