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To gain insight into the combined effects of nitrogen content in media and flagellate grazing on colony formation of Microcystis aeruginosa, we added Ochromonas sp. to M. aeruginosa cultured in different nitrogen content media for 7 days. Results showed that M. aeruginosa could be efficiently ingested by Ochromonas sp., no matter what nitrogen content media M. aeruginosa was cultured in. Colony formation was observed in M. aeruginosa in all Ochromonas sp. grazing treatments during the experiment. In contrast, M. aeruginosa populations in the controls were strongly dominated by unicellular and paired cell forms, and no colonies were observed. Among all Ochromonas sp. grazing treatments, the mean numbers of cells per particle of M. aeruginosa increased with decreased nitrogen concentration (except 0% N), therefore colony formation of M. aeruginosa can be enhanced under lower nitrogen conditions. This suggests that both nitrogen content and Ochromonas sp. grazing combine to affect M. aeruginosa colony formation. Three-way ANOVA showed a statistically significant interaction between time (day 1, 3, 5, and 7), treatment (with and without Ochromonas sp. grazing) and N content (0%, 10%, 25%, and 100% N) on the mean numbers of cells per particle, i.e. the extent of colony formation. At the end of the experiment, the influence of nitrogen content (except 0% N) on the numbers of cells per particle followed a rectangular hyperbolic response. The experiments demonstrated that there exists a combined effect of nitrogen concentration and flagellate grazing on colony formation of M. aeruginosa under laboratory conditions.
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