To address the issues of nitrite-oxidizing bacteria (NOB) reactivation and instability in traditional partial nitrification (PN) under low temperature and low NH4+-N conditions, while suspended algal-bacterial symbiotic systems with NOB inhibition potential suffer from severe light shading and are difficult to engineer, this study constructed a blue light irradiated photo-rotating biological contactor coupled with PN-algal symbiosis system to explore the stability of nitrogen transformation, microbial succession and metabolic regulation mechanisms under low-temperature and low-NH4+-N stress. Results showed that with 100 mg/L influent NH4+-N and at 30℃, PN-algal was rapidly initiated, with ammonia nitrogen conversion rate (NaRE) and nitrite accumulation rate (NiAE) stabilized at over 50 % and close to 100 % respectively. Under dual stress of gradually reducing the influent NH4+-N to 30 mg/L and the temperature to 20 °C, NaRE and NiAE remained at over 63.56 % and 92.74 % respectively, achieving long-term stable PN operation under low temperature and low NH4+-N conditions. Blue light continuously and selectively inhibited NOB while promoting algal growth (e.g., Parachlorella, Chlorella). The oxidative stress damage caused by low-NH4+-N was the primary role for the decline in system stress resistance. Microorganisms achieved synergistic resistance through dynamic remodeling of extracellular polymeric substances. The system was dominated by Nitrosomonas for nitritation, evolving into a new nitrogen transformation mode deeply coupled with cold-resistant microalgae and Candidatus_Brocadia. The key genes for nitrogen metabolism, photosynthesis and carbon metabolism were co-enriched. This study provides a novel reactor configuration and theoretical support for low NH4+-N water bodies with low-carbon autotrophic nitrogen removal.
