Low atmospheric pressure of plateau environments shapes microbial communities, nitrogen conversion, and carbon metabolism in biological nitrogen removal systems.

Wastewater treatment plants in high-altitude regions often exhibit unstable nitrogen removal under low atmospheric pressure, but the coupled impacts on oxygen transfer, microbial metabolism, and community adaptation remain poorly resolved. In this study, long-term bioreactor operation under different atmospheric pressures was performed to elucidate how low pressure reshapes biological nitrogen removal systems through changes in oxygen transfer, microbial metabolism, and community structure. Low pressure reduced oxygen solubility and gas-liquid/liquid-solid transfer, which suppressed nitrification and caused nitrite accumulation, while simultaneous nitrification-denitrification partly sustained total nitrogen removal. Multi-scale analyses integrating batch tests, enzyme activities, and metagenomics showed a consistent shift from oxidative to more electron-efficient pathways, with strengthened denitrification and expanded carbon metabolism that enhanced the use of carboxylic acids and amino acids and secured carbon and electron supply. The microbial community reorganized toward denitrifying polyphosphate-accumulating organisms (DPAOs), denitrifying glycogen-accumulating organisms (DGAOs), and conventional denitrifiers, with stronger functional associations despite a simpler network structure. These findings explain performance deterioration under plateau atmospheric conditions and indicate feasible control points to sustain nitrogen removal in high-altitude wastewater treatment systems.