Influences of metal ions on microcystin-LR degradation capacity and dynamics in microbial distribution of biofilm collected from water treatment plant nearby Kasumigaura Lake

•MC-LR degradation by biofilm was inhibited by Mn2+ and Cu2+, but not affected by Zn2+.•MC-LR can be degraded only after a lag phases of 2 days and 8 days for Zn2+ and Cu2+ cases.•The abundance of MC-LR degradation bacteria over biofilm was reduced by Mn2+.•The abundance of MC-LR degradation bacteria over biofilm was not affected by Zn2+.•The abundance of MC-LR degradation bacteria over biofilm was enhanced by Cu2+.

Microcystins-LR (MC-LR) which is a kind of potent hepatotoxin for humans and wildlife can be biodegraded by microbial community. In this study, the capacity of biofilm in degrading MC-LR was investigated with and without additional metal ions (Mn2+, Zn2+ and Cu2+) at the concentration of 1 mg L−1. The results indicated that the degradation rate of MC-LR by biofilm was inhibited by introduced Mn2+ and Cu2+ during the whole culture period. MC-LR cannot be degraded until a period of culture time passed both in the cases with Zn2+ and Cu2+ (2 and 8 days for Zn2+ and Cu2+, respectively). The results of mlrA gene analysis showed that the abundance of MC-LR degradation bacteria (MCLDB) in the microbial community under Mn2+ condition was generally lower than that under no additional metal ion condition. Meanwhile, a two days lag phase for the proliferation of MCLDB occurred after introducing Zn2+. And a dynamic change of MCLDB from Cu2+ inhibited species to Cu2+ promoted species was observed under Cu2+ condition. The maximum ratio of MCLDB to overall bacteria under various conditions during culture process was found to follow the tendency as: Cu2+ > Zn2+ ≈ no additional metal ion (Control) > Mn2+, suggesting the adverse effect of Mn2+, no obvious effect of Zn2+ and positive effect of Cu2+ on the distribution ratio of MCLDB over the biofilm.