Methanogenesis couples with arsenic methylation in urban interface biofilms under arsenic-phosphorus decoupling...

Urban interface biofilms represent understudied microenvironments where atmospheric deposition, microbial colonization, and stormwater runoff intersect, yet their role in contaminant transformation and greenhouse gas dynamics remains poorly understood.

Here, we investigated arsenic-phosphorus biogeochemistry and microbial functional gene dynamics across 18 urban interface sites.

We discovered a striking arsenic-phosphorus decoupling pattern, with industrial sites exhibiting high arsenic (92.1 mg kg^-1) but low phosphorus (322 mg kg^-1) concentrations, contrasting with control sites showing the opposite pattern. This decoupling, driven by differential rain-washing dynamics (24.5% As loss vs. 13.5% P loss), created unique selective pressure that drove co-enrichment of arsenic methylation (arsM) and high-affinity phosphate transporter (pstS) genes (r = 0.88, p < 0.001). Methylated arsenic species (MMA + DMA) comprising 18%-27% of total arsenic in stormwater runoff provided direct evidence of active arsM-mediated biotransformation. Most significantly, we found strong coupling between arsenic methylation and methanogenic potential, with arsM showing remarkable correlations with mcrA (r = 0.99) and dsrB (r = 0.98). Microcosm incubations confirmed this pattern, revealing CH₄ production rates positively correlated with arsM abundance while CO₂ flux showed an inverse trend, suggesting arsenic contamination shifts microbial carbon metabolism toward anaerobic pathways. Microbial community analysis revealed selective enrichment of arsenic-tolerant genera (Acinetobacter, Pseudomonas) and reduced alpha diversity at contaminated sites.

These findings establish urban interfaces as previously unrecognized hotspots where arsenic transformation and greenhouse gas production are mechanistically coupled, with important implications for understanding urban biogeochemical cycles and their environmental impacts.