The interactions between bacteria and algae may play a significant part in the formation and development of algal blooms. The bloom-forming cyanobacterium Microcystis occurs mainly as colonial form in natural waters, and thus it is necessary to study the interaction between bacteria and colonial Microcystis. This paper aimed to investigate effects of the cultivable bacteria attached to Microcystis colonies on the colony size and growth of colonial Microcystis aeruginosa. Eleven bacterial strains were isolated from M. aeruginosa colonies collected from Lake Taihu. Among these bacteria, seven bacterial isolates significantly influenced the colony size of M. aeruginosa, and four bacterial isolates significantly influenced the growth rate of M. aeruginosa. Four isolates, related to the Exiguobacterium, Delftia, Bacillus and Stenotrophomonas, significantly decreased the colony size of M. aeruginosa by 36-51%, and two isolates, assigned to the Chryseobacterium and Pseudomonas chengduensis, significantly increased the M. aeruginosa colony size by 89% and 63%, respectively, while these isolates had no effect on the cyanobacterial growth rate. Two isolates, belonged to the Rheinheimera and Pseudomonas, significantly decreased the growth rate of M. aeruginosa by 47% and 36%, respectively, and one bacterial strain related to Aeromonas increased the cyanobacterial growth rate by 22%, while these isolates had no effect on the cyanobacterial colony size. One isolate belonged to Sphingomonas was found to significantly increase the colony size of M. aeruginosa by 80% and significantly decrease the growth rate of M. aeruginosa by 21%. PCR-denaturing gradient gel electrophoresis (PCR-DGGE) analysis showed that all of the bacterial isolates were able to colonize M. aeruginosa colonies. Our data suggested that the bacteria attached to Microcystis colonies might influence the cyanobacterial colony size and growth, and thus influence the formation and development of Microcystis blooms. ARTICLE HISTORY

The bloom-forming cyanobacterium Microcystis occurs mainly as colonial aggregates under the natural conditions. This paper investigated the hydrophobicity and iron coagulation of extracellular polymeric substances (EPSs) from colonial Microcystis in order to understand the impact of EPS on the water treatment process. The higher contents of dissolved EPS (dEPS) and bound EPS (bEPS, mucilaginous matrix around the cells), lower dEPS/bEPS ratio and greater negative zeta potential of bEPS and dEPS were found in colonial Microcystis compared with unicellular Microcystis. XAD resin fractionation analysis indicated that the hydrophobicity could be ranked in an order as follows: bEPS > dEPS > dissolved extracellular organic matter (dEOM) for all the Microcystis strains. Correlation analysis showed that there was a statistically significant correlation between the amounts of carbohydrate and dissolved organic carbon in the hydrophobic fraction of EOM (dEOM, dEPS and bEPS), indicating that the hydrophobicity of Microcystis EOM might be related to carbohydrate. The coagulation experiment showed that for each colonial Microcystis strain, the removal efficiency of bEPS was higher than that of dEPS within the pH range from 3 to 10. The implications of the EPS characteristics were further discussed with respect to water treatment.