Extracellular synthesis of selenium nanospheres by Bacillus mycoides strain SeITE01

A bacterial strain (SeITE01), isolated from the rhizosphere of the selenium hyperaccumulator legume Astragalus bisulcatus [1] and identified as Bacillus mycoides, was studied for its ability to efficiently reduce selenite to elemental selenium in aerobic conditions and consequently to produce elemental selenium nanospheres. The isolate exhibited significant tolerance to selenite (SeO32-) up to 30mM in oxyanion concentration. SeITE01 was incubated with both 0.5 and 2mM Na2SeO3, performing the complete reduction of selenite respectively within 12 and 24 hours. The strain converted 91% of the initial selenite added to the culture medium into elemental selenium with cultures developing a deep red color characteristic of crystalline Se0. Characterization of red Se0 precipitate by using transmission electron microscopy (TEM), scanning electron microscopy (SEM) and UV-Vis spectroscopy revealed the presence of extracellular spherical nanoparticles. The sizes of these nanoparticles range from 200 to 250 nm in bacterial cultures after 24 hours of exposure to selenite. Moreover, after 48 hours, the nanoparticles reach a diameter of 1μm. EDX analysis of the same particles revealed the characteristic peaks of selenium absorption at 1.37 keV (SeLα), 11.22 keV (SeKα) and 12.49 keV (SeKβ), respectively. Selenite reduction activity was observed mainly in membrane proteins and even in the exoenzymatic fraction. Therefore, a hypothetical mechanism for the synthesis of selenium nanoparticles has been proposed.