Effects of exogenous short-chain N-acyl homoserine lactone on denitrifying process of Paracoccus denitrificans

Transcriptome analysis expands underlying mechanisms of quorum sensing mediating heterotrophic nitrification-aerobic denitrification process at low temperature

Quorum sensing (QS) could regulate the behavior of microbial communities and help them resist adverse low-temperature environments. A newly isolated heterotrophic nitrification-aerobic denitrification (HN-AD) bacterium, strain YB1107, exhibited strong tolerance to harsh cold environments, removing 93.5 % of ammonia within 36 h and achieving a maximum specific growth rate of 0.28 h-1 at 10 °C. Strain YB1107 secreted large amounts of N-butanoyl-L-homoserine or N-octanoyl-L-homoserine lactones in response to cold stimuli. The add-back experiments indicated that these two signaling molecules jointly manipulated microbial physiological behavior by improving ammonia oxidation and biofilm formation, while inhibiting aerobic denitrification. The transcriptome analysis revealed that QS systems enhanced the cold resistance of HN-AD bacteria by promoting nitrogen assimilation and reducing dissimilation through regulating related genes. This study provided new molecular insights into how QS mediated HN-AD at low temperatures and laid the foundation for the potential applications of psychrophilic HN-AD bacteria in wastewater treatment.

Meta-analyzing the mechanism of pyrogenic biochar strengthens nitrogen removal performance in sulfur-driven autotrophic denitrification system: Evidence from metatranscriptomics

Sulfur-driven autotrophic denitrification (SAD) exhibits significant benefits in treating low carbon/nitrogen wastewater. This study presents an eco-friendly, cost-effective, and highly efficient method for enhancing nitrogen removal performance. The addition of biochar prepared at 300 °C (BC300) notably increased nitrogen removal efficiency by 31.60 %. BC300 concurrently enhanced electron production, the activities of the electron transfer system, and electron acceptors. With BC300, the ratio of NADH/NAD+ rose 2.00±0.11 times compared to without biochar, and the expression of NAD(P)H dehydrogenase genes was markedly up-regulated. In the electron transfer system, BC300 improved the electroactivity of extracellular polymeric substances and the activities of NADH dehydrogenase and complex III in intracellular electron transfer. Subsequently, electrons were directed into denitrification enzymes, where the nar, nir, nor, and nos related genes were highly expressed with BC300 addition. Significantly, BC300 activated the Clp and quorum sensing systems, positively influencing numerous gene expressions and microbial communication. Furthermore, the O%, H%, molar O/C, and aromaticity index in biochar were identified as crucial bioavailable parameters for enhancing nitrogen removal in the SAD process. This study not only confirms the application potential of biochar in SAD, but also advances our comprehension of its underlying mechanisms.

The regulation effect of preventing soil nitrogen loss using microbial quorum sensing inhibitors

Preventing soil nitrogen (N) losses driven by microbial nitrification and denitrification contributes to improving global environmental concerns caused by NO3--N leaching and N2O emission. Quorum sensing (QS) signals regulate nitrification and denitrification of N-cycling bacteria in pure culture and water treatment systems, and mediate the composition of ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) in activated sludge. However, whether disrupting QS could prevent soil N losses remains unclear. This study explored the feasibility of applying quorum sensing inhibitors (QSIs) as an innovative strategy to reduce N losses from agricultural soils. The two QSIs, penicillic acid and 4-iodo-N-[(3S)-tetrahydro-2-oxo-3-furanyl]-benzeneacetamide (4-iodo PHL), were more effective in reducing N losses than traditional inhibitors, including N-(n-butyl) thiophosphoric triamide and 3,4-dimethylpyrazole phosphate. After 36 days of aerobic incubation, penicillic acid and 4-iodo PHL inhibited nitrification by 39% and 68%, respectively. The inhibitory effects are attributed to the fact that 4-iodo PHL decreased the abundance of archaeal and bacterial amoA genes, as well as the relative abundance of Candidatus Nitrocosmicus (AOA), Candidatus Nitrososphaera (AOA), and Nitrospira (nitrite-oxidizing bacteria/comammox), while penicillic acid reduced archaeal amoA abundance and the relative abundance of Nitrosospira (AOB) and the microbes listed above. Penicillic acid also strongly inhibited denitrification (33%) and N2O emissions (61%) at the peak of N2O production (day 4 of anaerobic incubation) via decreasing nitrate reductase gene (narG) abundance and increasing N2O reductase gene (nosZ) abundance, respectively. Furthermore, the environmental risks of QSIs to microbial community structure and network stability, CO2 emissions, and soil animals were acceptable. Overall, QSIs have application potential in agriculture to reduce soil N losses and the associated effect on climate change. This study established a new method to mitigate N losses from the perspective of QS, and can serve as important basis of decreasing the environmental risks of agricultural non-point source pollution.

