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.a-and P-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: ( l ) determinatlon of the enzymatic activities in copepods (mainly Acartia clausi); (2) enzymat~c activity in fecal pellets;(3) short-and long-term grazing experiments; and (4) degradab~lity of free glucosidase in seawater.aand P-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 aand P-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 pm filtration using Nuclepore filters, thereby leading to an underestimation of dissolved enzymes by -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.