Abstract:
The soluble selenium oxyanion, selenite (Se(IV)) is toxic and it bioaccumulates at low concentrations which is
why its presence in aquatic systems has to be regulated. Pseudomonas stutzeri NT-I is one of the numerous
microorganisms capable of reducing Se(IV) into the relatively less toxic Se(0). Previous studies using strain
NT-I have mainly focused on Se(IV) reduction occurring in the presence of bacterial biomass for the entirety of
the reaction. However, it has been reported that some pseudomonas strains are capable of secreting
biomolecules or metabolites into their surrounding environments which aid Se(IV) reduction. In this study,
aerobic batch reduction of 2 mM Se(IV) was carried out in two stages, that is, firstly in the presence of
bacterial biomass for 1 h (in order to initiate the reaction) and secondly in the absence of biomass by use of
the bacterial supernatant (cell-free extract), post biomass removal. This was done to investigate the selenium
reducing capabilities of the cell-free extracts. Obtained results showed that the total Se(IV) reduction within the
first hour prior to biomass removal was rapid, translating to an average reduction rate of 0.45 mmol.h-1 and a
22 % reduction in the Se(IV) concentration. However, Se(IV) reduction still occurred post biomass removal by
use of the cell-free extract, albeit at a slower rate. The average reduction rate was 0.01 mmol.h-1 and the
Se(IV) removal was 58 %. From these results, it was concluded that the bacterial biomass likely secreted
metabolites which remained in the medium after the removal of biomass. It is these secreted metabolites
which are thought to possess selenite reducing capabilities.