Nitrogen-loss and carbon-footprint reduction by plant-rhizosphere exudates

Low-carbon approaches to agriculture constitute a pivotal measure to address the challenge of global climate change. In agroecosystems, rhizosphere exudates are significantly involved in regulating the nitrogen (N) cycle and facilitating belowground chemical communication between plants and soil microbes to reduce direct and indirect emissions of greenhouse gases (GHGs) and control N runoff from cultivated sites into natural water bodies. Here, we discuss specific rhizosphere exudates from plants and microorganisms and the mechanisms by which they reduce N loss and subsequent N pollution in terrestrial and aquatic environments, including biological nitrification inhibitors (BNIs), biological denitrification inhibitors (BDIs), and biological denitrification promoters (BDPs). We also highlight promising application scenarios and challenges in relation to rhizosphere exudates in terrestrial and aquatic environments.

Facilitating nitrification and biofilm formation of Vibrio sp. by N-acyl-homoserine lactones in high salinity environment

The N-acyl-homoserine lactones (AHLs)-mediated quorum-sensing (QS) system played a crucial role in regulating biological nitrogen removal and biofilm formation. However, the regulatory role of AHLs on nitrogen removal bacteria in high salinity environment has remained unclear. This study evaluated the roles and release patterns of AHLs in Vibrio sp. LV-Q1 under high salinity condition. Results showed that Vibrio sp. primarily secretes five AHLs, and the AHLs activity is strongly correlated with the bacterial density. Exogenous C10-HSL and 3OC10-HSL were found to significantly enhance ammonium removal, while making a minor contribution to the growth rate. Both the C10-HSL and 3OC10-HSL promoted the biofilm formation of Vibrio sp. with an enhancement of 1.64 and 1.78 times, respectively. The scanning electron microscope (SEM) and confocal laser scanning microscope (CLSM) observations confirmed the biofilm-enhancing effect of AHLs. Further analysis revealed that AHLs significantly improved bacterial self-aggregation and motility, as well as the level of extracellular polymeric substances (EPS). These findings provide significant guidance on construction of nitrification system at high salinity.

New insights into exogenous N-acyl-homoserine lactone manipulation in biological nitrogen removal system against ZnO nanoparticle shock

The effects and mechanisms of three N-acyl-homoserine lactones (AHLs) (C₄-HSL, C₆-HSL, and C₁₀-HSL) on responses of biological nitrogen removal (BNR) systems to zinc oxide nanoparticle (NP) shock were investigated. All three AHLs improved the NP-impaired ammonia oxidation rates by up to 50.0 % but inhibited the denitrification process via regulating nitrogen metabolism-related enzyme activities. C₄-HSL accelerated the catalase activity by 13.2 %, while C₆-HSL and C₁₀-HSL promoted the superoxide dismutase activity by 26.6 % and 18.4 %, respectively, to reduce reactive oxygen species levels. Besides, the enhancements of tryptophan protein and humic acid levels in tightly-bound extracellular polymeric substance by AHLs were vital for NP toxicity attenuation. The metabonomic analysis demonstrated that all three AHLs up-regulated the levels of lipid- and antioxidation-related metabolites to advance the system's resistance to NP shock. The "dual character" of AHLs emphasized the concernment of legitimately employing AHLs to alleviate NP stress for BNR systems.

