Inhibition Of Ammonia Oxidation Research Articles

Disinfectant-induced ammonia oxidation disruption in microbial N-cycling process in aquatic ecosystem after the COVID-19 outbreak

Anthropogenic activities significantly impact the elemental cycles in aquatic ecosystems, with the N-cycling playing a critical role in potential nutrient turnover and substance cycling. We hypothesized that measures to prevent COVID-19 transmission profoundly altered the nitrogen cycle in riverine ecosystems. To investigate this, we re-analyzed metagenomic data and identified 60 N-cycling genes and 21 host metagenomes from four urban reaches (one upstream city, Wuhan, and two downstream cities) along the Yangtze River. Our analyses revealed a marked decrease in the abundance of bacterial ammonia monooxygenase genes, as well as in the host, ammonia-oxidizing autotrophic Nitrosomonas, followed by a substantial recovery post-pandemic. We posited that discharge of sodium hypochlorite (NaOCl) disinfectant may be a primary factor in the reduction of N-cycling process. To test this hypothesis, we exposed pure cultures of Nitrosomonas europaea to NaOCl to explore the microbial stress response. Results indicated that NaOCl exposure rapidly compromised the cell structure and inhibited ammonia oxidation of N. europaea, likely due to oxidative stress damage and reduced expression of nitrogen metabolism-related ammonia monooxygenase. Using the functional tagging technique, we determined that NaOCl directly destroyed the ammonia monooxygenase protein and DNA structure. This study highlights the negative impacts of chlorine disinfectants on the function of aquatic ecosystems and elucidates potential mechanisms of action.

Effects of 3-year biochar application on carbon sequestration, nitrogen retention and nitrate leaching of fluvo-aquic soil profiles in vegetable rotation fields

The impacts of biochar application on the carbon (C) and nitrogen (N) cycles in soil profiles in vegetable fields have rarely been reported. A three-year field experiment (CK, control; BC, biochar; N, nitrogen fertilizer; BCN, biochar and nitrogen fertilizer) was conducted in fluvo-aquic soil with a wild cabbage-Chinese cabbage rotation to investigate biochar effects on soil organic carbon (SOC) sequestration, N retention, and nitrate (NO3−) leaching in the soil profile (topsoil, 0–20 cm; subsoil, 20–50 cm; third-layer soil, 50–100 cm). The results showed that the biochar-induced increase in topsoil SOC under N fertilization was greater for Chinese cabbage season than for wild cabbage season. Excluding biochar-N, biochar application caused an extra increase in topsoil total nitrogen (TN) under N fertilization. Biochar decreased the NO3−-N concentration of third-layer soil solution, particularly under N fertilization, indicating that biochar reduced the potential source of NO3− leaching. Under N fertilization, biochar reduced the abundances of carbon-degrading bacteria (Streptomyces, Bacillus, Mycobacterium, and Sphingomonas) and genes (sga, xylA, lig, and pgu) in the topsoil, as well as the abundances of ammonia-oxidizing bacteria (Nitrosospira and Nitrosomonas) and gene (amoA) in the third-layer soil. However, biochar increased the abundance of ammonia-assimilating bacteria (Rhodococcus) and gene (glnA) in the topsoil. Biochar inhibited the microbial function associated with C degradation by affecting soil TN, thus enhancing topsoil SOC sequestration. Biochar promoted the soil microbial function related to ammonia assimilation by affecting dissolved organic carbon (DOC), subsequently enhancing ammonia assimilation. Moreover, biochar initially inhibited ammonia oxidation in the third-layer soil by increasing soil DOC, subsequently affecting nitrite oxidation and ultimately reducing soil NO3− leaching. Biochar significantly increased the N use efficiency of vegetables in the last two years. This study provides insights into biochar effects on the changes in soil microbial function, which promoted SOC sequestration, enhanced N retention and mitigated nitrate leaching in a vegetable rotation field.

Optimization of aeration strategies for intensified nitrogen and phosphorus removal in an aerated electrolysis-assisted submerged fixed bed reactor treating recirculating mariculture effluent

Nutrients such as nitrogen (N) and phosphorus (P) discharged from mariculture wastewater pose a serious threat to the environment. In this study, an electrochemically assisted submerged fixed bed bioreactor (E-SFBBR) with intermittent aeration was used to explore the effect of aeration strategies (aeration time, aeration rate and aeration position) on the removal of N and P. Phosphate (PO43--P) removal was maintained above 95% under different aeration strategies. Although PO43--P removal predominantly occurred in the cathode areas, the aeration position affected the main functional area for PO43--P removal by influencing the pattern of DO distribution within E-SFBBR; and chemical precipitation with iron ions was the main removal pathway as revealed by XPS analysis. E-SFBBR with upper aeration (i.e., UA-E-SFBBR), aerated for 6 h d−1 at 0.8 L min−1 showed the best N removal performance, i.e., 83.78 ± 4.67%. The cathode area mainly contributed to N removal through mixotrophic denitrification. Aeration position plays a greater role in affecting N removal compared to aeration time and rate. Upper aeration favored the growth of heterotrophic denitrification bacteria (f_Rhodobacteraceae and Marinicella) in the cathode through providing more anaerobic environment, resulting in a better N removal performance. Besides, results of N transformation pathway predicted by PICRUSt 2.0 revealed that upper aeration promoted the ammonia assimilation but inhibited ammonia oxidation and NO3--N reduction processes in the cathode area. This study provided a theoretical basis for the application of aerated E-SFBBR in mariculture wastewater treatment.

