Effect And Potential Mechanism Of Antibiotics On Nitrogen And Phosphorus Removal From Aquaculture Wastewater By Sesuvium Portulacastrum

Aquaculture products and aquaculture have greatly contributed to ensuring global food security and providing essential nutritional support for human beings.China is the largest aquaculture country globally,but the rapid development of the aquaculture industry has also brought a series of ecological and environmental problems,especially the discharge of aquaculture wastewater has a great impact on the ecological environment of the coastal waters.Water purification through the absorption of nitrogen and phosphorus in wastewater by plants is an important way of wastewater treatment.However,the residual pesticides,antibiotics,and other drugs in wastewater had obvious interference effects on the removal effect of plants,and the degree of influence and mechanism of action varies with different pollutants.Therefore,starting from the actual situation of aquaculture wastewater pollution,taking specific antibiotic residues as a simulation scenario.To elucidate how antibiotic residues affected plant removal of nitrogen and phosphorus from wastewater and the underlying mechanisms,which was of great scientific significance for optimizing the existing treatment methods of aquaculture wastewater and improving the process.This study selected the coastal plant-Sesuvium portulacastrum as the test plant and a typical sulfonamide antibiotic-sulfadiazine（Sulfadiazine,SDZ）in aquaculture wastewater as the target compounds.In the presence of 0mg/L,1 mg/L,5 mg/L,and 50 mg/L SDZ,assessed changes in nitrogen and phosphorus content in aquaculture wastewater,the growth indicators of S.portulacastrum,the changes of rhizosphere microbial communities,and analyzed the effect of SDZ on the root genes of S.portulacastrum,and to initially reveal its potential mechanism of action.The main results are as follows:（1）The exposure of SDZ inhibited the removal effect of S.portu-lacastrum on nitrogen and phosphorus in aquaculture wastewater.After 28 days of exposure,SDZ had the most obvious effect on total nitrogen and ammonium removal rate.Total nitrogen decreased from 87.5%to 76.4%-22.1%,ammonium decreased from 81.9%to66.3%-8.5%.Followed by nitrogen and total phosphorus,the total phosphorus decreased from 99.6%to 99.4%-85.5%,and the nitrogen decreased from 97.9%to 94.8%-82.8%.（2）SDZ exposure inhibited the growth of S.portulacastrum.In the SDZ treatment group,the fresh weight,root number,and moisture content of S.portulacastrum decreased significantly,and leaf enzyme activities（MDA,SOD,POD,CAT）increased.The root activity showed rising and then falling,and SDZ significantly reduced the root activity in the 50 mg/L SDZ treatment group.（3）SDZ exposure changed the microbial community structure of water samples and rhizosphere.With the increase of SDZ concentration,the diversity and richness of water samples and rhizosphere microorganisms increased first and then decreased.In the water samples and rhizosphere microorganisms,the relative abundance of Clostridia,Bacteroidia,Gammaproteobacteria,and Bacill was the highest,and the existence of SDZ affected the relative abundance of the flora.According to CCA analysis,it was found that the water samples and rhizosphere microorganisms were regulated by different environmental factors（TN,NH₄⁺-N,NO₃^--N,TP）.FAPROTAX functional prediction found that SDZ reduced the relative abundances of nitrate reduction,nitrogen respiration,nitrate respiration,and nitrite respiration related to nitrogen cycling functions in water samples and rhizosphere microorganisms.（4）SDZ exposure affected gene expression in S.portulacastrum roots.RNA-seq analysis revealed that the effect of high concentration SDZ（5 mg/L）exposure on the transcript level of S.portulacastrum root system was significantly higher than that of low concentration SDZ（1mg/L）.Further,GO functional annotation and KEGG pathway enrichment analysis were performed on the differentially expressed genes.GO functional annotation results showed that the differentially expressed genes possessed multiple molecular functions,included translation factor activity,RNA-binding peptidase activity,hydrolase activity,oxidoreductase activity,GTPase activity.KEGG pathway enrichment analysis showed that differentially expressed genes were significantly enriched in pathways,included carbon metabolism and amino acid biosynthesis.The changes in gene expression in these pathways might affect the growth and development of plants.To sum up,the residual SDZ inhibited the removal of nitrogen and phosphorus from S.portulacastrum.The potential mechanisms might include:1)The presence of SDZ could significantly affect the expression of root genes of S.portulacastrum,negatively influenced the physiological reactions,affected the growth and development,and ultimately reduced the absorption of nitrogen and phosphorus by plants.2)The existence of SDZ changed the microbial community structure in water samples and the rhizosphere.SDZ reduced the relative abundance of microbial communities related to nitrogen cycle function in water samples and rhizosphere microorganisms,thus affected the biological nitrogen removal.