Potential roles of acyl homoserine lactones (AHLs) in nitrifying bacteria survival under certain adverse circumstances.

Potential roles of quorum sensing (QS) in nitrifying bacteria activity and ecology, particularly under adverse circumstances have been rarely reported. Herein, eight lab-scale nitrification sequencing batch reactors, with or without adding acyl homoserine lactones (AHLs) were operated under adverse circumstances respectively. The results indicated that the introduction of AHLs significantly enhanced the nitrogen removal efficiency in the presence of nitrification inhibitors (dicyandiamide, DCD), accelerated the low temperature (10 °C) group into stable stage, and improved the utilization efficiency of AHLs in these two groups. Community analysis and qPCR further confirmed that AHLs significantly increased the abundance of nitrifying bacteria in low temperature group and DCD group, especially AOB. For normal condition (28 °C, pH = 8) or low pH level (5.5), however, the AHLs had no significant effect. Canonical correspondence analysis showed that nitrifying bacteria positively responded to AHLs, indicating that adding AHLs was an effective strategy to regulate nitrification process. However, under acid conditions, the effect of this regulatory mechanism was not significant, indicating that the influence of pH on the system was greater than that of AHLs. This study demonstrated that exogenous AHLs could enhance the competitiveness of nitrifying bacteria to utilize more resource and occupy space under some adverse environmental conditions.