Frequency of dividing cells and viability of Microcystis aeruginosa in sediment of a eutrophic reservoir

Blooms of Microcystis aeruginosa occurred in summer months for several years in the reservoir of Grangent (France). Disappearance of planktonic colonies in autumn does not necessarily mean the end of the life cycle for cyanobacteria, and part of them sink down to the sediment. Frequency of dividing cells (FDC) of benthic M aeruginosa was measured under experimental hypolimnetic conditions in winter and in spring (darkness and temperature = 4 or 8degreesC) to show overwintering survival and difference between seasons. Enzymatic activity (viability) of benthic cyanobacteria was controlled in situ in winter and spring. Our study revealed that in winter (4degreesC), benthic colonies continued to divide, with low FDC (mean % = 13.49) and low enzymatic activity (mean ratio carboxyfluorescein diacetate [CFDA] fluorescence: autofluorescence = 1.27). In contrast, when temperature increased in spring (8degreesC), the viability values significantly increased (mean ratio = 3.78), and a rhythmic increase of FDC appeared every 8 h with maximum of ca. 27.5 %. This corresponds to an ultradian rhythm of division, which provides strong evidence that benthic cells were under the control of an endogenous component synchronized by an internal clock. Our results suggest that hypolimnetic temperature plays an important role in the return of the active form of M aeruginosa in spring.