Extracellular reduction of selenite by a novel marine photosynthetic bacterium

A novel purple nonsulfur bacterium strain NKPB030619, which has resistance to over 5 mM selenite, was isolated from a marine environment. An initial concentration of 1.1 mM selenite, added to the medium, was decreased to under 0.05 mM within 5 days. The color of the cell suspension turned red within 2 days. The red coloration gradually decreased and black precipitates appeared during 2 weeks of cultivation. Under these conditions, two main types of deposit were formed extracellularly. These deposits were thought to contain red amorphous selenium and black vitreous selenium. The selenite reduction to elemental selenium in this bacterium was induced by the introduction of light and L-malic acid under anaerobic conditions. These results suggest that selenite reduction is coupled with photosynthesis and L-malic acid can serve as the indirect electron donor for its reduction. Phylogenetic analysis based on the 16S rDNA sequence showed that NKPB0360619 belongs to the alpha subdivision of Proteobacteria and is classified into the Rhodobacter species. The highest similarity of 86.2% was observed with R. sphaeroides.