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Wild Microcystis have highly diverse colonial structures and sizes, including variable colony geometry, cell arrangement, and diameter. These structural and dimensional variations may play an important role in continual, frequent Microcystis blooms during summer and autumn, the cause of which still remains unclear. Here, laboratory cultures and field investigations were applied to assess mechanisms that drive variation in structure and size, as well as factors that influence diversity. The results demonstrated that colonies grew to large sizes at the expense of their structure being loose and inhomogeneous. Furthermore, colonies may spontaneously change structure to relieve the constraints of size in return. Influencing factors (nutrient limits and turbulent shear) tended to promote these variations. Our work highlights that the diversity of Microcystis colonies may be a result of structural variations as survival strategies for gaining a higher upper size limit. Therefore, during seasonal successions, large colonies commonly have porous or loosely arranged structures, such as in M. aeruginosa. Additionally, this study hypothesized three possible transition routes for better understanding structural diversity and variations in Microcystis. [ABSTRACT FROM AUTHOR]

• Ca2+ causes rapid Microcystis aggregation to form a colony through cell adhesion. • Decreasing Ca2+ switches colony formation method from cell adhesion to division. • Laboratory formed colonies were morphologically similar to wild M. ichthyoblabe colonies. Large Microcystis colonies can lead to the rapid formation of surface accumulations, which are a globally significant environmental issue. Laboratory studies have shown that Ca2+ can quickly promote non-classical Microcystis colony formation via cell-adhesion, but our knowledge of the changes in the morphology of these colonies during subsequent long-term culture with Ca2+ is limited. In this study, a 72-day cultivation experiment was conducted to determine the long-term effects of Ca2+ on Microcystis colony formation. Laboratory results indicate that Ca2+ causes Microcystis to rapidly aggregate and form a colony through cell adhesion, then colony formation by cell-adhesion lost dominance, owing to the decrease in Ca2+ concentrations caused by precipitation/complexation. Although the initial colony morphology by cell adhesion is sparse, the newly divided cells, without separating from the mother cells, constantly fill the gaps in the original colony at Ca2+ concentrations >40 mg L−1 for a long time, which creates colonies on day 72 with a morphology similar to that of M. ichthyoblabe in Lake Taihu. If the Ca2+ levels in Lake Taihu continue to increase, Microcystis growth rate will decrease only slightly, while the colony proportion of total biovolume and biomass will increase. Moreover, higher Ca2+ concentrations do not affect microcystin content, but promote the content of bound extracellular polysaccharides (bEPS), enabling formation of larger colonies, which may promote Microcystis surface accumulation. [ABSTRACT FROM AUTHOR]

• Light attenuation in most colonies followed Lambert-Beer law. • The maximum thickness and cell concentration are in nearly inverse proportion. • M. aeruginosa and M. wesenbergii have advantages for light usage. • Cells in reticular or loose colony may receive more light. Colony formation provides excellent advantages for the dominance of Microcystis. However, studies on microenvironments during the process of colony formation are rare, especially regarding intra–colony light usage. This study analyzed the attenuation of light intensity in Microcystis colonies, where most objects followed Lambert-Beer law (I = I 0 10 - 1.256 × 10 - 3 bc ). Intra–colony light limited the maximum thickness of the colony (B Max = 4.3 × 10 5 c − 1 ) and thus affected colony size. Field data showed that the colony size for M. ichthyoblabe was small and limited to approximately 300 μm, while larger colonies were mainly formed by M. aeruginosa and M. wesenbergii respectively. These results imply that the strategies used by morphospecies to allow colonies to tolerate intra–colony light limitation might be different; M. aeruginosa benefited from a reticular growth pattern, and M. wesenbergii colonies were large (500 μm), obtaining a large thickness by lowering cell concentration. The results obtained in this work suggest that M. aeruginosa and M. wesenbergii had more advantages regarding intra–colony light usage, colony size level and bloom formation ability in summer and autumn. [ABSTRACT FROM AUTHOR]