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 phos phatase (APA) was studied in the pelagic zones of two German lakes. Th e obtained results supported the existence of an enzymatic mechanism o f phosphorus-regeneration in pelagic lake water, which appears to be d istinct 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 inhibit ion of activity by inorganic phosphorus (Pi) was observed. However, th e 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 capaciti es of the plankton community at high substrate concentrations. At thes e conditions the majority of the planktonic uptake of the released Pi occurred in the algal size fraction. However, the coupling between rel ease and uptake became much stronger at lower substrate concentrations , coinciding with an increased percentage of bacterial uptake. The obt ained results suggest that. regeneration of orthophosphate by the 5'-n ase system may be more significant for the phosphorus dynamics of the whole planktonic community as compared to Pi liberation by APA activit ies. However, considerable uncertainty is left with respect to its qua ntitative role in the P-cycle of lakes because available methods for d etermination of 5'-nase activity are still subject to methodological r estrictions.