Application of the frequency of dividing cells technique to estimate the in situ growth rate of Microcystis (Cyanobacteria)

1. Diel patterns of the frequency of dividing cells (FDC) of the bloom‐forming cyanobacteria Microcystis were investigated using both culture strains and natural populations. 2. In laboratory experiments, diel division cycles were examined twice in a 24‐h light/dark cycle during time‐course batch incubations of six culture conditions using two strains (morphospecies) of Microcystis (M. aeruginosa and M. wesenbergii). While both strains clearly showed phased cell division in the light period during the logarithmic growth phase, the peaks of FDC became unclear towards the stationary phases. Some dividing cells were always found in the dark period regardless of whether or not division had paused at the same time. 3. This result implied the inadequacy of applying the model of McDuff & Chisholm Limnology and Oceanography (1982) vol. 27 , pp. 783–787] directly to calculate the duration of cell division. Modified equations are proposed to calculate the duration of cytokinesis as a terminal event, in which the FDC values at night are regarded as 0% and all FDC values are subtracted by the minimum FDC value. 4. The diel FDC in natural populations of M. aeruginosa and M. wesenbergii were examined at five sites from a harbour to several distances offshore in Lake Biwa. While both species showed phased cell division patterns in the daytime at the harbour, no peaks in FDC were discernible in the samples taken from the offshore sites. These results strongly suggested that Microcystis growth was higher inshore than offshore. The in situ growth rates were estimated using the new equations.