The nZVI loaded biochar and mineral enhanced immobilization of Cd in sediments.• Bacterial composition, richness and diversity were changed after immobilization. • DOM presented autogenetic characteristics with bacterial community changes. • BC (nZVI/BC) possessed higher immobilization performance than minerals.

Microcystis, a genus of cyanobacteria that is dominant in eutrophic lakes, occurs mainly as colonial morphs under natural conditions but as single cells in laboratory cultures. Recent studies have suggested that Microcystis-bacteria interactions significantly influence Microcystis morphology, but the underlying mechanism remains unclear. In this study, a total of 48 strains of heterotrophic bacteria were purified from Microcystis mucilage. Five bacteria, Aeromonas veronii, Enterobacter aerogenes, Exiguobacterium acetylicum, Bacillus cereus and Shewanella putrefaciens, can induce unicellular Microcystis to form colonies. Heterotrophic bacteria stimulated Microcystis growth and induced the production of extracellular polymeric substances in coculture treatments. Extracellular polymeric substances, such as extracellular polysaccharides (EPS), were responsible for the mucilage formation in colonial Microcystis. We analysed extracellular metabolic compounds produced by Microcystis aeruginosa and Microcystis wesenbergii using gas chromatography mass spectrometry. Filtrate extracts from coculture treatments indicated that some compounds, such as 2-dodecen-1-yl(-) s u c c i n i c a n h y d r i d e a n d b e n z o i c a c i d , 2 , 3 -bis[(trimethylsilyl)oxy]-, trimethylsilyl ester, might play a significant role in colonial M. aeruginosa or M. wesenbergii formation. Our data suggested that the interaction of Microcystis and heterotrophic bacteria was crucial for the formation of Microcystis colony and outbreak of Microcystis blooms.

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