A COMPARATIVE-STUDY ON 5'-NUCLEOTIDASE (5'-NASE) AND ALKALINE-PHOSPHATASE (APA) ACTIVITIES IN 2 TAKES

Phosphorus regeneration by 5'-nucleotidase (5'-nase) and alkaline phosphatase (APA) was studied in the pelagic zones of two German lakes. The obtained results supported the existence of an enzymatic mechanism of phosphorus-regeneration in pelagic lake water, which appears to be distinct from APA and is presumed to be represented by 5'-nucleotidases of bacterial origin. These enzymes exhibit a high substrate-affinity and a high specificity for nucleotides. In contrast to APA, no inhibition of activity by inorganic phosphorus (Pi) was observed. However, the enzymes could be strongly inhibited by Ni2+ ions and to some extent by Glucose-6-Phosphate. As was exemplified with ATP as substrate, the Pi released by these enzymes was much in excess of the uptake capacities of the plankton community at high substrate concentrations. At these conditions the majority of the planktonic uptake of the released Pi occurred in the algal size fraction. However, the coupling between release and uptake became much stronger at lower substrate concentrations, coinciding with an increased percentage of bacterial uptake. The obtained results suggest that. regeneration of orthophosphate by the 5'-nase system may be more significant for the phosphorus dynamics of the whole planktonic community as compared to Pi liberation by APA activities. However, considerable uncertainty is left with respect to its quantitative role in the P-cycle of lakes because available methods for determination of 5'-nase activity are still subject to methodological restrictions.