The Role of Quorum Sensing Molecules in Bacterial-Plant Interactions

Quorum sensing (QS) is a system of communication of bacterial cells by means of chemical signals called autoinducers, which modulate the behavior of entire populations of Gram-negative and Gram-positive bacteria. Three classes of signaling molecules have been recognized, Al-1, Al-2, Al-3, whose functions are slightly different. However, the phenomenon of quorum sensing is not only concerned with the interactions between bacteria, but the whole spectrum of interspecies interactions. A growing number of research results confirm the important role of QS molecules in the growth stimulation and defense responses in plants. Although many of the details concerning the signaling metabolites of the rhizosphere microflora and plant host are still unknown, Al-1 compounds should be considered as important components of bacterial-plant interactions, leading to the stimulation of plant growth and the biological control of phytopathogens. The use of class 1 autoinducers in plants to induce beneficial activity may be a practical solution to improve plant productivity under field conditions. In addition, researchers are also interested in tools that offer the possibility of regulating the activity of autoinducers by means of degrading enzymes or specific inhibitors (QSI). Current knowledge of QS and QSI provides an excellent foundation for the application of research to biopreparations in agriculture, containing a consortia of AHL-producing bacteria and QS inhibitors and limiting the growth of phytopathogenic organisms.

Performance enhancement and population structure of denitrifying phosphorus removal system over redox mediator at low temperature

The development of denitrifying polyphosphate accumulating organisms (DPAOs) presents a strategy to carbon competition between denitrifying bacteria and phosphorus removing bacteria. However, low temperature inhibits the rate of enzyme-catalyzed and substrate diffusion during denitrifying phosphorus removal (DPR). Therefore, the present study assessed the addition of NQS (100 μmol/L) for enhancing the removal of TP and TN in DPR reactors operated at alternating anaerobic and anoxic phases and different influent phosphate concentrations. The results showed that the removal efficiency of TP and TN in NQS-DPR system at 10 °C were 99.9% and 42.0%, respectively, which were 2.1 and 2.0 times higher than that of DPR system. Adding NQS significantly alleviated the increase of pH under anoxic condition and decreased the ORP value of the reactor, which in turn enhanced the PHAs accumulation process. The determination of functional genes (nirK, narG and phoD) showed that Dechloromonas, Lentimicrobium, and Terrimonas were the dominant functional bacteria in NQS-DPR system at 10 °C with the relative abundance of 3.09%, 2.99% and 2.28%, respectively. This study can provide valuable information for the effects of the addition of the redox mediator on denitrifying phosphorus removal technology.

Advances in AHLs-mediated quorum sensing system in wastewater biological nitrogen removal: mechanism, function, and application

Biological nitrogen removal process is to convert organic nitrogen and ammonia nitrogen into nitrogen via a series of reactions by microorganisms, and is widely used in wastewater treatment for its costless, high-effective, secondary pollution-free characteristics. Quorum sensing (QS) is a communication mode for microorganisms to regulate bacteria's physiological behaviors in response to environmental changes. N-acyl-homoserine lactones (AHLs)-mediated QS system is widespread in nitrogen removal-related functional bacteria and promotes biological nitrogen removal performance by regulating bacteria behavior. Recently, there has been an increasingly investigated AHLs-mediated QS system in wastewater biological nitrogen removal process. Consequently, the AHLs-mediated QS system is considered a promising regulatory strategy in the biological nitrogen removal process. This article reviewed the QS mechanism in various nitrogen removal-related functional bacteria and analyzed its effect on biological nitrogen removal performance. Combined with the application research of the QS system for enhanced biological nitrogen removal, it further put forward some prospects and suggestions which are of practical significance in practical application.

