Biotechnological approach to selenite detoxification through the formation of Se0 nanoparticles by means of a Bacillus mycoides strain isolated from the rhizosphere of Astragalus bisulcatus

Selenium is a trace element commonly found in the earth’s crust. It belongs to the Group 16 (chalcogens) of the Periodic Table and occurs in a variety of oxidation states in the environment. In particular, the predominant Se species in oxic conditions are the oxyanions selenite (SeO32 - ) and selenate (SeO42 - ), with the former exerting the highest toxicity. Interestingly, the ability to reduce SeO32 - into the non - toxic elemental form is widespread among microorganisms. The present work investigates on the re duction mechanisms of selenite to zero valent selenium nanoparticles by Bacillus mycoides SeITE01, a bacterial strain isolated from the rhizosphere of the Se - hyperaccumulator legume Astragalus bisulcatus. The strain SeITE01 exhibits resistance to SeO32 - up to 25 mM and is capable of complete reduction of 0.5 and 2.0 mM SeO32 - within 12 and 24 hours, respectively. SeITE01 also demonstrated to convert 91% of the selenite initially added to the growth medium into elemental selenium, with cultures developing a deep red color characteristic of crystalline monoclinic Se0. However, Se0 production was delayed respect to selenite depletion in the culture medium. Characterization of red Se0 precipitate by using transmission electron microscopy, scanning electron micro scopy and UV - Vis spectroscopy revealed the presence of extracellular spherical nanoparticles. In few cases, also intracellular nanoparticles were detected. Size of such selenium nanoparticles range from 50 to 400 nm in diameter, according to the different incubation times. SeITE01 protein fractions were assayed for selenite reduction activity, which can be associated to membrane proteins and spent culture medium after NADH addition. On the basis of the results gained so far, two different mechanisms for the synthesis of selenium nanoparticles have been proposed. The involvement of proteins/peptides able to extracellularly reduce selenite and a possible reduction of selenite by membrane reductases