Cr(VI) stress-driven enrichment of rhizosphere microbial functions and metabolic regulation synergistically reduce...

Chromium (Cr) speciation in soil significantly impacts its bioavailability. Rhizosphere microorganisms, as key drivers of Cr(VI) valence transformation, undergo functional gene enrichment and metabolic adaptation under environmental stimuli. This study investigated the microbial detoxification and reduction of Cr(VI) in rhizosphere soil through a pot experiment with gradient Cr(VI) exposures. Integrated analyses of 16S rRNA sequencing, qPCR, and non-targeted metabolomics revealed that Cr(VI) stress reduced bacterial species diversity but drove the enrichment of tolerant bacterial communities. The dominant bacterial genus Sphingomonas and several rare genera were significantly enriched, while key Cr(VI) reductase genes (chrR and nfsA) were upregulated. This microbial restructuring was accompanied by the upregulation of specific sugars and amino acids, activating metabolic pathways such as ABC transporters and amino acid biosynthesis to support Cr(VI) detoxification and reduction. The Cr(III) produced by reduction is fixed in the precipitated state of Cr-iron hydroxide (increasing by 6.6-15.4 mg·kg^-1 dry soil) and the organically bound state (increasing by 1.4-12.2 mg·kg^-1 dry soil), which reduces Cr bioavailability. Our results provided a theoretical basis for the natural remediation of soil Cr(VI) pollution.