Seasonal Variations in Microbial Community Structure and Functional Potential in a High-Altitude Membrane Bioreactor System

This study investigated seasonal variations in treatment performance, extracellular polymeric substances (EPS), microbial community structure, and functional potential in a full-scale high-altitude (3650 m) membrane bioreactor (MBR) wastewater treatment plant using shotgun metagenomic sequencing. Water temperature ranged from 9.9 °C in winter to 16.8 °C in summer, while pH remained neutral (7.1–7.6). The system maintained stable removal of suspended solids (>93%), chemical oxygen demand (COD) (effluent 13–21 mg/L), and 5-day biological oxygen demand (BOD5) (effluent < 2.8 mg/L), and ammonium nitrogen (NH4+-N) removal remained high (effluent < 0.2 mg/L) throughout the year. Notably, total nitrogen (TN) removal efficiency decreased significantly in winter, whereas total phosphorus (TP) removal was less seasonally sensitive. EPS peaked in winter (protein 49.8 mg/L, polysaccharide 9.2 mg/L), mainly driven by the protein fraction, indicating an adaptive response to low-temperature stress. Microbial richness and diversity were highest in spring and summer, decreased sharply in autumn, and partially recovered in winter through cold-adapted taxa. Dominant phyla (Chloroflexota, Pseudomonadota) and genera (Candidatus Villigracilis, Nitrospira) maintained nutrient removal through seasonal functional complementarity. Metagenomic analysis revealed higher carbon, nitrification, and denitrification gene abundances in spring and summer, while the relative abundances of nirK and nosZ shifted in autumn and winter. Phosphorus metabolism genes remained abundant, with pstS peaking in autumn. Overall, the high-altitude MBR exhibited strong adaptability through seasonal regulation of EPS, microbial communities, and metabolic functions, ensuring stable and efficient operation.