Side-stream blue light process washes out nitrite oxidizing bacteria via asymmetric photoprotection for partial...

Selective suppression of nitrite-oxidizing bacteria (NOB) while preserving ammonia-oxidizing bacteria (AOB) is essential for achieving stable partial nitrification (PN) as a pretreatment for the downstream anammox process, yet nitrite accumulation under low-ammonia municipal wastewater conditions remains difficult to achieve using conventional suppression strategies. This study demonstrated a substrate-free side-stream blue light irradiation strategy that maintained high ammonium removal and achieved an average nitrite accumulation rate (NAR) of 57.9% during stable period. The selective NOB suppression mechanisms were elucidated through batch experiments and multi-omics analyses across four mechanistic levels: 1) Under substrate-free pretreatment, ammonia monooxygenase (AMO) maintained a protective resting conformation. Conversely, nitrite oxidoreductase (NXR) appeared more vulnerable to blue light stress, contributing to 59.3% ATP depletion and a 72.1% reduction in catalase activity. 2) MAG-based protein sequence alignment identified PYP- and LOV-related photosensory features in Nitrosomonas enabling proactive light signal transduction, whereas Nitrospira possessed only a non-functional PYP homolog and a damage-responsive photolyase, conferring asymmetric photoadaptation capacity. 3) AOB mounted targeted upregulation of amoCAB subunits and core metabolic genes, while NOB suffered broad transcriptional disruption, notably the downregulation of the sufC gene which impaired Fe-S cluster assembly required for NxrB-mediated electron transfer. 4) Continuous blue light pressure drove directional community restructuring, resulting in a 4.4-fold enrichment of Nitrosomonas and a 7.6-fold reduction of Nitrospira. These findings support side-stream blue light irradiation as a mechanistically grounded NOB-control strategy for developing upstream PN units before anammox in low-ammonia wastewater treatment.