Metagenomic analysis reveals the microbial response to petroleum contamination in oilfield soils

The response of the microbes to total petroleum hydrocarbons (TPHs) in three types of oilfield soils was researched using metagenomic analysis. The ranges of TPH concentrations in the grassland, abandoned well, working well soils were 1.16 × 102–3.50 × 102 mg/kg, 1.14 × 103–1.62 × 104 mg/kg, and 5.57 × 103–3.33 × 104 mg/kg, respectively. The highest concentration of n-alkanes and 16 PAHs were found in the working well soil of Shengli (SL) oilfield compared with those in Nanyang (NY) and Yanchang (YC) oilfields. The abandoned well soils showed a greater extent of petroleum biodegradation than the grassland and working well soils. Α-diversity indexes based on metagenomic taxonomy showed higher microbial diversity in grassland soils, whereas petroleum-degrading microbes Actinobacteria and Proteobacteria were more abundant in working and abandoned well soils. RDA demonstrated that low moisture content (MOI) in YC oilfield inhibited the accumulation of the petroleum-degrading microbes. Synergistic networks of functional genes and Spearman's correlation analysis showed that heavy petroleum contamination (over 2.10 × 104 mg/kg) negatively correlated with the abundance of the nitrogen fixation genes nifHK, however, in grassland soils, low petroleum content facilitated the accumulation of nitrogen fixation genes. A positive correlation was observed between the abundance of petroleum-degrading genes and denitrification genes (bphAa vs. nirD, todC vs. nirS, and nahB vs. nosZ), whereas a negative correlation was observed between alkB (alkane- degrading genes) and amo (ammonia oxidation), hao (nitrification). The ecotoxicity of petroleum contamination, coupled with petroleum hydrocarbons (PH) degradation competing with nitrifiers for ammonia inhibited ammonia oxidation and nitrification, whereas PH metabolism promoted the denitrification process. Moreover, positive correlations were observed between the abundance of amo gene and MOI, as well as between the abundance of the dissimilatory nitrate reduction gene nirA and clay content. Thus, improving the soil physicochemical properties is a promising approach for decreasing nitrogen loss and alleviating petroleum contamination in oilfield soils.

Role of Nitric Oxide in Hydroxylamine Oxidation by Ammonia-Oxidizing Bacteria.

An important role of nitric oxide (NO) as either a free intermediate in the NH3 oxidation pathway or a potential oxidant for NH3 or NH2OH has been proposed for ammonia-oxidizing bacteria (AOB) and archaea (AOA), respectively. However, tracing NO metabolism at low concentrations remains notoriously difficult. Here, we use electrochemical sensors and the mild NO scavenger 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide (PTIO) to trace apparent NO concentration and determine production rates at low micromolar concentrations in the model AOB strain Nitrosomonas europaea. In agreement with previous studies, we found that PTIO does not affect NH3 oxidation instantaneously in both Nitrosospira briensis and Nitrosomonas europaea, unlike inhibitors for ammonia oxidation such as allylthiourea and acetylene, although it effectively scavenged NO from the cell suspensions. Quantitative analysis showed that NO production by N. europaea amounted to 3.15% to 6.23% of NO2- production, whereas N. europaea grown under O2 limitation produced NO equivalent to up to 40% of NO2- production at high substrate concentrations. In addition, we found that PTIO addition to N. europaea grown under O2 limitation abolished N2O production. These results indicate different turnover rates of NO during NH3 oxidation under O2-replete and O2-limited growth conditions in AOB. The results suggest that NO may not be a free intermediate or remain tightly bound to iron centers of enzymes during hydroxylamine oxidation and that only NH3 saturation and adaptation to O2 limitation may lead to significant dissociation of NO from hydroxylamine dehydrogenase. IMPORTANCE Ammonia oxidation by chemolithoautotrophic ammonia-oxidizing bacteria (AOB) is thought to contribute significantly to global nitrous oxide (N2O) emissions and leaching of oxidized nitrogen, particularly through their activity in nitrogen (N)-fertilized agricultural production systems. Although substantial efforts have been made to characterize the N metabolism in AOB, recent findings suggest that nitric oxide (NO) may play an important mechanistic role as a free intermediate of hydroxylamine oxidation in AOB, further implying that besides hydroxylamine dehydrogenase (HAO), additional enzymes may be required to complete the ammonia oxidation pathway. However, the NO spin trap PTIO was found to not inhibit ammonia oxidation in AOB. This study provides a combination of physiological and spectroscopic evidence that PTIO indeed scavenges only free NO in AOB and that significant amounts of free NO are produced only during incomplete hydroxylamine oxidation or nitrifier denitrification under O2-limited growth conditions.

