Quorum Sensing And Application In Aerobic Denitrification

Nitrogen compounds discharged into the environment cause serious problems such as the eutrophication of rivers and deterioration of water sources,as well as hazards to human health.Of the main nutrients in nitrogen,nitrate pollution has become a critical environmental issue that requires technical and policy interventions in quorum sensing.Quorum sensing（QS）,which is the most widely explored cell-cell communication mechanism,small chemical signaling molecules are involved in chemical communications between microbes.When the signal molecule concentration reaches a certain threshold,it will bind to its specific receptor,thereby regulating the expression of related genes and some physiological behaviors.In view of the increasingly serious nitrate pollution,biological method is currently used to remove nitrogen.Although the application of QS in the environmental field has been increasing,no studies focus on the improvement of denitrification by QS disruption.It was found that las and rhl QS systems had an effect on anaerobic denitrification.Our previous research also found that aerobic denitrification was regulated by las and rhl QS systems.Therefore,this article focuses on the effects of QS on the aerobic denitrification of Pseudomonas aeruginosa as well as its regulation,and meanwhile explores the response of las and rhl QS systems to oxygen and NO₃ˉ.From the perspective of QS,a control strategy for aerobic denitrification was proposed to provide a theoretical basis for the treatment of nitrate wastewater.The following conclusions were obtained:（1）The QS mutant strain had a higher nitrate removal capacity than the wild type strain（WT）.After supplementing the corresponding signal molecules,the mutant strain has no significant difference in the amount of nitrate removal from WT.It found that the las and rhl QS systems of can inhibit the aerobic denitrification of P.aeruginosa PAO1.It found that under aerobic conditions the regulation of las and rhl QS systems on denitrification occurred at the transcription level by affecting the expression of denitrification functional genes.And there may not be a hierarchy between las and rhl QS systems.QS inhibits aerobic denitrification by negatively regulating DNR,so DNR plays a key role in this process.（2）As the initial nitrate concentration in the medium increased,the nitrate removal of theΔlas I was always higher than that of WT,but the nitrate removal of theΔrhl I was similar to that of WT at high concentrations.When the initial nitrate concentration in the medium increased to 2000 mg/L,the rhl system seemed to have no effect on denitrification.At this time,there was no significant difference in aerobic denitrification activity and denitrification function gene expression between theΔrhl I and the WT.High concentration of nitrate（2000 mg/L）had no significant effect on the las system,but had a greater effect on the rhl system.Under the condition of 2000 mg/L nitrate concentration,the secretion of rhl system signal molecules was greatly reduced,while the secretion of virulence factors regulated by rhl system was also reduced,and the effect on denitrification was weakened.（3）With the increase of dissolved oxygen（DO）concentration in the culture medium,the nitrate removal ofΔlas I andΔrhl I was always higher than that of WT.The las and rhl QS systems had inhibited the denitrifying enzyme activity and the transcriptional expression of denitrifying functional genes of P.aeruginosa PAO1.It found that the change of DO didn’t affect the inhibition of denitrification by QS.However,the reduction of DO could inhibit the secretion of signal molecules and virulence factors in rhl system,and it had no significant effect on las system.These phenomena may be related to the respiratory electron transfer chain.QS inhibits denitrification by regulating dissimilated nitrate respiratory regulators,but the regulatory relationship under different dissolved oxygen conditions needs to be further explored.（4）Bioaugmentation of P.aeruginosa can increase the nitrate removal in activated sludge process from 56±2%to 82±1%.Based on the establishment of bioaugmentation system,the effect of QS disruption on aerobic denitrification showed that the addition of quorum sensing inhibitor（QSI）can effectively inhibit the secretion of 3-oxo-C₁₂-HSL and C₄-HSL and block the microbial communication,thereby improving the aerobic denitrification efficiency of the reactor.After the addition of QSI,the nitrate in the reactor was almost completely removed,and the activities of nitrate reductase and nitrite reductase increased by 88±2%and 74±2%,respectively.In addition,the transcriptions of nap A,nir S,nor B,nor C,and nos Z were up-regulated by 2.3-fold,3.1-fold,3.2-fold,3.7-fold,and 3.3-fold,respectively,while QS mainly regulates denitrification at the transcription level.