Effects of the Anaerobic Respiration of Shewanella oneidensis MR-1 on the Stability of Extracellular U(VI) Nanofibers

Uranium (VI) is considered to be one of the most widely dispersed and problematic environmental contaminants, due in large part to its high solubility and great mobility in natural aquatic systems. We previously reported that under anaerobic conditions, Shewanella oneidensis MR-1 grown in medium containing uranyl acetate rapidly accumulated long, extracellular, ultrafine U(VI) nanofibers composed of polycrystalline chains of discrete meta-schoepite (UO(3)·2H2O) nanocrystallites. Wild-type MR-1 finally transformed the uranium (VI) nanofibers to uranium (IV) nanoparticles via further reduction. In order to investigate the influence of the respiratory chain in the uranium transformation process, a series of mutant strains lacking a periplasmic cytochrome MtrA, outer membrane (OM) cytochrome MtrC and OmcA, a tetraheme cytochrome CymA anchored to the cytoplasmic membrane, and a trans-OM protein MtrB, were tested in this study. Although all the mutants produced U(VI) nanofibers like the wild type, the transformation rates from U(VI) nanofibers to U(IV) nanoparticles varied; in particular, the mutant with deletion in tetraheme cytochrome CymA stably maintained the uranium (VI) nanofibers, suggesting that the respiratory chain of S. oneidensis MR-1 is probably involved in the stability of extracellular U(VI) nanofibers, which might be easily treated via the physical processes of filtration or flocculation for the remediation of uranium contamination in sediments and aquifers, as well as the recovery of uranium in manufacturing processes.