Ecological damage of submerged macrophyte Myriophyllum spicatum by cell extracts from microcystin (MC)- and non-MC-producing cyanobacteria, Microcystis

To explore how decomposed Microcystis-dominant cyanobacterial blooms affect submerged macrophytes, the submerged plant Myriophyllum spicatum was exposed to cell extracts from microcystin (MC)- and non-MC-producing Microcystis strains in a laboratory experiment. Results showed that both Microcystis cell extracts exerted obvious damages to plant biomass, photosynthesis, primary and secondary metabolism measures, and resistance of plant antioxidant systems, with MC-producing Microcystis having stronger effects due to the presence of MCs. Cyanotoxins other than MCs responsible for the negative effects from both Microcystis strains needs further identification. The Shannon diversity and Chao1 indices of epiphytic and planktonic bacteria were decreased by the cell extracts from both Microcystis strains. However, epiphytic and planktonic bacterial communities responded differently to Microcystis cell extracts at the genus level. The dominant genera of planktonic bacteria including Enterobacter, Pseudomonas, and Novosphingobium from phylum Proteobacteria, Chryseobacterium from phylum Bacteroidetes, and Microbacterium from Actinobacteriota in the treatments with cell extracts were previously reported to have strains with algicidal and MC-degrading capabilities. Bacterial genes associated with energy production and conversion, amino acid transport and metabolism, and inorganic ion transport and metabolism, were more abundant in both treatments than the control for planktonic bacteria, but less abundant for epiphytic bacteria. We speculate that planktonic bacterial communities have the potential to use and degrade substances derived from Microcystis cell extracts, which may be beneficial for M. spicatum to alleviate damages from Microcystis. Further research is needed to verify the structure and function dynamics of epiphytic and planktonic bacteria in the interaction between cyanobacteria and submerged macrophytes.