Inhibition of <i>Microcystis aeruginosa</i> by submerged macrophytes and epiphytic biofilms.

To investigate the potential role of epiphytic biofilms in the inhibition of submerged macrophytes on <i>Microcystis aeruginosa</i>, we compared the effect of crude extracts from <i>Myriophyllum</i> <i>spicatum</i> on the growth and photosynthesis pigments of <i>M. aeruginosa</i> under conditions of retaining and removing biofilms. The results showed that there was a significant enhancement in inhibitory effects of plant extracts against <i>M. aeruginosa</i> when retaining epiphytic biofilms. By the end of the experiment, the inhibition rate reached up to 85.6% for the extract equivalent to a plant fresh weight of 10.0 g·L^-1 within groups containing biofilms, whereas the value was 6.6% within group lacking the biofilms. Non-targeted metabolomics analysis revealed significant difference in the metabolite composition between extract with and without biofilms. The group containing biofilms exhibited the presence of multiple metabolites known for their algicidal activity. Moreover, the epiphytic biofilms of <i>M. spicatum</i> hosted a diverse array of bacteria and eukaryotes that had been recorded with algicidal effects, as evidenced by high-throughput sequencing of 16S rRNA and 18S rRNA. These microorganims primarily belonged to the phyla of Proteobacteria, Actinobacteriota, Firmicutes, Bacteroidota, Chytridiomycota, and Ciliophora. In conclusion, epiphytic biofilm harbors diverse algicidal microorganisms and cyanobacterial inhibitory metabolites, which could be invovled in the host plant's inhibition of <i>M. aeruginosa</i>.