Key physiological traits and chemical properties of extracellular polymeric substances determining colony formation in a cyanobacterium

Colony formation of cyanobacteria is crucial for the formation of surface blooms in lakes. However, the underlying mechanisms of colony formation involving in physiological and cell surface characteristics remain to not well be established. Six cyanobacterial Microcystis strains (including both unicellular and colonial ones) were employed to estimate the influences of their physiological traits and the composition of extracellular polymeric substances (EPS) on colony or aggregate formation. Results show that raising the number of the photosynthetic reaction center and light-harvesting antenna in the PSII and reducing the growth rate were the major physiological strategies of Microcystis to produce excess EPS enhancing colony formation. Tightly bound EPS (T-EPS) was responsible for colony formation, which approximately accounted for 50% of the total amount of EPS. Five fluorescent components (protein-, tryptophan-, and tyrosine-like components and two humic-like components) were found in the T-EPS, although the amounts of these components varied with strains. Importantly, colonial strains contained much higher tyrosine-like substances than unicellular ones. We suggest that tyrosine-like substances might serve as a crosslinking agent to connect other polymers in EPS (e.g., proteins or polysaccharides) for colony formation. Our findings identified key physiological traits and chemical components of EPS for colony formation in Microcystis , which can contribute to a better understanding on the formation of Microcystis blooms.