Outbreaks of Microcystis blooms can affect growth of submerged plants, which in turn can inhibit cyanobacterial growth. Microcystin (MC)-producing and non-MC-producing Microcystis strains typically coexist in Microcystis-dominated blooms. However, the interaction between submerged plants and toxin-producing Microcystis compared with non-toxin producing strains is not clear.
Plant-Microcystis co-culture experiments showed that the MC-producing Microcystis strain had a higher resistance to negative impacts by the co-cultured submerged plant M. spicatum than the non-MC-producing strain. By contrast, the plant M. spicatum was impacted more by the MC-producing Microcystis than the non-MC-producer. The MC cell quotas in the strain were significantly higher in the coculture treatment (the PM+ treatment, p < 0.05), indicating that the production and release of MCs might be a key factor responsible for the reduced impact of M. spicatum.
When the submerged plant M. spicatum was exposed to cell extracts from microcystin (MC)- and non-MC-producing Microcystis, both Microcystis cell extracts exerted obvious damage to aquatic plant biomass, photosynthesis, primary and secondary metabolism measures, and resistance of plant antioxidant systems. However, MC-producing Microcystis had stronger effects presumably due to the presence of MCs. The epiphytic and planktonic bacteria diversity decreased when exposed to both Microcystis cell extracts. However, epiphytic and planktonic bacterial communities responded differently to Microcystis cell extracts at the genus level. We speculate that planktonic bacterial communities have the potential to utilise and degrade substances derived from Microcystis cell extracts, which may be beneficial for M. spicatum to alleviate damage from Microcystis.
Overall, this study indicated that the capacity to produce MCs, as well as the density of Microcystis should be taken into account when attempting to re-establish submerged vegetation to undertake remediation works. Further research is needed to verify the structure and function dynamics of epiphytic and planktonic bacteria in the interaction between cyanobacteria and submerged macrophytes.