Two types of bound extracellular polysaccharides and their roles in shaping the size and tightness of Microcystis colonies

Extracellular polysaccharide (EPS) can be distinguished into soluble or bound types and significantly contributes to colony formation in Microcystis. Depending on the binding strength with cells, the bilayer structure of bound EPS contains loosely or tightly bound EPS (LB-EPS or TB-EPS) and their roles in shaping the size and tightness of Microcystis colonies deserve further investigation. In this study, the influences of two types of bound EPS on the size and tightness of Microcystis colonies were investigated after a series of pretreatment to obtain LB-EPS retaining or stripped samples. Results showed that cells with LB-EPS formed large and loose colonies. Furthermore, the ratios of LB-EPS to TB-EPS, which indicate the size and tightness of the colonies, were higher in the retaining groups than in the stripped groups. Our findings also provide evidence that calcium enrichment is conducive to colony formation in Microcystis. This study provides new insights into the formation and enlargement of Microcystis colonies, which contributes to a better understanding on the role of EPS in Microcystis aggregation and morphology changes.