Abstract: Microorganisms play a key role in selenium cycling. Many microorganisms could reduce Se(IV) and Se(VI) to less toxic Se(0) with the formation of Se nanoparticles (SeNPs), which are eventually exploitable in the fields of environmental remediation, biosensors, and medicine. It is of great significance to optimize the conditions of selenium enrichment for the application of selenium-enriched microorganisms. In this study, the strain WNJ40 isolated from Seladen soil was identified by sequence analysis of 16S rDNA technique, physiological and morphological characteristics. The effect of sodium selenite on the strain was observed by scan electron microscopy (SEM). Based on single-factor experiments, the response surface methodology was performed to design, optimize, and verify the selenium enrichment condition of strain WNJ40. The results showed that: (1) The strain WNJ40 was identified as Serratia marcescens. (2) Results of SEM analysis showed that spherical aggregations with diameters of 100-800 nm were produced when strain WNJ40 grew in the medium containing sodium selenite. (3) The optimal fermentation conditions were determined as: 8% of inoculation, 120 r·min^-1 of rotation speed, and 58 h of fermentation time. Under the optimization conditions, selenium enrichment by the strain was248.70 mg·L^-1, which was 22.74% higher than that before the optimization. Our results provide a theoretical and practical basis for the application of the strain.

Key words: Serratia marcescens, response surface methodology, selenium enrichment, optimization.