The Regulation Of Exogenous N-Acyl Homoserine Lactone On The Denitrifying Process Of Paracoccus Denitrificans

Paracoccus denitrificans, as a denitrifying bacteria, can reduce NO3- to NO, N2O or N2 in either aerobic or anaerobic conditions due to the ability of utilizing both oxygen and nitrogenous oxides. In quorum sensing (QS) system, bacteria can regulate bacteria behaviors at population level by releasing and sensing autoinducers that accumulate to a threshold. In many gram-negative bacteria, QS system is driven by N-acylated homoserine lactone (AHL) molecules. In this study, we report effects of exogenous AHLs signal molecules C6-HSL (N-hexanoyl-DL-homoserine lactone), C14-HSL (N-Tetradecanoyl-DL--homoserine lactone) and C16-HSL (N-hexadecanoyl-L-Homoserine Lactone) on denitrifying process of Paracoccus denitrificans respectively under aerobic and anaerobic growth. The denitrification activity of the cells was monitored by measuring denitrification products (nitrate, nitrite, and nitrous oxide), and individual mRNA levels for the nitrate, nitrite, nitric oxide and nitrous oxide reductases. The main results are as follows:(1) Effects of exogenous short-chain signal molecule C6-HSL on denitrifying process of P. denitrificans2 μM C6-HSL has little effects on cell density under either anaerobic or aerobic culture conditions, and the nitrate reduction activity was appeared slightly affected by C6-HSL.2 μM C14-HSL supressd P. denitrificans anaerobic growth but has little effects on cell density and nitrate, nitrite reduction under aerobic culture conditions. C6-HSL significantly decresed the transcription of nirS, norC and nosZ in aerobic culture.Transcriptional level of nir, norB and norC in the C14-HSL treated sample was also repressed in aerobic condition, and N2O accumulation activity in P. denitrificans was suppressed by C6-HSL and C14-HSL. In anaerobic groups, the rate of nitrate consumption was suppressed due to AHL addition. C6-HSL increased mRNA relative amount of nirS, norB and norC, while the transcription of nirS, norB, norC and nosZ was up-regulated by the presence of C14-HSL signal under the anaerobic condition.As a result, N2O accumulation activity in P. denitrificans was suppressed by C6-HSL and C14-HSL under aerobic condition. In contrast, exogenous C6-HSL stimulated the production of N2O, meanwhile the addition of C14-HSL continuously enhanced the relative rate of N2O accumulation and reduction under anaerobic condition.The results suggested that C6-HSL and C14-HSL significantly affected the activities of nitrite reductase and nitric oxide reductase in P. denitrificans and the regulation of denitrification by quorum sensing may be important in N2O release.By comparison, long-chain AHL C14-HSL appeared to have more significantly effect than short-chain AHL C6-HSL on anaerobically denitrifying process of P. denitrificans.(2) Effects of added C16-HSL on denitrifying process of P. denitrificans.2 μM C16-HSL has negative effects on cell density under either anaerobic or aerobic culture conditions.N2O production and reduction activity in P. denitrificans was suppressed by C16-HSL under aerobic condition, while the transcriptional level of nirS, norB and norC was down-regulated to over 50% by the addition of C16-HSL signal. Under anaerobic condition, C16-HSL significantly increased mRNA relative amount of nirS, norB, norC and nosZ.Meanwhile, N2O accumulation and reduction was simultaneously enhanced in C16-HSL treated groups.Compared with C6-HSL and C14-HSL experiments, the results suggested that long-chain AHLs appeared to have more significantly effect on anaerobically denitrifying process of P. denitrificans.