Photosynthetic production and exoenzymatic degradation of organic matter in the euphotic zone of a eutrophic lake

Photosynthetic production of organic matter, and its exoenzymatic decomposition were studied in the euphotic zone of a naturally eutrophic lake during early spring phytoplankton bloom, and after its breakdown. Phytoplankton were the major biomass producers when algae were actively growing, and the algal fraction (>3.0 μm) contributed on average 75–80% to the total biomass of microplankton. When the phytoplankton bloom began to decline bacterial biomass increased rapidly and, at the end of the bloom, bacteria contributed 48.7–69.98% to the total biomass of microplankton. The high bacterial abundance during phytoplankton bloom breakdown followed the highest rates of glucose uptake, and the highest rates of alkaline phosphatase, leucine-amino-peptidase, β-galactosidase and β-glucosidase activities. The majority of enzyme activity was associated with the bacterial size fraction of seston. The activities of free (dissolved in water) exoenzymes were negligible. The synthesis of bacterial exoenzymes was under control of an induction/derepression mechanism, and depended on the amount of easily assimilable substrates, and/or the presence of polymeric organic compounds in the water, which served as substrates for exoenzymatic hydrolysis. The tight metabolic coupling between bacterial exoenzymatic hydrolysis and uptake of low molecular weight substrates, and its ecological significance is discussed.