Humic acid inhibits colony formation of the cyanobacterium Microcystis at high level of iron

Colony formation is a key process for the occurrence of Microcystis blooms. In order to inhibit colony formation of Microcystis at high level of iron using humic acid, unicellular Microcystis aeruginosa was cultivated in laboratory treated with varying concentrations of iron and humic acid. Our results showed that the extracellular polysaccharides (EPS) content and average colony size increased from 0.57 pg cells−1 and 4.0 μm to 0.93 pg cells−1 and 26.1 μm, respectively, while iron concentration increased from 0.68 mg L−1 to 6.8 mg L−1, suggesting that high level of iron stimulated EPS secretion and induced unicellular Microcystis to form colonies. Transcriptome analysis showed that two genes described as glycosyltransferases (BH695-2217 and BH695-3696) were significantly up-regulated while EPS content increased with increasing iron concentration indicating that iron may regulate the expression of genes involved in polysaccharide synthesis. When treated with 10 mg C L−1 humic acid at high level of iron, the EPS content and average colony size decreased by 35.5% and 56.3%, respectively, revealing that humic acid inhibited EPS secretion under high level of iron condition, and ultimately inhibited colony formation of Microcystis. Our results suggested that humic acid could be used as an agentia inhibiting large colony formation of Microcystis and thereby reducing the occurrence of Microcystis blooms. •High level of iron stimulated EPS secretion and colony formation of Microcystis.•Iron may regulate the expression of genes involved in polysaccharide synthesis.•Humic acid inhibited EPS secretion and colony formation at high level of iron.