Quorum sensing systems regulate heterotrophic nitrification-aerobic denitrification by changing the activity of nitrogen-cycling enzymes

Heterotrophic nitrification-aerobic denitrification (HNAD) is essential in diverse nitrogen-transforming processes. How HNAD is modulated by quorum sensing (QS) systems is still ambiguous. The QS system in Pseudomonas aeruginosa manipulates colony behavior. Here, we described the influence of the Pseudomonas quinolone signal (PQS) and N-acyl-l-homoserine lactone (AHL) on HNAD. The HNAD of P. aeruginosa was inhibited by the oversecretion of PQS. AHL- or PQS-deficient P. aeruginosa mutants had a higher ability for nitrogen removal. QS inhibited heterotrophic nitrification mainly via controlling the activity of nitrite oxidoreductase (NXR) and the depressed aerobic denitrification by regulating the catalytic abilities of nitric oxide reductase (NOR), nitrite reductase (NIR), and nitrate reductase (NAR). The addition of citrate as the sole carbon source increased the nitrogen removal efficiency compared with other carbon sources. Nitrite, as the sole nitrogen source, could be used entirely with only the moderate concentration of PQS contained. AHL and PQS controlled both nitrification and denitrification, suggesting that QS plays an important role in nitrogen cycle under aerobic conditions.