Successful sulphide-driven partial denitrification: Efficiency, stability and resilience in SRT-controlled conditions

Partial denitrification is emerging as a valuable solution for NO₂^- supply in Anammox systems. When reduced sulphur compounds are used as electron donors, S-driven Partial Autotrophic Denitrification (PAD) can also be achieved, allowing for an integrated autotrophic nitrogen (N) and sulphur (S) removal from liquid and gaseous streams. The aim of the present work was to maximise NO₃^- reduction to NO₂^- coupled with complete HS^- oxidation, by the selective control of influent S/N ratio and sludge retention time (SRT). A 2.5-L chemostat was operated for 115 days and three operational phases were tested at decreasing SRT of 40, 23 and 13 h, testing S/N ratios in the range of 0.5-1 gS/gN. Successful sulphide-driven PAD was achieved and lead to average NO₂^- conversion efficiencies as high as77±17% at all the conditions tested, with the highest value of 99% at the lowest S/N of 0.58 gS/gN and SRT of 23 h. Respirometric tests showed that NO₃^- uptake rate was stable at 90±10 mgN/gVSS/h, when NO₃^- was present as sole electron acceptor or at NO₂^- levels as high as 120 mgN/l; on the contrary, NO₂^- uptake rates were very sensitive to the applied conditions. Metabarcoding analyses revealed that the microbial community was highly enriched in Sulphur Oxidizing Bacteria (SOB>80%) and stable S-limiting conditions appeared to favour Thiobacillus over Sulfurimonas genus. A preliminary assessment of N₂O potential emission was also performed. To the best of our knowledge, this is the first work evaluating the synergic effect of SRT and influent S/N ratio on nitrite accumulation in highly SOB-enriched systems and the NO₂^- conversion efficiencies achieved are among the highest reported in literature.

Metagenomics reveals functional species and microbial mechanisms of an enriched thiosulfate-driven denitratation consortia

In this study, thiosulfate-driven denitratation (TDD) system was successfully established under optimal S/N molar ratio of 1.00, with nitrite accumulation efficiency (NAE) of 82.24 ± 17.09%. This work highlighted that thiosulfate significantly preferred the reduction of nitrate than nitrite. However, after the depletion of thiosulfate, the in-situ formed intermediate product element sulfur (S⁰) served as the main electron donor, and significantly favored the reduction of nitrite than nitrate, which constrained nitrite accumulation and nitrate removal. In addition, metagenomic sequencing revealed that the functional denitratation species might be Thiobacillus_sp._65-29, but the occurrence of Nir-annotated species would decrease nitrite accumulation. Under S/N ratio of 1.00, the decreased abundant Nir-annotated species (e.g., Thiobacillus_sp.), as well as the down-regulated quorum sensing interactions between Nar- and Nir-annotated species were key microbial metabolisms of high NAE in the TDD system. Overall, this work provides new sight into the metagenome-base functional species and metabolic potential of thiosulfate-driven denitratation.

Realizing the role of N-acyl-homoserine lactone-mediated quorum sensing in nitrification and denitrification: A review

Nitrification and denitrification are crucial processes in the nitrogen cycle, a vital microbially driven biogeochemical cycle. N-acyl-homoserine lactone (AHL)-mediated quorum sensing (QS) is widespread in bacteria and plays a key role in their physiological status. Recently, there has been an increase in research into how the AHL-mediated QS system is involved in nitrification and denitrification. Consequentially, the AHL-mediated QS system has been considered a promising regulatory approach in nitrogen metabolism processes, with high potential for real-world applications. In this review, the universal presence of QS in nitrifiers and denitrifiers is summarized. Many microorganisms taking part in nitrification and denitrification harbor QS genes, and they may produce AHLs with different chain lengths. The phenotypes and processes affected by QS in real-world applications are also reviewed. In wastewater bioreactors, QS could affect nitrogen metabolism efficiency, granule aggregation, and biofilm formation. Furthermore, methods commonly used to identify the existence and functions of QS, including physiological tests, genetic manipulation and omics analyses are discussed.

