Stimulation of peptidase activity in nutrient gradients in the Baltic Sea

In the Baltic Sea, peptidase and alkaline phosphatase activities always show the same pattern in nutrient gradients: increasing enzyme activities with decreasing dissolved inorganic nitrogen (DIN) and phosphate. The field experiments reported here suggest that peptidase activity may be regulated by DIN in a manner similar to that of alkaline phosphatase by phosphate. In nutrient gradients established in mesocosms, peptidase activity increased 5- to 7-fold, whereas the alkaline phosphatase activity increased between 29- and 65-fold. Parallel to the enzyme activities, phytoplankton biomass (measured as chlorophyll a), organic phosphorus and nitrogen increased, but the closest correlation was found between peptidase and alkaline phosphatase activity (r=0.90; n=18; p<0.01). The addition of nitrate, and nitrate plus phosphate, at the time where nutrients were depleted and both enzyme activities were high, did not result in a decrease of peptidase activity. Rather, the peptidase activity was highest after simultaneous treatment with phosphate and nitrate. The addition of 1 mg l−1 alkaline phosphatase caused a 6-fold increase of the peptidase activity after 96 h. Several mechanisms of the stimulation of peptidase activity by alkaline phosphatase are discussed.