EXTRACELLULAR, LOW-AFFINITY BETA-N-ACETYLGLUCOSAMINIDASES LINKED TO THE DYNAMICS OF DIATOMS AND CRUSTACEANS IN FRESH-WATER SYSTEMS OF DIFFERENT TROPHIC DEGREE

Extracellular hydrolysis of 4-methylumbelliferyl beta-N-acetylglucosam inide was measured in the oligomesotrophic Piburger See and the eutrop hic Rimov reservoir during spring and summer phytoplankton blooms. res pectively. Total enzymatic activity (TEA) ranged between 0.2 and 19.1 nmol l(-1) h(-1) in the reservoir and between 0.8 and 12.4 nmol l(-1) h(-1) in the lake. High-affinity (K-m < 1 mu mol l(-1)) and low-affini ty (K-m > 100 mu mol l(-1)) enzymes were kinetically identifiable in m ost samples from both localities. The low-affinity enzyme activity (LE A) usually accounted for >60% (mean: 80%) of TEA. LEA and diatom bioma ss significantly correlated over time in the reservoir epilimnion (r(s ) = 0.578) and in the lake metalimnion (r(s) = 0.862). As diatoms poss ess chitin and take up its monomer, N-acetylglucosamine, two explanati ons of the observed relationships are suggested: extracellular beta-N- acetylglucosaminidase activity partly originates either from ectoenzym es of chitinolytic bacteria attached to diatom cells or from ectoenzym es of diatoms, enabling them to take up N-acetylglucosamine from ambie nt amino sugars instead of synthesizing it de novo. A significant posi tive correlation of LEA with crustacean abundance was found in the lak e epilimnion (r(s) = 0.850), apparently reflecting the growing spring populations of frequently moulting juvenile crustaceans. A possible co ntribution of chitinolytic bacteria, accompanying the crustacean popul ations, to LEA is discussed.