The allelopathy of submerged macrophytes on cyanobacteria is one of the key mechanisms for the successful restoration of submerged vegetation in eutrophic water bodies. However, the response and underlying mechanism of epiphytic microbial communities to the allelopathic cyanobacteria-inhibition of host-submerged macrophytes remain unclear. Therefore, three submerged macrophytes commonly used in ecological restoration, were selected to co-culture with Microcystis to explore the dynamic response and potential mechanism of epiphytic microbial communities during allelopathic Microcystis-inhibition of host plants under different interaction stages (before, during and after Microcystis-inhibition) and different Microcystis densities (high and low). The main results are as follows:
The diversity of epiphytic bacteria and eukaryotic algae increased, while the diversity of other eukaryotes decreased. During the recovery period, the diversity of eukaryotic algae and other eukaryotes returned to the level of the control group, while bacterial diversity decreased yet remained higher than that of the control group. The complexity of the epiphytic microbial network in the treatment groups increased, and the assembly process of each microbial community was mainly regulated by deterministic factors, with the bacterial community being the main responsive group. Further through the screening of algicidal bacteria and metabolomic analysis, it was found that the relative abundance of algicidal bacteria and the expression levels of metabolites were enhanced in the inhibition stage but decreased in the recovery period.
The results of co-culture experiments involving three submerged macrophytes and Microcystis at different densities showed that the diversity and community structure of the epiphytic microbiota on the surface of the three submerged macrophytes changed. The complexity of epiphytic microbial networks associated with E. nuttallii and V. natans increased, and the role of deterministic selection processes in community assembly was enhanced during interactions with Microcystis. By screening algicidal bacteria and conducting metabolomics analysis, we found that the relative abundance of epiphytic algicidal bacteria associated with H. verticillata and E. nuttallii was significantly higher in the high-density treatment group, compared to the low-density treatment group. The resluts indicated that epiphytic microbial community of submerged macrophytes, especially the epiphytic bacteria, may be involved in the allelopathic cyanobacteria-inhibition process of the host plants. Future studies should integrate multi-omics approaches and synthetic flora functional verification experiments to clarify whether submerged macrophytes recruit beneficial bacteria, such as algicidal bacteria, in the planktonic environment via metabolic regulation.