Morphological response of Microcystis aeruginosa to grazing by different sorts of zooplankton

In the experiment we investigated the effect of grazing by different sorts of zooplankton on the induction of defensive morphology in the cyanobacterium Microcystis aeruginosa. The results showed that protozoan flagellate Ochromonas sp. grazing could induce colony formation in M. aeruginosa, whereas M. aeruginosa populations in the control and the grazing treatments of copepod Eudiaptomus graciloides, cladoceran Daphnia magna, and rotifer Brachionus calyciflorus were still strongly dominated by unicells and paired cells and no colony forma occurred. In the protozoan grazing treatment, the proportion of unicells reduced from 83.2% to 15.7%, while the proportion of cells in colonial form increased from 0% to 68.7% of the population at the end of the experiment. The occurrence of a majority of colonial M. aeruginosa being in the treatment with flagellates, indicated that flagellate grazing on solitary cells could induce colony formation in M. aeruginosa. The colonies could effectively deter flagellate from further grazing and thus increase the survival of M. aeruginosa. The colony formation in M. aeruginosa may be considered as an inducible defense against flagellate grazing under the conditions that toxin cannot deter flagellate from grazing effectively.