Effects of Mixed Allelochemicals on the Growth of Microcystis aeruginosa, Microcystin Production, Extracellular Polymeric Substances, and Water Quality

Meta-analysis to identify inhibition mechanisms for the effects of submerged plants on algae

Submerged plants inhibit algae through shading effects, nutrient competition, allelopathy, and combinations of these mechanisms. However, it is unclear which mechanism is dominant, and how the inhibition intensity results from the traits of the plant and algae. In this study, we performed meta-analysis to quantitatively identify the dominant mechanisms, evaluate the relationship between inhibition intensity and the species and functional traits of the submerged plants or algae, and reveal the influences of external environmental factors. We found that allelopathy caused stronger inhibition than the shading effect and nutrient competition and dominated the combined mechanisms. Although the leaf shapes of the submerged plants influenced light availability, this did not change the degree of algae suppression. Algal species, properties (toxic or nontoxic) and external environmental factors (e.g., lab/mesocosm experiments, co-/filtrate/extract culture, presence or absence of interspecific competition) potentially influenced inhibition strength. Cyanobacteria and Bacillariophyta were more strongly inhibited than Chlorophyta, and toxic Cyanobacteria more than non-toxic Cyanobacteria. Algae inhibition by submerged plants was species-dependent. Ceratophyllum, Vallisneria, and Potamogeton strongly inhibited Microcystis, and can potentially prevent or mitigate harmful algal blooms of this species. However, the most common submerged plant species inhibited mixed algae communities to some extent. The results from lab experiments and mesocosm experiments both confirmed the inhibition of algae by submerged plants, but more evidence from mesocosm experiments is needed to elucidate the inhibition mechanism in complex ecosystems. Submerged plants in co-cultures inhibited algae more strongly than in extract and filtrate cultures. Complex interspecific competition may strengthen or weaken algae inhibition, but the response of this inhibition to complex biological mechanisms needs to be further explored. Our meta-analysis provides insights into which mechanisms contributed most to the inhibition effect and a scientific basis for selecting suitable submerged plant species and controlling external conditions to prevent algal blooms in future ecological restoration of lakes.

Allelopathic inhibition of the extracts of Landoltia punctata on Microcystis aeruginosa

To study the allelopathic effect of the extracts of Landoltia punctata, the changes of cell density of Microcystis aeruginosa were measured. The anti-algae allelopathic effect of different organic solvent extracts of L. punctata was evaluated, and the physiological, biochemical indexes were determined to discuss the mechanism of algal inhibition. The results showed that the petroleum ether, dichloromethane and ethyl acetate extracts showed various inhibitory effects on M. aeruginosa. Among them, ethyl acetate extract was the most strongly allelopathic part with the semi-effect concentration(EC₅₀) of 59.6 mg L^-1, the central polarity part of inhibitory activity. The contents of chlorophyll a(Chl a) and phycobiliproteins(PBPs) of M. aeruginosa were decreased under the concentration of 200 mg L^-1 ethyl acetate extract, which indicated that the photosynthesis of M. aeruginosa was inhibited. The consent of microcystins was lower compared to control under 200 mg L^-1. The contents of superoxide dismutase(SOD), malondialdehyde(MDA) and hydrogen peroxide(H₂O₂) of cell pellets were firstly increased and then decreased, which suggested that the algal cells were seriously damaged by oxidation. The results indicated that the extracts of L. punctata had inhibitory effect on M. aeruginosa, and the ethyl acetate extract was the central part of the inhibitory substances, which affected photosynthesis and caused peroxidation damage to inhibit cell proliferation. These findings will be helpful for exploration and application of allelopathic effects of L. punctata in harmful algae control.

Effects of suspended solids on cyanobacterial bloom formation under different wind fields

Wind waves and suspended solids (SS) generated by the resuspension of sediments are ubiquitous characteristics of lake ecosystems. However, their effects on phytoplankton remain poorly elucidated in shallow eutrophic lakes. Laboratory experiments were carried out to investigate the responses of Microcystis aeruginosa to SS under static (wind speed of 0 m/s) and breeze (wind speed of 3 m/s) conditions. Results showed that 50 mg/L SS can promote the growth of M. aeruginosa, accelerate the formation of colonies, and increase the floating rate under no-wind conditions. Comparing with static environment, breeze can significantly increase the growth rate of M. aeruginosa and benefit the formation of larger colonies of algae cells. Driven by wind and SS, the buoyancy of the cyanobacteria community in different experimental groups was obviously different. The specific performance was that low SS concentration and breeze were in favor of the floating of cyanobacteria, while high SS concentration went against the floating of algal cells. As a conclusion, wind speed of 3 m/s and 20-50 mg/L SS have a synergistic effect on the formation of cyanobacterial blooms. This study can provide an improved current understanding of bloom formation and turbidity management strategies in shallow eutrophic lakes.