Characteristics And Biotransformation Of Selenium Nanoparticles In Rahnella Aquatilis HX2

Selenium is an essential trace element that plays important role in people health.The selenium-deficient areas where human health risk has become one of the environmental ecological problems account for 72%of the total area in China.In nature,bacteria were found to be able to transform inorganic selenium into selenium nanoparticles(SeNPs)with more bioactivity and lower toxicity compared with inorganic selenium.Previous study suggested that the strain Rahnella aquatilis HX2 could convert selenate and selenite into SeNPs and promote the absorption and transformation of selenium in maize.In this study,the metabolisms of different selenium species were analysed in strain HX2.The key factors on the SeNPs biotransformation were investigated,and the physical and chemical characteristics of SeNPs were clarified simultaneously.This study will provide a new evidence for the application of R.aquatilis in the production of selenium nanoparticles as a nutritional supplement and in management of environmental Se pollution.Selenium metabolism in HX2 and its effects on cellular morphology were investigated.The results showed that strain HX2 can tolerate up to 3.5 mM selenomethione(Se-Met),80 mM selenite[Se(IV)],and 580 mM selenate[Se(VI)].The selenite and selenate were transformed into SeNPs with average dimensions of 242 and 218 nm,respectively.The zeta potentials of SeNPs produced by HX2 in the treatment of Se(IV)and Se(VI)were-31.8 and-29.6 mV,respectively.Cellular membrane was damaged,flagella were inhibited,and cell sizes were increased in the treatments of selenium.Results of real-time quantitative PCR(RQ-PCR)analysis suggested that CysA and CysP involved in the transport regulation of selenate and selenite,GrxA and YnWE related to the selenium reduction,and the morphological changes of HX2 cells treated with selneium can be regulated by FliO,FlgE,FliC,RodA,and SulA.Additionally,ZntA,Ada,and GntK involved in the detoxification,self-repair,and energy metabolism in strain HX2.Fermentation process of SeNPs was optimized.SeNPs yield was increased by 72.9%.The results of the key factors on SeNPs biotransformation in HX2 indicated that nitrate reductase was involved in the biotransformation.Glutathione(GSH),L-cysteine(Cys),and NADH promoted the biosynthesis of SeNPs,and riboflavin had no effect on SeNPs biotransformation in HX2.The SeNPs biosynthesized in HX2 were isolated and purified for physical and chemical characterization.The results of dynamic light scattering(DLS),Zeta potential,and Raman scattering showed that SeNPs produced by HX2 were stable and spherical with an average of 291.5 nm.The crystalline forms of SeNPs were amorphous and imbued with monoclinic crystal structure.The results of X-ray photoelectron energy spectrum(XPS)indicated that the biological macromolecules were binding to surface of SeNPs,and SeNPs contained Se0 and Se2-species on surface of SeNPs produced by HX2.