From colonial clusters to colonial sheaths: analysis of Microcystis morphospecies in mesocosm by imaging flow cytometry

The alarming increase in the frequency of blooms of Microcystis in freshwater lakes and reservoirs occurs worldwide, with major implications for their ecosystem functioning and water quality. We applied FlowCAM-based imaging flow cytometry together with PCR and sequencing to get a comprehensive picture of the seasonal development of Microcystis community in a long-term running lake mesocosm experiment. The IFC analysis with manual taxonomic classification confirmed early findings with a machine learning algorithm that some Microcystis morphospecies completely disappeared and re-appeared along the mesocosm experiment timeline. This observation supports the hypothesis of the main transition pathways of colonial Microcystis . For the first time, colonial mucilaginous envelopes or sheaths were reported as separate entities, and not as a part of Microcystis colonies. The colonial sheaths may contain a few single Microcystis cells and reach significant numbers (thousands) during a cyanobacterial bloom. We also found that non-identifiable colonial small clusters of Microcystis cells are an important stage in the complex mosaic of a Microcystis bloom and are associated with the development of colonial forms. Our findings were validated by the principal component analysis coupled with the constructed associative matrices. We hypothesize that colonial sheaths may be crucial at Microcystis spp. dispersal and represent one of the stages of colonies development.