Semi-starvation fluctuation driving rapid partial denitrification granular sludge cultivation in situ by microorganism exudate metabolites feedbacks

In this study, semi-starvation fluctuation driving PD granules cultivation in situ by microorganism exudate metabolites feedbacks was firstly investigated. The PD granules of high nitrite production were cultivated with an excellent mean nitrate-to-nitrite transformation rate (NTR) of 56.39% in just 30 days. The granules size was improved from the initial size of 0.09 ± 0.01 mm in diameter to a size above 2 mm when the extracellular polymeric substance (EPS) content increased from 80.21 ± 10.20 mg/g MLVSS to 777.00 ± 22.13 mg/g MLVSS. Acyl-homoserine lactone signals (AHLs) ultimately increased ten-fold more than the initially through 30 days of cultivation. Meanwhile, Thauera had been identified as the main function bacteria of PD, which enriched from 0.47% to 10.67%. Results demonstrated that AHLs, EPS, PD bacteria and the PD granules cultivation were closely associated. Semi-starvation fluctuation produced oligotrophic stress on bacterial community, a part of bacteria would be eliminated on starvation for oligotrophic stress and AHLs of bacteria regarded as distress signals resulted in the rapid formation of PD granules. A mechanism for PD granular cultivation with semi-starvation fluctuation was proposed from the aspect of oligotrophic stress. A better strategy for rapid PD granules cultivation was obtained and it could be useful for the mainstream granule-based PD combined with the anammox process application in the future.

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.

Undesirable effects of exogenous N-acyl homoserine lactones on moving bed biofilm reactor treating medium-strength synthetic wastewater

Exogenous AHLs are gradually reported to facilitate biofilm growth, however, whether they play a universal role in promoting biofilm formation and pollutants removal remains to be investigated. The pollutant removal, biofilm properties, microbial community and the distribution of AHLs were investigated in three lab-scale MBBRs by continuous dosing 100 nM N-Hexanoyl-L-homoserine lactone (C6-HSL) and N-Octanoyl-L-homoserine lactone (C8-HSL) in synthetic wastewater under normal nutrition (40 mg/L NH₄⁺-N with C/N = 20). Results showed that adding AHLs didn't affect organics removal and exogenous C6-HSL even significantly suppressed NH₄⁺-N removal by 0.44-20.29% after 16 days (p < 0.05). The introduction of AHLs both facilitated biofilm growth and extracellular polymeric substances secretion while suppressed ATP production especially during the stable operation period, with 48.96% by C6-HSL (p < 0.05) and 27.25% by C8-HSL, respectively. Exogenous AHLs inhibited the proliferation of Chryseobacterium, resulting in improvement in biofilm growth and it probably mediated ATP synthesis through regulating the release of 3OHC12-HSL in aqueous phase. Organics removal and biofilm growth were mainly attributed to the combined actions of multitudinous AHLs in biofilm phase rather than that in aqueous phase. The counterintuitive conclusions obtained in this study highlighted the importance of legitimately applying exogenous AHLs to accelerate biofilm formation and the start-up of MBBR in wastewater treatment.

Paracoccus denitrificans can utilize various long-chain N-acyl homoserine lactones and sequester them in membrane vesicles

Many gram-negative bacteria utilize N-acyl homoserine lactone (AHL) signals to communicate with each other. Once they have been released, these signals are assumed to be shared among the population in the local environment. In contrast to this canonical quorum-sensing (QS) model, recent study in Paracoccus denitrificans showed that they can traffic their signals to each other via membrane vesicles (MVs). Here, we demonstrate that various long-chain AHLs inhibited cell aggregation in P. denitrificans, whereas the short-chain AHLs alone did not. Furthermore, MVs released from P. denitrificans were able to take up the long-chain AHLs from the environment into MVs. The AHLs associated with MVs triggered gene expression in P. denitrificans, indicating their role in QS. Our results suggest that P. denitrificans can sequester the AHL produced by other bacteria and deliver the signals to themselves via MVs. Utilizing the signals from other bacteria may be advantageous for P. denitrificans to reach the threshold QS concentration in a polymicrobial community in which the population of its own species is relatively low.

