Study On Removal Of Antibiotic Resistance Genes In Dairy Farm Wastewater By Pistia Stratiotes

With the large-scale development of China’s dairy industry,the manure generated in the breeding process has brought great pressure to the environment.In particular,there are problems such as large emissions,complex components and difficult treatment and utilization of dairy wastewater.Moreover,the antibiotic resistance genes（ARGs）contained in the wastewater will cause potential harm to the environment.Although anaerobic and aerobic wastewater treatment processes were set up in dairy farms,the cost and energy consumption of these processes were very high,and the traditional treatment methods that rely on microbial proliferation may lead to ARGs enrichment.Therefore,low-cost and high-efficiency plant treatment technology has attracted widespread attention.The floating plant“Pistia stratiotes”has strong absorption and reproduction speed,so it was widely used to purify a variety of polluted water.However,there was a lack of research on the tolerance and purification effect of Pistia stratiotes to high-concentration livestock and poultry wastewater at present.Therefore,this paper takes Pistia stratiotes as the research object,and studies the tolerance and purification effect of Pistia stratiotes to nitrogen,phosphorus and organic matter in wastewater through simulation experiments.The growth and decline of different types of ARGs in the treatment of cattle wastewater by Pistia stratiotes were clarified,and the driving effects of different factors on the change of ARGs were analyzed.The main conclusions were as follows:（1）With the increase of nitrogen,phosphorus and organic matter stress concentration,the biomass of Pistia stratiotes increased first and then decreased.The activities of three antioxidant enzymes and chlorophyll in Pistia stratiotes increased first and then decreased under stress.The tolerance concentrations of nitrogen,phosphorus and COD_cr were 50 mg/L,35 mg/L and 2500 mg/L,respectively.The acute lethal concentrations of nitrogen and phosphorus（death within 3 days）were 200 mg/L.Pistia stratiotes as a whole showed good purification effect for conventional pollutants in three kinds of actual wastewater in dairy farms.The growth efficiency of raw sewage was the highest,and the removal rates of total nitrogen,ammonia nitrogen,total phosphorus and COD_cr were 84%～92%,90%～96%,30%～93%and67%～77%,respectively,while the purification effects of anaerobic and oxidation pond effluents decreased slightly.（2）Sixteen antibiotic resistance genes（ARGs）and three mobile genetic elements（MGEs）selected in this study were common in three kinds of dairy wastewater,and the relative abundance ratio was MGEs（70.98%）>str-ARGs（12.96%）>sul-ARGs（8.99%）>tet-ARGs（4.11%）>bla-ARGs（2.30%）.In the process of Pistia stratiotes treating different cattle farm wastewater,the absolute abundance of most ARGs showed positive removal effect.Pistia stratiotes showed positive removal effects on 16 ARGs in raw water and oxidation pond wastewater,and the absolute abundance（lg value）decreased by 0.25（erm A and fex A）～3.66(bla _TEM-1)and 0.61（ISCR1）～3.51（str B）,respectively.In the treatment of low concentration anaerobic wastewater by Pistia stratiotes,16 kinds of ARGs and MGEs all showed positive removal effect,and the absolute abundance（lg value）decreased by 0.09（fex A）～4.07（str B）.In the treatment of high concentration anaerobic wastewater by Pistia stratiotes,only 9/16 of ARGs showed positive removal.（3）The high-throughput sequencing technology was used to analyze the variation of microbial communities in the wastewater before and after Pistia stratiotes treatment.The results showed that the number of OTU units and the number of specific OTU units in the three kinds of wastewater after 20 d treatment were significantly higher than those in the non-plant treatment group.Alpha diversity showed that the diversity and abundance of microorganisms in the wastewater treated by Pistia stratiotes were increased,and Beta diversity showed that the microbial communities in the wastewater treated by Pistia stratiotes were significantly different from those in the non-plant treatment group.At the phylum level,Proteobacteria（32.01%～73.78%）,Firmicutes（9.63%～33.48%）,Bacteroidetes（5.93%～20.03%）and Actinobacteriota（0.72%～5.80%）were the main microorganisms.At the genus level,uncultured,Acinetobacter,unidentified and Rombsia were the main microorganisms.The relative abundance of Proteobacteria and Firmicutes decreased,while the relative abundance of Bacteroidetes and Actinobacteriota increased after Pistia stratiotes treatment,and the relative abundance of Cyanobacteria increased in non-plant treatment（up to 50.52%）,At the genus level,the relative abundance of uncultured and unidentified unknown bacteria increased by 11.65%and 17.85%,respectively,while the relative abundance of Acinetobacter and Rombsia decreased.It is worth noting that the Flavobacterium genus increased by 12.9%-13.85%after Pistia stratiotes treatment of low concentration raw water and oxidation pond wastewater.（4）The main driving factor analysis results of ARGs change showed that the explanatory rates of dominant phyla and bacterial-binding environmental factors(nitrogen and phosphorus content,COD_cr,p H and EC)for ARGs change were 77.67%and 78.57%,respectively.The network analysis showed that the change of relative abundance of ARGs and the explanatory rates of TP in environmental factors and Proteus in microbial community for ARGs change were the highest.Therefore,it can be concluded that the removal of ARGs by Pistia stratiotes was jointly driven by microbial change and environmental factor change.