The effect of nitrogen on the reduction of selenite to elemental selenium by Pseudomonas stutzeri NT-I

Abstract

Selenium is a ubiquitously occurring natural mineral, however, an upsurge in anthropogenic sources has caused its release into the environment to aggravate thereby exceeding the proposed World Health Organisation (WHO) limit of 40 μg.L-1. The soluble selenium oxyanions, selenate (SeO4 2-), and selenite (SeO3 2-), cause acute and chronic toxicity to living organisms in aquatic environments. In addition, they readily bio-accumulates, even at low concentrations. Therefore, the attenuation of effluent concentrations to acceptable levels prior to discharge into local water bodies is necessary. Selenium bioremediation is emerging as one of the most cost-effective treatment options. In this article, the selenium reducing bacterium Pseudomonas stutzeri NT-I was studied for selenite reduction. Aerobic batch reduction experiments were carried out with the bacteria isolate suspended in glucose supplemented mineral salt media (glucose-MSM). The experiments were performed under previously determined optimum conditions of pH 7, temperature of 37 °C, salinity of 5 g.L-1 and glucose of 10 g.L-1. To determine the effect of nitrogen addition on the reduction of selenite to elemental selenium (Se(0)), the reduction experiments were performed in the presence and absence of 1.604 g.L-1 ammonium chloride (nitrogen source). In the reduction of 0.5 mM SeO3 2-, average biomass based selenite reduction rates of 0.0014 mmol.(g.h)-1 in the presence of nitrogen as compared to 0.0012 mmol.(g.h)-1 in the absence of nitrogen were measured. For a higher concentration such as 10 mM SeO3 2-, average biomass based selenite reduction rates of 0.05 mmol.(g.h)-1 in the presence of nitrogen as compared to 0.031 mmol.(g.h)-1 in the absence of nitrogen were measured. This indicated that the increased initial reduction rate was a result of both increased biomass amount as well as increased biomass activity in response to increased selenite concentration.