Characterization and significance of β‐glucosidase activity in lake water

Kinetics of β ‐glucosidase activity ( β GlcA) and bacterial activity (glucose uptake, thymidine incorporation) and cell numbers were measured in the euphotic zone and in the water column of Pluβsee during spring phytoplankton bloom development and after its breakdown. Heterotrophic bacteria were the major producers of the enzyme. Activity of free β ‐glucosidase, unassociated with microbial cells, was negligible. β GlcA displayed a distinct temporal and spatial distribution pattern in lake water. β GlcA was low when algal populations grew actively, but during the algal bloom breakdown β GlcA increased rapidly. The increase in β GlcA was proportional to the abundance of bacteria and to their heterotrophic uptake of glucose, as well as to bacterial production, measured by the thymidine incorporation method. In contrast with its response to pH, β ‐glucosidase exhibited no obvious adaptation to ambient temperature of lake water. β GlcA produced by aquatic bacteria was under control of a repression‐induction mechanism, and synthesis was derepressed when the level of directly assimilable hexoses (glucose or galactose) fell below a critical level. The tight relationship between the rates of β GlcA and glucose uptake indicated the existence of a specific, coupled hydrolysis‐uptake system in lacustrine bacteria.