The released polysaccharide inhibits cell aggregation and biofilm formation in the cyanobacterium Synechocystis sp. PCC 6803

Cyanobacteria secrete exopolysaccharide (EPS), which can be intimately associated with the cell surfaces (known as capsular polysaccharide, CPS), or released into the surrounding environment (released polysaccharide, RPS). The aim of this study was to explore the role of EPS in cell aggregation and biofilm formation in the unicellular cyanobacteriumSynechocystissp. strain PCC 6803. Three mutants (Delta sll5043, Delta slr1063and Delta slr1076) were obtained through disrupting three putative glycosyltransferase genes (sll5043,slr1063andslr1076) separately. Decreased contents and altered monosaccharide composition of both RPS and CPS were observed in the three mutants compared with the wild type (WT). RPS from the WT (WT-RPS) had high emulsification activity, while RPSs from the mutants had no emulsification activity. Unlike the WT, the three mutants formed cell aggregates and biofilms. Compared with the WT, the three mutants possessed lower cell hydrophobicity and less negative cell surface charge, which could result from the altered production of CPS. 40 mu g ml(-1)WT-RPS significantly decreased cell hydrophobicity of the three mutants, and significantly inhibited cell aggregation and biofilm formation, while the RPS from each mutant itself had no effect on cell hydrophobicity of the mutant, and did not affect cell aggregation and biofilm formation. The inhibitory effect of WT-RPS on cell aggregation and biofilm formation was thus suggested to be related to its ability to reduce cell hydrophobicity. The difference in cell aggregation and biofilm formation between the WT and the three mutants could be related to differences in cell hydrophobicity, cell surface charge and physicochemical properties of RPS. Our results in this study indicated that both CPS and RPS affected cell aggregation and biofilm formation inSynechocystissp. strain PCC 6803, and WT-RPS inhibited cell aggregation and biofilm formation.