Pseudomonas sp. LY2 enhanced soil nitrate reduction to ammonium and arsenite oxidation via nitrogen-arsenic coupling

The microbial mechanism of dissimilatory nitrate (NO₃^-) reduction to ammonium (NH₄⁺) (DNRA) coupled with arsenite (As(III)) oxidation is controversial. This study elucidated the mechanisms of nitrogen-arsenic coupling in Pseudomonas sp. LY2 through incubation experiments and transcriptomic analyses, with validation via a soil microcosm incubation experiment. The results showed that the strain converted 100.0 mg L^-1 of NO₃^--N into 58.0 mg L^-1 of NH₄⁺-N while oxidizing 37.5 % of 50.0 mg L^-1 As(III), concurrently improving electron transfer efficiency by 0.4 μg O₂ min^-1 g^-1 within 72 h. Transcriptomic analysis revealed that As(III) induced 1.4-1.5-fold up-regulation of DNRA key genes (nirB and nirD), respectively, and 1.2-fold up-regulation of As(III) oxidation genes (aioA and aioB), followed by the promotion of the deamination of organic nitrogen via the up-regulation of aspA, dadA, and guaD genes, then producing NH₄⁺-N. Simultaneously, As(III) influenced electron transfer pathways by down-regulating conventional electron transport chain genes (nuo, sdh, pet, cyo, and cyd) and up-regulating antioxidant genes (sodA, ahpCF, and tpx). Adding Pseudomonas sp. LY2 to As-contaminated soil promoted the production of NH₄⁺-N (8.4 mg kg^-1) and decreased the soil available As(III) (1.4 mg kg^-1) and nitrous oxide emissions (280.6 μg kg^-1). This study identifies Pseudomonas sp. LY2 is a functional strain for nitrogen conservation and arsenic detoxification, providing a theoretical and practical basis for bioremediation of arsenic-contaminated soils.