Oxygen modulates arsenic bioaccessibility by shaping microbial community structure and functional gene expression...

Soil arsenic speciation is influenced by redox arsenic microbes, which are affected by environmental factors, particularly oxygen. However, the regulation of microbial communities and arsenic transformation by oxygen remains unclear. In contrast to prior research that often focuses on single aspects such as gene expression or microbial composition, our work integrates community structure, functional gene abundance, and arsenic speciation shifts under controlled oxygen conditions, revealing system-level mechanistic linkages. Results showed that the increase in bioaccessible arsenic content in non-sterilized soil was 1.80%-8.04% under aerobic conditions and 2.87%-10.89% under anaerobic conditions. Oxygen availability significantly influenced the speciation of arsenic in the water-soluble fraction (F_Ws), with As(V) (93%) predominating under aerobic conditions and As(III) (94.78%) under anaerobic conditions. These differences were attributed to distinct microbial community structures shaped by the two oxygen conditions. Aerobic conditions supported a more diverse (Shannon index 7.8) microbial community with dominant genera Lysobacter (16.28%) and Sphingomonas (16.76%). In contrast, under anaerobic conditions, the community was less diverse (Shannon index 7.0) and predominated by Clostridiales (25.89%) and Sporolactobacillus (22.62%). This microbial community variation corresponded with differences in arsenic redox gene abundance. Under aerobic conditions, the abundance of the arsenite oxidase gene (aioA) was upregulated 32.40 times more than arsenate reductase (arsC), while arsC was 111.57 times higher than aioA under anaerobic conditions. These findings underscore the regulatory role of oxygen in microbially mediated arsenic mobilization and transformation in soil, highlighting that variations in microbial community composition and functional gene abundance under contrasting oxygen conditions specifically drive arsenic biotransformation and dynamics, deepen our understanding of arsenic biogeochemical cycling under oxygen conditions, and provide important theoretical support for the risk management and control of arsenic-contaminated soil.