Combined effects of amoxicillin and copper on nitrogen transformation and the microbial mechanisms during aerobic composting of cow manure

Pollutants in livestock manure have a compound effect during aerobic composting, but research to date has focused more on single factors. This study investigated the effects of adding amoxicillin (AMX), copper (Cu) and both (ACu) on nitrogen transformation and the microbial mechanisms in cow manure aerobic composting with wheat straw. In this study, compared with CK, AMX, Cu, and ACu increased NH3 cumulative emissions by 32.32%, 41.78% and 8.32%, respectively, due to their inhibition of ammonia oxidation. Coexisting AMX and Cu decreased the absolute abundances of amoA/ nxrA genes and increased the absolute abundances of nirS /nosZ genes, but they had an antagonistic effect on the changes in functional gene abundances. Pseudomonas and Luteimonas were enriched during the thermophilic and cooling periods due to the addition of AMX and ACu, which enhanced denitrification in these two groups. Moreover, adding AMX and/or Cu led to more complex bacterial networks, but the effect of the two pollutants was lower than those of the individual pollutants. These findings provide theoretical and experimental support for controlling typical combined pollution with antibiotics and heavy metals in livestock manure.

An overlooked influence of reactive oxygen species on ammonia-oxidizing microbial communities in redox-fluctuating aquifers

Reactive oxygen species (ROS) are ubiquitous in O2-perturbed aquifers, but their role in shaping ammonia-oxidizing microbial communities is not clear. This study examined the dynamic responses of ammonia-oxidizing microorganisms (AOMs) in redox-fluctuating aquifers to ROS via field investigation and in-lab verification using transcriptomes/ metatranscriptome and RT-qPCR. Ammonia-oxidizing archaea (AOA) dominated recharge aquifers with lower ROS levels, whereas ammonia-oxidizing bacteria (AOB) and heterotrophic nitrifying aerobic bacteria (HNB) predominated in discharge areas with higher ROS levels. Similar succession in AOM enrichments was found in that the dominant AOMs changed from AOA Nitrosopumilus to AOB Nitrosomonas with increasing ROS. Ammonia oxidation and antioxidant capacity differed significantly among three AOM isolates exposed to ROS. ROS decreased the amoA gene expression of AOA strain Nitrososphaera viennensis PLX03, accompanied by inhibited ammonia oxidation capacity. By contrast, the catalase and superoxide dismutase activities of the AOB strain Nitrosomonas oligotropha PLL12 and HNB strain Pseudomonas aeruginosa PLL01 increased, and the antioxidant genes katG, sodA, ahpC, and ahpF were significantly upregulated. These results demonstrate that ROS exert an important influence on AOMs in redox-fluctuating aquifers. This study improves our understanding of the ecological niches of AOMs in surface/subsurface environments.

Microplastics affect soybean rhizosphere microbial composition and function during vegetative and reproductive stages

Microplastics (MPs) are emerging contaminants in agricultural soil, whereas their effects on the rhizosphere microbial ecosystems and biogeochemical nitrogen cycles during plant growth remain unknown. Here, a 70-day greenhouse experiment was carried out with black and fluvo-aquic soil to evaluate the influence of polyamide (PA), polyethylene (PE), polyester (PES), and polyvinyl chloride (PVC) MPs on the bacterial communities and functions in the soybean rhizosphere. The PA treatment consistently affected the rhizobacterial alpha diversity in the fluvo-aquic soil at soybean vegetative and reproductive growth stages, whereas the PE, PES, and PVC treatments had a short-term effect on the bacterial alpha diversity. At two growth stages, 6 and 23 biomarkers were consistently abundant in the PA treatment in the black soil and fluvo-aquic soil, respectively, and order Rhizobiales was found to be a biomarker for PA MPs contamination in both soils. Additionally, PA treatment decreased bacterial network complexity and tightness, whereas the effects of the PE, PES, and PVC on bacterial co-occurrence patterns varied depending on the soil types. Furthermore, PES and PVC treatments inhibited ammonification processes in the soybean rhizosphere, and PE could temporarily inhibit ammonia oxidation and denitrification processes according to the variations of N-cycling gene abundances. These effects on soil N-cycling also varied with soil types and soybean growth stages. This study provides profound information for understanding of MPs residues on the assembly of the soybean rhizosphere communities and function during plant development.

