LAUSR

Microalgae, including cyanobacteria, may excrete cell-bound exopolysaccharides (bEPS). Up to now the linkage between microalgal bEPS and flocculation is not fully understood. The aim of this paper was to explore the role of bEPS in flocculation of the cyanobacterium Synechocystis sp. strain PCC6803 by comparing the flocculation behaviors of the wild type (WT) and a bEPS-deficient mutant Δsll5043. In the ferric chloride and chitosan flocculation experiments, the bEPS deficiency exerted similar impacts on flocculation: a decreased optimum flocculant dosage, a decreased flocculation efficiency at the optimum flocculant dosage, and a decreased flocculation efficiency at the high flocculant dosages (0.50–0.65 mM for ferric chloride and 1.25–2.00 mg L−1 for chitosan). The proportion of small flocs in total flocs for the Δsll5043 was higher than that for the WT for both flocculation experiments with 0.55 mM ferric chloride and 1.25 mg L−1 chitosan. Our results indicated that the altered bEPS production of Synechocystis sp. strain PCC6803 had a strong impact on cyanobacterial flocculation with ferric chloride and chitosan. The affected flocculation behaviors included not only the optimum flocculant dosage but also the flocculation efficiency and the floc size. The deficiency of bEPS resulted in a less negative cell surface charge and lower cell hydrophobicity in Synechocystis 6803, both of which have been suggested to affect cell flocculation.