Influent COD/N regulatemicrobial intercellular communication influencing partial  nitrification performance
Achieving nitrite accumulation in mainstream municipal wastewater has significanteconomic and ecological benefits. Microbial intercellular communication mediated byacyl-homoserine lactones (AHLs) could regulate the performance of activated sludgesystems, and nitrogen transformation is also influenced by AHLs. Theoretically,intercellular communication mediated by AHLs could impact nitrite accumulation;however, the relevant mechanisms remain unclear. This study explored nitrogentransformation processes and interspecies interactions under different COD/N ratios,identified crucial AHLs and microorganisms that promoted partial nitrification, andvalidated these findings through external AHLs addition tests. The results showed thata nitrite accumulation ratio (NAR) of 65.8% was achieved at a COD/N of 3, with 3OXO-C12-HSL and C6-HSL identified as critical AHLs. External addition tests indicatedthat C6-HSL promoted the activities of ammonia-oxidizing bacteria (AOB) and nitriteoxidizing bacteria (NOB), while 3-OXO-C12-HSL inhibited their activities. Under theCOD/N ratio of 3, key quorum sensing species such as Thauera and Ca.Accumuliacterwere enriched, altering intercellular communication and inhibiting the positive feedbackregulation between C6-HSL and NOB. Consequently, the metabolism of DNAreplication, oxidative phosphorylation, glycolysis, and carbon fixation processes weresuppressed in NOB, leading to nitrite accumulation. This study provided a theoreticalbasis for the formation of partial nitrification through community self-regulation inmunicipal wastewater treatment and offered a potential strategy for reducing carbonemissions from wastewater treatment.