Calcium promotes formation of large colonies of the cyanobacterium Microcystis by enhancing cell-adhesion.

Large Microcystis colonies can lead to the rapid formation of surface accumulations, which are a globally significant environmental issue. Laboratory studies have shown that Ca 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 Ca is limited. In this study, a 72-day cultivation experiment was conducted to determine the long-term effects of Ca on Microcystis colony formation. Laboratory results indicate that Ca 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 Ca 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 Ca concentrations >40 mg L for a long time, which creates colonies on day 72 with a morphology similar to that of M. ichthyoblabe in Lake Taihu. If the Ca 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 Ca 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.