Pseudomonas stutzeri NT-I : optimal conditions for growth and selenate reduction

Abstract

In this study, Pseudomonas stutzeri NT-I growth and selenate reduction were examined using aerobic batch experiments. Optimal growth conditions were determined in a mineral salt medium in the presence of background selenium. Optimal conditions for the reduction of selenate to selenite and elemental selenium was identified using harvested cells in a mineral salt medium. The reduction profiles of selenium were monitored using selenite as indicator species. A glucose and nitrogen independent maximum biomass concentration of 0.64 g/L dry cell weight was measured for all glucose concentrations above 2 g/L, signifying the presence of a population density control mechanism. Optimal growth conditions for the culture were obtained at a pH of 7, temperature of 37 °C, a salinity of 10 – 20 g/L NaCl, and a background selenium concentration of 5 mM. Optimal selenium reduction rates were observed at a temperature of 37 °C, pH 7 – 8 and salinity less than 5 g/L NaCl. The similarity of conditions for maximum growth and selenium reduction rates provide evidence that optimal operation can be achieved for both parameters simultaneously, a requirement for continuous operation. The microbe was capable of practically complete reduction of up to 4 mM selenate in less than 3 h of operation, translating to a volumetric reduction rate of between 0.2 mM/h (for 0.5 mM selenate) and 1.33 mM/h (for 4 mM selenate). The increasing mass-based reduction rates of between 0.006 mmol/g.h (for 0.5 mM selenate) and 0.1 mmol/g.h (for 4 mM selenate) indicate that the increased reduction rate was a result of both increased biomass and increased biomass activity with increased selenate concentration. Results from the study demonstrate the potential of the organism Pseudomonas stutzeri NT-I for the biological remediation of selenate and subsequent removal from the environment.