From colonial clusters to colonial sheaths: Imaging flow cytometry analysis of Microcystis morphospecies dynamics in mesocosm and links to CyanoHABs management

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. The dominance of Microcystis is tightly related to colonial formation by Microcystis. However, studies of the colonial development of Microcystis morphospecies are rare. This research applied FlowCAM-based imaging flow cytometry to analyze the development of Microcystis morphospecies in the mesocosms mimicking eutrophic shallow lakes and the effect of temperature changes. A significant positive association was found between M. ichtyoblabe, M. aeruginosa, and M. smithii colonies, particularly in the high-temperature tanks, suggesting that these morphospecies belong to one ecocluster, which supports the hypothesis of the central transition pathways of colonial Microcystis. The small colonial clusters of Microcystis cells represented an important stage in the sequence of Microcystis bloom and were associated with the development of colonial forms. A correlation analysis showed that the higher pH was positively correlated with the abundance of M.wesenbergii independently of temperature changes. The colonial sheaths' abundances increased following a maximum of M.wesenbergii abundance, reaching significant numbers (thousands), and a majority of sheaths contained at least some Microcystis cells. We hypothesize that colonial sheaths may be crucial at Microcystis spp. dispersal and represent an obligatory stage of colonies development. The sheaths may protect Microcystis cells against environmental stress factors, improve cell survival at low nutrient levels, and participate in Microcystis dispersal and spreading. Our findings can be applicable to early CyanoHAB detection and management of Microcystis dispersal.