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The diversity and succession pattern of cyanobacteria, particularly picocyanobacteria and bloom-forming cyanobacteria, were examined monthly in a eutrophic lake (lake Chaohu) in China using a combination of light microscopy and 16S ribosomal RNA (rRNA) sequence analysis. The results showed that both picocyanobacteria and bloom-forming cyanobacteria have high levels of diversity. Microcystis and Anabaena were the two predominant bloom-forming genera, and two obvious shifts occurred between them from spring to winter. Anabaena was dominant in spring, then it was rapidly replaced by Microcystis in summer and became dominant again in late autumn and early winter. Apart from water temperature, three forms of dissolved nitrogen (NO3-N, NO2-N, and NH4-N) were important driving factors for their seasonal succession, as demonstrated by redundancy analysis. Clone libraries and sequence analysis revealed that picocyanobacteria (mainly Synechococcus-like) are also important cyanobacteria members in lake Chaohu. All 8 picocyanobacterial phylotypes belonged to the Cyanobium clade. The phylotypes could be further grouped into at least 7 distinct clusters, and 4 of these clusters do not belong to any of the previously described clusters. Picocyanobacteria accounted for more than 70% (percentage in the clone library) in March and April but only accounted for less than 10% from June to October during the Microcystis bloom. The relative abundance of picocyanobacteria was positively correlated with the mass ratio of dissolved inorganic nitrogen and phosphorus (r=0.965, P<0.01, n=10) and ammonium concentration (r=0.721, P<0.05, n=10).
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