Remediation Of Toxic Microcystis Bloom And Its Associated Biological Pollution By Constructed Wetland Planted With Alisma Orientale,Combined With Mechanism Analysis

Objective: For the remediation of toxic Microcystis and its associated antibiotics resistance genes（ARGs）,component optimizaiton for constructed wetlands（CWs）were carried out to improve the remediation potential of target pollutants.The remediation effect and mechanism of CWs on eutrophic lake water polluted by Microcystis were explored.Methods: The substrates screening were performed from blast furnace slag,biochar,and sawdust,and then the best one was used to construct mini-CW unit,combined with herb Alisma orientale.The residual concentration of microcystin（MC）-LR,nitrogen and phosphorus in CWs effluent were detected.The relative abundances of dominant ARGs（sul1,sul2 and intl1）,Microcystis and MC-degrading gene mlr A in effluent and substrate DNA were quantitatively determined by q PCR.Through 16 S diversity sequencing and metagenome sequencing of effluent and substrate respectively,the changes of microbial diversity,structure and function,the enrichment of functional genes related to carbon,nitrogen and phosphorus cycling were analyzed,while as the dominant phylum involved these processes.Results: Approximately 98.46 % of the Microcystis was removed by the sawdust-based CW only within 2 d,whereby < 0.37 μg/L of residual microcystin（MC）-LR was detected,with removal efficiency of > 96.47%,which is potentially caused by the higher relative abundance of MC-degrading gene mlr A.Lower target ARGs accumulations in the sawdust-based CW may be attributed to the lower intl1 relative abundance and microbial functions mobile element contents which could influence horizontal gene transfer.In sequential batches for the treatment of eutrophic lake water,using sawdust and Alisma orientale to construct mini-CWs,and setting up unplanted CWs as control to investigate the role of Alisma orientale in the removal of target pollutants.The efficiencies for removal of Microcystis and MCLR by planted CWs ranged from 92.00 to 95.88% and 86.48 to 94.82%,respectively,significantly（P<0.05）higher than unplanted ones.Less accumulation of target ARGs was also found in planted CWs.All CWs owned good capacity to remove nitrogen and phosphorus,however,planting remarkably improved ammonia nitrogen removal.In all CWs,Bacteroides,Patescibacteria,Acinetobacter,Flavobacterium,Cloacibacterium and Acinetobacter lwoffii were increased significantly（P<0.05）,Firmicutes,Cyanobacteria,Planctomycetes,Verrucomicrobia,Exiguobacterium,Pirellula,Cyanobium＿PCC-6307 were decreased significantly（P<0.05）.The results of bacterial community function prediction and gene abundance showed that,in the effluent of planted CWs,species richness and diversity were increased significantly（P<0.05）,while the pathogenicity was reduced.Functional genes related to denitrification in planted CWs were significantly enriched,which might further promote nitrogen removal,and species annotation showed that dominant phylums involved in nitrogen metabolism mainly included Proteobacteria,Verrucomicrobia,Acinetobacter and Planctomycetes.The similar enrichments for genes involed in phosphorus and carbon cycling were found in all CWs.The main phylums involved in phosphorus metabolism were Proteobacteria,Verrucomicrobia,Chlorobi,Chloroflexi,Bacteroides,and the genes encoding glycoside hydrolases were most abundant for carbon metabolism.Conclusion: Sawdust and Alisma orientale are suitable fo miniCWs construction.The presence of Alisma orientale could obviously enhance the removal ability of Microcystis and MC-LR,and enrich mlr A and denitrification related genes,and slow down the spread of resistance,and remove target pollutants by promoting the cycling of carbon,nitrogen and phosphorus,which could provide material selection guidance and theoretical basis for the application of constructed wetlands in the remediation of eutrophic water polluted with toxic Microcystis.