Study On The Mechanism Of Microbial Transmembrane Transformation Of Selenium And Nitroaromatic Compounds

The species and concentrations of pollutants are growing sharply owing to the rapid development of industry.Microbes can transform toxic metal ions and organic pollutants to products with low toxicity due to their diverse metabolic activities.Therefore,improving the bioconversion efficiency of pollutants has become one of the hot topics in the field of environmental science and engineering.During the biotransformation process,membranes protecting cells from external stimuluses play significant roles in the substances exchange with the outside environment.However,the transmembrane transport and the biotransformation of pollutants at membranes are not well studied,leading to the difficulties in metabolic regulation and insufficient utilization of such a process in pollutant degradation.This dissertation aims to illuminate the transportation and transformation mechanism of inorganic ions and organic pollutants at membranes for enhancing pollutants transformation.For inorganic ions,we explored the transmembrane transport mechanism of selenite and proposed strategies to improve selenite transport and enhance the recovery of Se as valuable products.For organic pollutants,we investigated the biotransformation mechanism of 2,4-dichloronitrobenzene at membranes,and this mechanism was also applied to the reduction of other nitroaromatic compounds.Main contents and results of this dissertation are as follows:1.Explored the mechanism of inhibited selenite transformation by phosphate in E.coli.Phosphate shares similar structure with selenite,hence these two ions may compete with each other to use the same transport channel.The suppressed removal of selenite by phosphate was revealed by the selenite biotransformation in the presence of phosphate.At low concentration of phosphate,selenite was transformed to Se0 nanoparticles.Along with the increase of phosphate concentration,selenite transformation was further inhibited and the main products changed to organo-Se with high toxicity.Both the high and low affinity phosphate transporters were involved in the selenite transformation with the low one playing the dominant role by knock-out experiment.These results show that selenite transformation was inhibited by phosphate via competing transport channel,providing new insights to the biotransformation of selenite by prokaryote.2.Regulated the removal of selenite at low concentration by E.coli.The biotransformation efficiency of selenite is low when selenite is at low concentration.To address the issue,the phosphate transporter was overexpressed as it can transport selenite into cells.By conducting experiments with the strains overexpressing phosphate transporter,the uptake of selenite was significantly improved.Further,the ingested selenite was transformed to Se0 nanoparticles.The system stability was strengthened by regulating the synthesis of glutathione.These findings offer new possibilities to deal with wastewater with flue gas desulfurization effluents and recovery of Se.3.Regulated the co-transformation process of selenite and cadmium ion.The coexistence of selenium and heavy metal ions in actual wastewater is more common,and the coexistence of selenium and cadmium has been reported.By inhibiting the expression of the Cd(Ⅱ)efflux system,the intake of cadmium was significantly increased without affecting the intake of selenium,but the conversion of selenium to CdSxSe1-x quantum dots was promoted.The generated bio-quantum dots exhibited high biocompatibility,concentrated size distribution(3.82±0.53 nm)and long lifetime(45.6 ns),suggesting the potential to be utilized for bioimaging.This study not only illuminate the role of Cd(Ⅱ)export system in the biotransformation of Cd(Ⅱ)and selenite,but also afford a strategy to recovery Cd and Se as valuable products simultaneously.4.Investigating the reduction mechanism of 2,4-dinitrochlorobenzene(DNCB)by Shewanella.In response to the current unclear understanding of the terminal reductase of nitrobenzene pollutants in electrochemically active bacteria,the reduction process of DNCB by Shewanella oneidensis MR-1 is analyzed.DNCB was reduced to 4-chloro-1,3-benzenediamine with 4-chloro-3-nitroaniline and 2-chloro-5-nitro-benzamine as primary intermediates.Intracellular reductases such as old yellow enzyme,NfnB and azoreductase participate in the reduction of DNCB,but their contribution was relatively small.CymA in the inner membrane played a key role in the reduction process of DNCB,and didn’t not rely on the extracellular electron transfer pathway to achieve the reduction of DNCB.The role of CymA as terminal reductase for DNCB was further confirmed by both in vivo and in vitro experiments.Moreover,CymA could also catalyze the reduction of other aromatic pollutants with nitro groups in S.oneidensis MR-1.This work brings new perspectives to the microbial reduction of a nitroaromatic compounds and offers a feasible way to the bioremediation of environment polluted by these chemicals.