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The utilization of allelochemicals to inhibit algal overgrowth is a promising approach for controlling harmful algal blooms (HABs). Quercetin has been found to have an allelopathic effect on algae. However, its responsive mechanism needs to be better understood. In the present study, the inhibitory effects of different quercetin concentrations on M. aeruginosa were evaluated, and the inhibition mechanisms were explored. The results demonstrated that quercetin significantly inhibited M. aeruginosa growth, and the inhibitory effect was concentration-dependent. The inhibition rate of 40 mg L−1 quercetin on algal density reached 90.79% after 96 h treatment. The concentration of chlorophyll-a (chl-a) in treatment groups with quercetin concentrations of 10, 20, and 40 mg L−1 decreased by 59.74%, 74.77%, and 80.66% at 96 h, respectively. Furthermore, quercetin affects photosynthesis and damages the cell membrane, respiratory system, and enzyme system. All photosynthetic fluorescence parameters, including the maximum photochemical quantum yield (Fv/Fm), the actual photochemical quantum yield (YII), the maximum relative electron transfer rate (rETRmax), and light use efficiency (α), exhibited a downtrend after exposure. After treatment with 20 mg L−1 quercetin, the nucleic acid and protein content in the algal solution increased, and the respiration rate of algae decreased significantly. Additionally, superoxide dismutase (SOD) activities significantly increased as a response to oxidative stress. In comparison, the activities of ribulose 1,5-biphosphate carboxylase (Rubisco) and phosphoenolpyruvate carboxylase (PEPC) decreased significantly. These results revealed that quercetin could inhibit M. aeruginosa by affecting its photosynthesis, respiration, cell membrane, and enzymic system. These results are promising for controlling M. aeruginosa effectively.

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How to control the frequent occurrence of cyanobacteria, especially the outbreak of toxin-producing Microcystis aeruginosa, has been a subject of constant research. This investigation focused on the effect of Moina mongolica on restricting M. aeruginosa blooms under different variables (temperature, light intensity, and salinity) and its growth at the molecular level. The results of batch experiments showed that the range of M. mongolica feeding rates was from 4.02 ± 0.81 × 103~182.23 ± 5.37 × 103 cells/ind·h in the whole experiment, where the highest feeding rates of larva M. mongolica and adult M. mongolica were 133.21 ± 5.24 × 103 vs. 182.23 ± 5.37 × 103 cells/ind·h at 30 °C, 85.88 ± 0.44 × 103 vs. 143.15 ± 14.07 × 103 cells/ind·h at 3000 lx and 88.18 ± 0.32 × 103 vs. 84.49 ± 4.95 × 103 cells/ind·h at 0‰ salinity, respectively. The results of transcriptomics further demonstrated that the response of M. mongolica to M. aeruginosa toxicity was caused by the downregulation of relevant functional genes (cell components, cell processes, metabolic processes, and protein complexes) and related signaling pathways (apoptosis, phagosome, lysosome, ribosome, oxidative phosphorylation, amino and nucleoside sugar metabolism, and PPAR signaling pathways). The findings show that M. mongolica can be released to low-salinity lakes and coastal areas (the subtropic and temperate zones) to prevent and inhibit M. aeruginosa blooms in the early summer phase. Additionally, the results achieved by the investigation will provide the relevant technology for inhibiting cyanobacteria blooms because M. mongolica even resists the produced toxin by M. aeruginosa.

The growing world population has produced an increasing demand for seafood, and the aquaculture industry is under corresponding pressure to fill this demand. The offshore ecology and environment are under significant threat with the continuous expansion of the scale and intensity of aquaculture. Integrated multi-tropic aquaculture (IMTA) is a healthy and sustainable mariculture model based on ecosystem-level management, and has become popular in recent years. It is an effective way to cope with the significant changes in offshore ecosystems under multiple stressors. Phytoplankton bacteria are essential to maintaining the marine ecosystem’s balance and stability. Investigating the changes in the community structure of marine planktonic bacteria can elucidate the impact of mariculture on the marine ecological environment. This study took the fish-shell IMTA system with natural macroalgae nearby as the object, and monitored the plankton community’s structure in the system’s surface seawater for four quarters from July 2020 to April 2021. The space–time distribution characteristics and influencing factors of the plankton community in the surface water were examined. The results showed no significant difference between the planktonic bacterial communities at different sampling sites. There was also no significant difference in the α-diversity index. However, the dominant species and abundance of planktonic bacteria at the sampling sites differed significantly. Proteobacteria and Bacteroides were the dominant groups of planktonic bacteria. The results of the distance-based redundancy analysis demonstrated that chemical oxygen demand, chlorophyll a, and dissolved oxygen constituted the primary environmental factors affecting the planktonic bacterial community structures. The heatmap also showed that NH4+- N, temperature, and salinity levels were also related to certain planktonic bacteria. This study preliminarily identified the distribution of the surface bacterial plankton community and its response to changes in environmental factors in the sea area near Xiasanhengshan Island. The results provide a preliminary basis for assessing the health and stability of the IMTA system in open sea areas.