Study On The Degradation And Mechanism Of Carbendazim By Microalgae

Carbendazim is a common fungicide.Carbendazim can cause embryonic toxicity,teratogenicity,mutagenicity,and anemia.Algae,as producers in the ecosystem,can use pollutants in the environment as nutrients for their growth and development through photosynthesis.In this paper,carbendazim was selected as the research object to study the biodegradation ability of microalgae to carbendazim.Firstly,the biological toxicity of carbendazim to different microalgae and the tolerance of microalgae to carbendazim were studied to determine the dominant algae species and the optimal degradation conditions.Then the co-degradation experiment of carbendazim was studied with dominant algae species.And the effects of different concentrations of inorganic salts（Na Cl）on the degradation of carbendazim from dominant algae were studied.Finally,The metabolic pathway of carbendazim was studied.Three microalgae（Chlorella sp.,Tetradesmus obliquus,and Microcystis aeruginosa）were selected to explore their tolerance and degradation rate to carbendazim at different concentrations.In the end,Tetradesmus obliquus was selected as the dominant species because its tolerance to carbendazim was better than that of Chlorella sp.and Microcystis aeruginosa,and its degradation rate was the highest.And the optimal initial concentration of carbendazim was determined to be 0.5 mg/L.To obtain the best degradation conditions,firstly,the culture medium with different initial p H was set up.It was found that the growth of Tetradesmus obliquus not only stopped or even died,but also lost the degradation ability of carbendazim under extreme environment（strong acid and strong base）.In combination with the growth conditions and degradation rate of carbendazim in different initial p H values,it is concluded that the initial p H for carbendazim degradation is 7-8.Then,the initial algae density was changed,and the growth status and degradation rate were observed.It was found that different initial algae density did not affect the degradation of carbendazim.Next,the effects of different light times on carbendazim degradation by Tetradesmus obliquus were investigated.It was found that when the light/dark ratio was 12 h:12 h,the growth condition was the best,and the degradation of carbendazim mainly occurred at night.Finally,the toxicity of carbendazim to dominant algae species was studied and the concentration of carbendazim was increased to 50.0 mg/L.When carbendazim concentration reached 50.0 mg/L,carbendazim inhibited the growth of Tetradesmus obliquus.Meanwhile high concentration of carbendazim inhibited the production of chlorophyll a and affected photosynthesis.Through the change of carotenoid,it was found high concentration of carbendazim can cause higher oxidative damage to cells.The co-degradation of carbendazim and other carbon sources（glucose）by Tetradesmus obliquus was investigated.Different concentrations of glucose were added to the medium,and it was found that the addition of glucose greatly promoted the growth of Tetradesmus obliquus,but reduced the degradation rate of carbendazim.This indicates that when multiple organic carbon sources are present in the medium,microalgae will preferentially degrade simple organic carbon sources that are easy to decompose.Then the effects of different concentrations of inorganic salts（Na Cl）on the degradation of carbendazim by Tetradesmus obliquus were studied.With the increase of Na Cl concentration,the growth and degradation of tetralogy obliquus were inhibited.Therefore,the inhibition of growth and even death of Tetradesmus obliquus may be related to the imbalance of element content and the increase of oxidative damage caused by the accumulation of reactive oxygen species（ROS）.The metabolizing pathway of carbendazim was further studied and the degraded algal fluid was analyzed by HPLC-MS.Three intermediates were identified in the supernatant and labeled as TP 126,TP 134 and TP 192,respectively.The structures of these three intermediates were preliminarily determined by the plasm-nucleus ratio and database.We can infer the metabolic pathway of carbendazim degradation by three intermediates.Carbendazim was first converted to TP 134 by decarboxylation and demethylation,and then to TP 126 by deamination.Finally,it was changed into CO₂and H₂O through a series of cracking reactions,and TP 192 may be the product of carbendazim hydrogenation.