Response Characteristics And Mechanisms Of Microalgal-bacterial Symbiosis System To Triclosan

Triclosan（TCS）,a typical toxic organic micropollutant,is frequently detected in wastewater.The Microalgal-bacterial symbiosis（MABS）system,consisting of microalgae and bacteria,has received much attention due to its advantages of high impact resistance,good pollutant treatment effect and low carbon and energy saving.However,there were few studies on the response characteristics and mechanisms of MABS systems to TCS.Therefore,this study systematically explored the response characteristics and mechanisms of MABS systems to TCS.The effects of TCS on the pollutant removal performance of MABS systems at different growth periods,the growth and activity of microorganisms,and the characters of extracellular and intracellular situations of microbes in MABS flocs were investigated,which provide some theoretical guidance and technical support for ensuring the safety of the water environment.The pollutant removal effect and growth characteristics of MABS system to TCS was firstly explored.The results showed that TCS could reduce the nitrogen and phosphorus removal efficiency of the MABS system.The inhibition rate of NH₄⁺-N and TN removal in the MABS system was up to 26.45%and 16.87%in the log phase.The NH₄⁺-N removal inhibition rate of the group with TCS after EPS exfoliating was more significant（5.77%increase）than that in the group with TCS only;while high TCS loading（500μg/L）resulted in a rebound of TN concentration in the MABS system during the stationary phase（16.77%increase）.TP removal in the MABS system during the log phase was inhibited by up to 34.66%,while the TP removal inhibition rate of the group with TCS after EPS exfoliating was 16.19%higher than that in the group with TCS only;TCS caused an increase in TP concentration in the effluent of the MABS system during the stationary phase（1.67-6.33 times）.In addition,TCS inhibited the biomass accumulation and reduced the microbial activity in the MABS system,while accelerating the decay of microalgae.The growth of microalgae in the logarithmic MABS system was inhibited by TCS,resulting in a maximum reduction of 286%in chlorophyll a growth rate.After exfoliating the EPS treatment,TCS inhibited the growth of microalgae and all microorganisms more strongly and accelerated the decay of microalgae in the MABS system during the stationary phase.TCS also inhibited microbial activity（microalgal photosynthetic activity and nitrifying bacterial activity）.Secondly,the effect of TCS on the properties of MABS flocs was discussed.TCS enhanced the secretion of EPS by MABS microorganisms and the decreased of floc surface charge,causing floc aggregation and increasing the number of functional groups in EPS.In the logarithmic MABS system,the secretion of LB-EPS and TB-EPS increased with TCS loading（0-100μg/L）（55.14-83.72 mg/g VSS and 136.33-192.54 mg/g VSS,respectively）,while the further increase of TCS load weakened the secretion of them（55.51 mg/g VSS and 136.09 mg/g VSS,respectively）,and the protein content increased with the increase of TCS load.The EPS secretion ability of the group with TCS after EPS exfoliating was stronger than that of the TCS only（1.11 times）,and the EPS secretion of the stationary MABS system was similar to that of the logarithmic MABS system.Due to the excessive secretion of EPS and the decrease of cell surface charge,the aggregation of MABS flocs was intensified.In addition,TCS can induce the increase of EPS functional groups.Finally,the response mechanism of MABS system to TCS was discussed.TCS altered the structure of microbial populations and induced oxidative stress in MABS systems,induced cell membrane permeability and enhanced nitrogen,phosphorus and TCS cellular uptake,which were further enhanced by adding TCS after EPS exfoliating.Among the bacterial populations,Rhodococcus increased in abundance in the experimental group with TCS,while SM1A02 and AAP99,which are associated with pollutants removal,decreased in abundance.Among the microalgal populations,Auxenochlorella and Parachlorella,which belong to Trebouxiophyceae,played an important role in nitrogen and phosphorus removal in the system and have high TCS tolerance.The increase of their abundance may be related to the removal of TCS in the system.SOD activity showed an increasing trend of low TCS load and decreasing trend of high TCS load,while TCS induced membrane lipid peroxidation,causing increased membrane permeability and increased MDA concentration,which was further aggravated by adding TCS after EPS exfoliating.In addition,the intracellular uptake of nitrogen by microorganisms increased from 46.64 mg TN/g VSS to 78.48 mg TN/g VSS,while the intracellular uptake in the group with TCS after EPS exfoliating was as high as 96.07 mg TN/g VSS;the intracellular uptake of phosphorus by microorganisms increased from 7.61mg TP/g VSS to 9.52 mg TP/g VSS,similar to that of nitrogen,while the intracellular uptake by the the group with TCS after EPS exfoliating reached 10.02 mg TP/g VSS;the cellular uptake of TCS at high TCS loading（500μg/L）increased to 41.04 times that of low TCS loading（20μg/L）,indicating that TCS enhanced the bioaccumulation effect of MABS flocs by inducing enhanced permeability of microbial cell membranes.In summary,TCS inhibited the removal of nitrogen and phosphorus by MABS system and biological growth,intensified EPS secretion and the degree of microbial aggregation,while changing the structure of microbial populations,causing cellular oxidative stress,resulting in enhanced membrane permeability and improving the bioaccumulation effect.