This study designed and operated two sequencing batch reactors (SBRs), denoted as reactors A and B, to investigate the effects of temperature variation on the performance and microbial community of activated sludge systems. Reactor A was operated for 206 days under five temperature stages: A1 (20 ℃), A2 (15 ℃), A3 (10 ℃), A4 (5 ℃), and A5 (cyclic oscillation between 5-35 ℃, changing every 3 days). Reactor B was operated for 146 days to examine the effects of elevated temperatures. During operation, sludge characteristics, water quality parameters, and viral abundances were regularly monitored, and metagenomic analyses were performed to assess changes in bacterial community structure and metabolic functions.Results showed that (1) temperature significantly influenced sludge physicochemical properties and pollutant removal efficiency. In reactor A, mixed liquor suspended solids (MLSS) increased at 15 ℃ but decreased and exhibited sludge bulking at 5 ℃, while MLSS in reactor B continuously declined under high temperatures. Both reactors maintained high COD and ammonia removal efficiencies (>95%), but total nitrogen (TN) removal rates differed (A: 51.9~81.6%; B: 69~80.7%). The total phosphorus (TP) removal in reactor A fluctuated during temperature oscillations, whereas TP removal in reactor B eventually dropped to zero. (2) The amplitude of temperature fluctuation affected viral abundance: minor variations caused slight changes, whereas large fluctuations led to marked declines; similarly, viral abundance decreased in reactor B under high temperatures. (3) Temperature shifts altered bacterial community composition and functional gene abundances. The core taxa were Pseudomonadota, Actinomycetota, and Bacteroidota (combined relative abundance > 50%). The denitrification gene nirS remained relatively stable and corresponded with consistently high TN removal efficiency, whereas nirK was more sensitive to heat stress. Decreases in the phosphorus metabolism genes ppk1 and ppk2 were associated with fluctuations or failures in TP removal.This study elucidates the impacts of temperature variation on microbial community dynamics and metabolic functions in activated sludge systems, providing insights for maintaining stable wastewater treatment performance under extreme temperature conditions.
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