Kinetics of microbial selenite reduction by novel bacteria isolated from activated sludge.

A total of three bacteria isolated from activated sludge of a wastewater treatment plant were found to reduce selenite to elemental selenium nanoparticles as both amorphous nanospheres and monoclinic nanocrystals. The three isolated strains, which are potential candidates for bioremediation of selenite-contaminated water sources, were designated as Citrobacter sp. NVK-2, Providencia sp. NVK-2A, and Citrobacter sp. NVK-6 based on 16S rRNA sequencing. Despite belonging to the same genus, the kinetics of selenite reduction by strain NVK-2 (Vmax  = 58.82 μM h-1 , Km  = 3737.12 μM) completely differed from that of strain NVK-6 (Vmax  = 19.23 μM h-1 , Km  = 1300.17 μM). The selenite reduction rate by strain NVK-2A (Vmax  = 9.26 μM h-1 , Km  = 3044.73 μM) was the slowest among the investigated microorganisms. The microbial selenite reduction rates according to various organic sources indicated that simple organic sources such as acetate and lactate were better than more complex organic sources such as propionate, butyrate, and glucose for selenite removal. Interestingly, the selenite reduction rate was significantly enhanced when the organic source was strategically divided into small portions and consecutively supplied to the culture.