Biofilm activity, ammonia removal and cell growth of the heterotrophic nitrifier,Acinetobactersp., facilitated by exogenousN-acyl-homoserine lactones

In the present study, the heterotrophic nitrification–aerobic denitrification strain, Acinetobacter sp. JQ1004, was treated with three typical N-acyl-homoserine lactone (AHL) molecules (C6-HSL, C8-HSL, and 3-oxo-C10-HSL) during the nitrogen removal process. The effects of AHLs on biofilm formation, flocculation, extracellular polymeric substance characteristics, and nitrogen removal were investigated. Findings revealed that low concentrations of these three AHLs could promote ammonia removal and cell growth as follows: C8-HSL > C6-HSL > 3-oxo-C10-HSL, whereas high concentrations suppressed nitrogen removal. Transcript levels of the amoA gene in the heterotrophic nitrification process were detected by real-time PCR, indicating that the addition of each AHL with 10 nmol L⁻¹ could stimulate expression of amoA. Notably, the addition of C6-HSL at relative lowly concentrations significantly accelerated biofilm formation and self-aggregation of strain JQ1004. Many microbial-flocs were observed between cells using scanning electron microscopy when strains were dosed with 10 nmol L⁻¹ C6-HSL. Excitation emission matrix spectra revealed that the addition of C6-HSL and C8-HSL at 10 nmol L⁻¹ did not change the components and structures of the extracellular polymeric substance (EPS), but the fluorescence intensity of various components increased substantially. However, the addition of 3-oxo-C10-HSL reduced the fluorescence strength of EPS, which had no remarkable effect on biofilm formation, self-aggregation, and nitrogen removal of the strain.

In the present study, the heterotrophic nitrification–aerobic denitrification strain, Acinetobacter sp. JQ1004, was treated with three typical N-acyl-homoserine lactone (AHL) molecules (C6-HSL, C8-HSL, and 3-oxo-C10-HSL) during the nitrogen removal process.

Microbial Photoelectrotrophic Denitrification as a Sustainable and Efficient Way for Reducing Nitrate to Nitrogen

Biological removal of nitrate, a highly concerning contaminant, is limited when the aqueous environment lacks bioavailable electron donors. In this study, we demonstrated, for the first time, that bacteria can directly use the electrons originated from the photoelectrochemical process to carry out the denitrification. In such photoelectrotrophic denitrification (PEDeN) systems (denitrification biocathode coupling with TiO₂ photoanode), nitrogen removal was verified solely relying on the illumination dosing without consuming additional chemical reductant or electric power. Under the UV illumination (30 mW·cm^-2, wavelength at 380 ± 20 nm), nitrate reduction in PEDeN apparently followed the first-order kinetics with a constant of 0.13 ± 0.023 h^-1. Nitrate was found to be almost completely converted to nitrogen gas at the end of batch test. Compared to the electrotrophic denitrification systems driven by organics (OEDeN, biocathode/acetate consuming bioanode) or electricity (EEDeN, biocathode/abiotic anode), the denitrification rate in PEDeN equaled that in OEDeN with a COD/N ratio of 9.0 or that in EEDeN with an applied voltage at 2.0 V. This study provides a sustainable technical approach for eliminating nitrate from water. PEDeN as a novel microbial metabolism may shed further light onto the role of sunlight played in the nitrogen cycling in certain semiconductive and conductive minerals-enriched aqueous environment.