Pii: S0038-0717(00)00171-1

Extracellular enzyme activities and the kinetics of glucose and leucine uptake were measured to study the role of different substrate pools for bacterioplankton in a shallow eutrophic lake. The study took place during the period of cyanobacterial bloom in late summer and its collapse in the autumn. Leucine aminopeptidase activity (LAP), b-glucosidase activity (b-Gluc), H-leucine incorporation (LI) and Cglucose incorporation (GI) were measured in Lake Võrtsjärv during the autumn of 1997 (September–October). The kinetic parameters Vmax and KM were determined for both enzymes using artificial fluorogenic substrates (leucine amino-methylcoumarin and methyl-umbelliferyl bglucose). Leucine and glucose uptake were measured using radiolabelled compounds. Abundance and production of bacterioplankton were also measured. Several environmental parameters including temperature, nutrient concentrations, seston content, and phytoplankton characteristics such as biomass, chlorophyll a concentration, and primary production were followed. The GI (Vmax) correlated positively with release of low molecular weight products of primary production, b-Gluc activity was more closely correlated with polymeric substrates released after breakdown of cyanobacterial bloom. LAP specific activity (i.e. activity per cell) increased towards the end of the experimental period and correlated more closely with the presence of specific populations than to the total number of bacteria. By the end of the experiment time, LI switched to a lower affinity system with higher specific Vmax and KM. Bacteria preferentially used available carbon (as dissolved glucose) over carbon sources derived from the exoenzymatic hydrolysis of polymers. However, both sources of leucine (dissolved and hydrolysis products) were used equally by bacteria. Phytoplankton (dominated by cyanophytes) was the main origin of readily available dissolved low molecular weight compounds. After a cyanobacterial bloom some populations of bacteria grew on the dead cell material of algae, but the total number of heterotrophic bacteria decreased from approximately 7 × 10 to 1:9 × 10 ml: q 2000 Elsevier Science Ltd. All rights reserved.