Dynamics and metabolic activity of the benthic cyanobacterium Microcystis aeruginosa in the Grangent reservoir (France)

Abstract

The dynamics of benthic colonies of Microcystis aeruginosa (cyanobacteria) play an important part in the formation of the summer bloom underlying many harmful effects. Became this benthic phase remains somewhat unknown, we developed an approach using flow cytometry to follow the esterase activity of this species in the Grangent reservoir. The esterase activity of benthic cyanobacteria extracted from two layers of sediment (0-2 cm and 2-4 cm deep) was measured weekly, as an indicator of viability, by flow cytometry with carbonyfluorescein diacetate. In parallel, the concentration of benthic cyanobacteria was estimated under epifluorescence microscopy and the water temperature and dissolved oxygen concentration were measured in the hypolimnion. Esterase activity increased gradually as spring temperatures increased. This metabolic reactivation allowed one part of the benthic cyanobacteria to regain the water column, entailing a simultaneous decrease of their number in the upper sediment (0-2 cm). Then, after a maximum in summer, esterase activity decreased while the number of colonies increased. This corresponded with the domination of the senescent planktonic form following the decline of the bloom.