Selenium solubilization by Bacillus sp. S01: Mechanistic insights and environmental implications in paddy soils

Selenium (Se) is an essential micronutrient for humans, and crop Se biofortification presents a global health strategy to ensure safe dietary Se intake. However, low Se bioavailability in paddy soils limits Se uptake by rice (Oryza sativa L.), hindering agronomic Se biofortification. Although microorganisms play a pivotal role in mediating Se transformation within soil biogeochemical cycles, the underlying mechanisms remain to be fully elucidated. In this study, a Se-tolerant bacterium, Bacillus sp. S01, was isolated from high-Se soil and demonstrated the ability to convert Se(0) into bioavailable Se species. Integrated metabolomic and genomic analyses putative Se(0)-solubilizing genes in strain S01, including sulfur assimilation-related genes (gene1757, gene2869, and gene1971). Heterologous expression confirmed that gene1757, gene2869, and gene1971 enhanced Se(0) dissolution in Escherichia coli. Soil microcosm and pot experiments revealed that inoculation with strain S01 increased soluble and exchangeable Se fractions while reducing residual Se content. Additionally, it significantly improved soil pH, enzyme activities (sucrase, acid phosphatase, catalase, urease), and reshaped the rhizosphere microbial community, with Bacillus, Fonticella, and Lutispora identified as key taxa driving Se activation and bioavailability. These changes collectively enhanced rice biomass, yield, and enhanced grain Se content by 91 %. In summary, strain S01 likely transform Se(0) into bioavailable forms via sulfur metabolism pathways while improving Se bioavailability through modulation of soil properties and rhizosphere microbiota. These findings advance our understanding of microbial Se cycling and highlight the potential of Se-solubilizing bacteria in sustainable Se biofortification.