STUDIES ON PHOSPHATASE-ACTIVITIES IN HUMIC LAKES

Microbial hydrolytic enzymes (MHE) were studied in acid lakes with different amounts of dissolved humic matter (DHM) in the Scandinavian boreal zone. Special attention was paid to acid phosphatase activities (APA), its distribution, kinetics, inhibitors, and modifiers. APA varied between 134-1706 nmol. L(-1).h(-1) (average 460 nmol. L(-1). h(-1)) with slightly higher values in acid humic waters compared to clear water lakes. Of the total APA, 60-80% was not particle bound and passed 0.2-mu m Nuclepore filters. V-max from enzyme kinetics gave higher values (636.5 nmol .L(-1). h(-1)) of APA in the acidified experimental Basin A of HUMOR/HUMEX-lake Skjervatjern compared to lower values (283.7 nmol. L(-1).h(-1)) in the control Basin B. After size fractionation, particle-bound APA (100-0.2 mu m size fraction) showed lower K-m values (6.1 mu M) in Basin A than in Basin B (17 mu M). Non-particle-bound APA (passing 0.2-mu m pore size filter) showed higher K-m (1.3 mu M) in Basin A compared to Basin B (0.6 mu M). APA in both basins of Skjervatjern was not inhibited by ambient P-PO4 concentrations. Inhibition by PO4-3 will exist only when the ambient P-PO4 concentration increases to 100 mu g P-PO4. L(-1) and more. Ambient DHM can act as modifier of APA kinetics by changing V-max and K-m. Nutrient enrichment can induce more APA activities only when C, N, P are added simultaneously. Single element enrichment alone has no effect on elevated APA activities. Phagotrophic microplankton are regarded as potential contributors to APA activities.