Microcystis bloom, a cyanobacterial mass occurrence often found in eutrophicated water bodies, is one of the most serious threats to freshwater ecosystems worldwide. In nature, Microcystis forms aggregates or colonies that contain heterotrophic bacteria. The Microcystis-bacteria colonies were persistent even when they were maintained in lab culture for a long period. The relationship between Microcystis and the associated bacteria was investigated by a metagenomic approach in this study. We developed a visualization-guided method of binning for genome assembly after total colony DNA sequencing. We found that the method was effective in grouping sequences and it did not require reference genome sequence. Individual genomes of the colony bacteria were obtained and they provided valuable insights into microbial community structures. Analysis of metabolic pathways based on these genomes revealed that while all heterotrophic bacteria were dependent upon Microcystis for carbon and energy, Vitamin B12 biosynthesis, which is required for growth by Microcystis, was accomplished in a cooperative fashion among the bacteria. Our analysis also suggests that individual bacteria in the colony community contributed a complete pathway for degradation of benzoate, which is inhibitory to the cyanobacterial growth, and its ecological implication for Microcystis bloom is discussed.

a b s t r a c t a r t i c l e i n f oAlthough the synthetic organic polymers have been widely applied as flocculant aids to improve solid-liquid separation efficiency, it is not very clear how the charge type and molecular weight of the polymers influence the characteristics of flocs. In this paper, floc characteristics, such as floc size, density, structure, strength and reversibility were investigated in details in terms of their response to the polyacrylamide (PAM) with different charge types and molecular weights. Laser light scattering, image analysis and settling technology were employed simultaneously. The results showed that floc size induced by anionic PAM with medium molecular weight (A130) and ultra high molecular weight (A115) was 419 µm and 438 µm respectively, correspondingly, floc size induced by cationic PAM with medium molecular weight (C448) and very high molecular weight (C498) was 731 µm and 796 µm. Meanwhile, floc fractal dimension was inversely proportional to polymer molecular weight and the values were between 1.91 and 2.40. In addition, floc density was affected by molecular weight of anionic PAM more dramatically than that of cationic PAM. For a given floc size, the density increased with molecular weight. Furthermore, flocs formed by cationic PAM showed much greater shear-resistant ability and reversibility than anionic PAM. Nevertheless, the poor reversibility of the flocs formed by high molecular weight was also observed. An explanation for the resultant floc characteristics was offered in terms of flocculation mechanisms.

a b s t r a c tMicrofiltration performance in coagulation-microfiltration hybrid systems (without settling process) has strong correlation with the floc structure. This study investigated the effect of Al speciation distribution (Al a , Al b and Al c ) of PACls (polyaluminum chlorides) on floc structure and the overall membrane performance. The floc properties, analyzed with small angle laser light scattering (SALLS), of different aluminum based coagulants, AlCl 3 (PACla, with high Al a species), polyaluminum chloride (PAClb, with high Al b species), commercial polyaluminum chloride product (PAClp, with similar amount of each Al species) were compared using synthetic waters with added humic acid and kaolin particles. The coagulated waters were then fed through a dead-end microfiltration system using an unstirred batch cell with 0.22 m PVDF flat sheet membrane. The results show that the property of flocs, largely depended on aluminum species, is the most influencing factor for membrane filtration behaviors. The floc sizes from different coagulants are in the following order PACla > PAClp > PAClb, which is consistent with the amount of Al a species in the PACls. The fractal dimension values are very similar for each PACls. The floc strength follows the order of PACla > PAClb > PAClp. Cake made of PACla-flocs shows a noticeable flux advantage followed by PAClp-flocs and PAClb-flocs, which agrees well with the order of floc size. However, the flux decline of PAClp-flocs is much worse than PACla-flocs although both of them have similar final steadystate floc size. The difference between the flux of PAClb-flocs and PAClp-flocs is not as significant as the difference between their floc sizes. This can be explained that the PAClp-flocs are much more fragile than PAClb-floc due to its weak floc strength. These results also suggest that floc strength is another very important factor influencing the filtration performance.