TitleEvaluation of extracellular, high-affinity beta-N-acetylglucosaminidase measurements from freshwater lakes: An enzyme assay to estimate protistan grazing on bacteria and picocyanobacteria
Publication TypeJournal Article
Year of Publication1996
AuthorsVrba, J, Šimek, K, Pernthaler, J, Psenner, R
JournalMicrobial Ecology

Protistan community grazing rates upon both bacterioplankton and autotrophic picoplankton were estimated using fluorescently-labeled prey and by measurement of extracellular hydrolysis of 4-methylumbelliferyl (MUF) beta-N-acetylglucosaminide in a eutrophic reservoir and an oligo-mesotrophic lake during phytoplankton blooms. In addition, enzyme methods were optimized in bacterivorous flagellate cultures by two enzyme assays, based on fluorometric detection of protistan digestive activity, which were compared and calibrated independently against flagellate bacterivory. Enzymatic hydrolyses of MUF beta-N,N’,N ’’-triacetylchitotriose and MUF beta-N-acetylglucosaminide were measured in cell-free (sonicated) and whole-cell (unsonicated) samples. The hydrolysis of both substrates, using the whole-cell enzyme assay at in situ pH, was correlated significantly with total grazing rate of Bodo saltans. Thus the whole-cell enzyme assay with MUF beta-N-acetylglucosaminide was used for freshwater samples. High-affinity (K-m 100 mu mol l(-1)) enzymes were distinguished kinetically in most samples from both systems studied. Activities (V-max) of the high-affinity enzyme varied from 0.24 to 1.43 nmol l(-1) h(-1). Protistan community grazing on bacterioplankton was in the range of 0.15-1.36 mu g C l(-1) h(-1) both for lake and reservoir, the differences being observed in grazing on picocyanobacteria (lake, 0.03-0.22 mu g C l(-1) h(-1); reservoir, 0.35-1.56 mu g C l(-1) h(-1)). The enzyme activities were correlated significantly with the protistan grazing both on bacterioplakton (r(s) = 0.62, P