PHYTOPLANKTON PRODUCTION AND BACTERIAL-ACTIVITY IN DECOMPOSITION OF ORGANIC-MATTER IN AN EUTROPHIC LAKE - THE CASE OF AMINOPEPTIDASE ACTIVITY

PhytopLankton primary production, bacterial heterotrophic activities ( production, uptake of [H-3]amino acids and [C-14]glucose) and aminopep tidase activity were studied in the water column and during the diel c ycle in highly eutrophicated Lake Mikolajskie. The microplankton activ ities changed both in the water column and throughout the day. The hig hest rate of phytoplankton primary production and extracellular releas e were noted in the surface of epilimnion. Heterotrophic activities (i .e. bacterial secondary production, uptake of [H-3]amino acids and [C- 14]glucose, aminopeptidase activity) reached maximal values in deeper waters of epilimnion and at the boundary between epi- and metalimnion. The reasons for spatial separation of bacterial and phytoplankton act ivity in water column are disscused. Diurnal variations of heterotroph ic activity exhibited similar pattern as did the changes of algal prod uction and chlorophyll(a) concentration. Only bacterial production inc reased markedly after the time of the highest algal activity. A tight relationship (''hydrolysis-uptake coupling system'') between activity of aminopeptidase and the rate of uptake of [H-3]amino acids was obser ved. The effect of different chemical compounds (glucose, leucine, amm onium chloride) on natural aminopeptidases was tested. In all suppleme nted samples decrease of enzyme affinity (increase K(m)) was noted. Th e activity of enzyme (V(max)) did not change. It is suggested that the tested compounds acted as competitive inhibitors of aminopeptidase ac tivity.