Algal Inhibition Efficiency And Ecology Effects Of Allelochemicals Sustained-Release Microspheres(ACs-SMs) And Water Purification Efficiency Of Submerged Forest System Regulated By ACs-SMs

Submerged forest system（SFS）is considered as an effective means to treat eutrophic waters.However,the competition between algae and grass may cause the failure of SFS construction.In this study,two kinds of phenolic acids allelochemicals（ACs）were selected and used for the preparation of ACs sustained-release microspheres（ACs-SMs）using slow-release microsphere technique.The factors influencing the preparation of ACs-SMs were investigated.Eco-friendly ACs-SMs with good embedding rate and sustained-release efficiency were prepared.The factors influencing the preparation effect of carbon modified ACs-SMs（CM@ACs-SMs）were further investigated.The sustained-release performance and mechanism of CM@ACs-SMs were investigated.The effects of CM@ACs-SMs on aquatic plants,animals and microbial communities in aquatic ecosystems were investigated to ensure that ACs-SMs can be used safely in aquatic ecosystems.The regulation mechanism of CM@ACs-SMs strengthening the construction of SFS was explored.Water purification efficiency,and nitrogen migration and transformation law of SFS regulated by ACs-SMs were clarified.The optimal preparation conditions of ACs-SMs were determined by single factor experiment and orthogonal experiment as follows:the concentrations of sodium alginate,allelochemicals,chitosan,calcium chloride,and vanillin were 2.5%（W/V）,800 mg/L,0.5%（W/V）,6%,and 1%（W/V）,respectively.The optimal embedding environment is as follows:p H=4.0,titration port size=1 mm,titration height=5 cm,cross-linking time=60 min.In this study,carbon materials（carbon nanotubes,biochar and activated carbon）were innovatively used to modify ACs-SMs.The results show that compared with carbon nanotubes and biochar,ACs-SMs modified by activated carbon have better performance.The optimal loading time of allelochemicals on activated carbon was 30 min,and the optimal addition amount of activated carbon was 0.3 g C/L SA.The first order sustained release kinetics model,Ritger-Peppas model and Higuchi model can describe the sustained release kinetics of carbon modified ACs-SMs,indicating that the sustained release modles of ACs were drug diffusion and skeleton dissolution.The optimal dosage of ACs-SMs was 500 mg/L as the initial algal density was 10⁶ cell/m L.ACs-SMs can effectively reduce the density of algae,reduce the content of Chl-a,stimulate the membrane lipid peroxidation of algae,damage the membrane system,produce MDA,and damage the photosynthetic system.Algae cells induced SOD,POD and CAT antioxidant enzymes to weaken the effect of allelochemicals sustained release microspheres on algae.The effects of ACs-SMs on aquatic animals,aquatic plants and microbial communities in aquatic ecosystems were investigated firstly.ACs-SMs had little effect on the morphological indexes of submerged plants（plant height,number of leaves,average length of leaves,average area of leaves and average biomass）,chlorophyll indexes（Chl-a and Chl-b）,redox status indexes（MDA,SOD,POD and CAT）and chlorophyll fluorescence indexes(F_v/F_o,F_v/F_m,q P and r ETR_max,etc.),even promoted the increase of some indexes at the later stage of the experiment,suggesting that ACs-SMs did not inhibit the growth of submerged plants,resulting in oxidative stress and photosynthetic system damage.Allelochemicals sustained-release microspheres decreased the richness and diversity of adherent microorganisms and rhizosphere microorganisms,and promoted the relative abundance of dominant bacteria.It should be noted that the leaves of submerged plants can reduce the influence of ACs-SMs on attached microorganisms,and the rhizosphere of plants is conducive to the increase of the number of bacteria that can promote plant growth.ACs-SMs did not affect the motility ability,the feeding,digestion and assimilation abilities and the number of intestinal microbe of Daphnia magna.In conclusion,ACs-SMs have good ecological security for aquatic animals,plants and microorganisms.Finally,the regulation mechanism of CM@ACs-SMs to enhance the construction of SFS and cooperate with SFS to inhibit algae was explored.When initial algae density was in high concentration（10⁶ cell/m L）,M.aeruginosa inhibited the growth of submerged plants,resulting in oxidative stress,membrane damage and photosynthetic system of submerged plants.The addition of ACs-SMs inhibited the growth of algae and reduced the inhibitory effect of M.aeruginosa on submerged plants.With the decrease of the initial algal density,the competitiveness of SFS gradually increased,and the growth of M.aeruginosa was inhibited and oxidative stress occurred,the cell membrane of the algae was damaged,and the photosynthetic system of the algae was destroyed.The addition of ACs-SMs was beneficial to the removal effect of TN,NO₃^--N and TP in SFS.The removal rates of TN,NO₃^--N and TP increased with the decrease of the initial density of M.aeruginosa.N assimilation experiments showed that the ¹⁵N content in both leaves and roots of submerged plants increased significantly,indicating that nitrogen was transferred from Microcystis aeruginosa to submerged plants.With the decrease of the initial algal density,the ¹⁵N in the leaves and roots of submerged plants increased gradually.In summary,ecologically friendly ACs-SMs with efficient embedding,long-term sustained-release and effective inhibition of algae were successfully prepared.It was proven that the ACs-SMs had little impact on aquatic plants,aquatic animals and microorganisms in SFS.Under the control of ACs-SMs,SFS was successfully constructed,realizing the transformation of the aquatic ecosystem from"algal"to"grass".