Efficient Microbial Production of Selenium Nanoparticles by Selenium-Tolerant Alcaligenes sp. HZ-9-3-3.

Selenium (Se) is a crucial trace element for human and animal health, contributing significantly in various physiological processes. However, its uneven global distribution leads to widespread deficiencies. Nano-selenium (selenium nanoparticles, SeNPs), with low toxicity and high bioavailability, emerges as a promising alternative, and microbial synthesis is the preferred method for its production. In this study, we collected samples from the selenium-rich region of Enshi, Hubei Province, China, and used sodium selenite (Na_₂SeO_₃) gradient screening to isolate microorganisms capable of tolerating high selenium concentrations and efficiently producing SeNPs. We identified nine selenium-tolerant strains that withstand over 0.115 M of sodium selenite. Among these, strain HZ-9-3-3, identified as Alcaligenes sp., tolerated up to 0.231 M and effectively converted selenite to elemental selenium. Optimization using single-factor experiments and central composite design (CCD) established optimal conditions: inoculum size of 12.7% (±0.1%), pH 7, temperature 28 °C, and sodium selenite concentration of 0.0289 M, achieving a conversion rate of 69.7% (±0.1%). The produced SeNPs exhibited moderate antioxidant activity, with IC₅₀ values of 1.5 mg/mL for DPPH radicals and 1.7 mg/mL for hydroxyl radicals, and a reducing power equivalent to 0.842 mg/mL of vitamin C. These findings highlight the potential of strain HZ-9-3-3 for efficient bioconversion of selenium and production of SeNPs with significant antioxidant properties.