Microplastics inhibit biofloc formation and alter microbial community composition and nitrogen transformation function in aquaculture

Biofloc technology, extensively used in intensive aquaculture systems, can prompt the formation of microbial aggregates. Microplastics (MPs) are detected abundantly in aquaculture waters. This study explored the effects of MPs on biofloc formation, microbial community composition and nitrogen transformation function in simulated biofloc aquaculture production systems. The formation process and settling performance of bioflocs were examined. High-throughput sequencing of 16S and 18S rRNA genes was used to investigate the microbial community compositions of bioflocs. Nitrogen dynamics were monitored and further explained from functional genes and microorganisms related to nitrogen transformation by metagenome sequencing. We found that the aggregates consisting of bioflocs and MPs were formed and the systems with MPs had relatively weak settling performance. No significant differences in bacterial diversity (p > 0.05) but significant differences in eukaryotic diversity (p < 0.05) were found between systems without and with MPs. Significant separations in the microbial communities of prokaryotes (p = 0.01) and eukaryotes (p = 0.01) between systems without and with MPs were observed. The peak concentration of nitrite nitrogen (NO2−-N) in systems with MPs was lower than that in systems without MPs (pControl/MPs Low = 0.02 and pControl/MPs High = 0.03), probably due to the low abundance of hao and affiliated Alphaproteobacteria_bacterium_HGW-Alphaproteobacteria-1 and Alphaproteobacteria_bacterium, but the high abundance of nxrA and affiliated Alphaproteobacteria_bacterium_SYSU_XM001 and Hydrogenophaga_pseudoflava that related to nitrification. The low concentration of NO2−-N in systems with MPs suggested that the presence of MPs might inhibit ammonia oxidation but promote nitrite oxidation by altering the microbial community structure and function. These results indicated that aggregates consisting of bioflocs and MPs could be formed in aquaculture water, and thus, inhibiting their settlement and altering nitrogen transformation function by affecting the microbial community composition.

Biological nitrification inhibition in the rhizosphere: determining interactions and impact on microbially mediated processes and potential applications

Nitrification is the microbial conversion of reduced forms of nitrogen (N) to nitrate (NO3-), and in fertilized soils it can lead to substantial N losses via NO3- leaching or nitrous oxide (N2O) production. To limitsuch problems, synthetic nitrification inhibitors have been applied but their performance differs between soils. In recent years, there has been an increasing interest in the occurrence of biological nitrification inhibition (BNI), a natural phenomenon according to which certain plants can inhibit nitrification through the release of active compounds in root exudates. Here, we synthesize the current state of research but also unravel knowledge gaps in the field. The nitrification process is discussed considering recent discoveries in genomics, biochemistry and ecology of nitrifiers. Secondly, we focus on the 'where' and 'how' of BNI. The N transformations and their interconnections as they occur in, and are affected by, the rhizosphere, are also discussed. The NH4+ and NO3- retention pathways alternative to BNI are reviewed as well. We also provide hypotheses on how plant compounds with putative BNI ability can reach their targets inside the cell and inhibit ammonia oxidation. Finally, we discuss a set of techniques that can be successfully applied to solve unresearched questions in BNI studies.

Nitrification inhibitor DMPP offsets the increase in N2O emission induced by soil salinity

To evaluate the effect of a nitrification inhibitor on reducing N2O emissions from saline soils, we undertook a field plot experiment with three salinity levels (non-saline, NS = 0.21 dS m−1; low saline, LS = 1.02 dS m−1; and high saline, HS = 5.23 dS m−1), factorially combined with two nitrification inhibitor treatments (with and without DMPP (3,4-dimethylpyrazole phosphate)). The low saline soil had the highest N2O flux peak and its cumulative N2O emissions were 2.2-fold those of the non-saline soil and 3.1-fold those of the high saline soil. Low salinity strongly inhibited nitrite oxidation and only slightly inhibited ammonia oxidation, which resulted in a high accumulation of NO2−-N and high N2O emissions. The nitrification inhibitor DMPP reduced cumulative N2O emissions (p < 0.05) by 61% in non-saline soil (reduction of 88.3 mg N m−2) and by 75% in low saline soil (reduction of 239.8 mg N m−2). DMPP offsets low salinity-induced high N2O emissions by inhibiting ammonia oxidation.

Inhibitory mechanisms and fate of the analgesic drug acetaminophen in nitrifying activated sludge

This work investigated the inhibitory effects and fate of acetaminophen (N-acetyl-p-aminophenol, APAP) on activated sludge (AS) under nitrifying and aerobic conditions. APAP disrupted the two-step biological nitrification process in a dose-dependent manner. 100 mg/L APAP inhibited ammonia oxidation (the first step of nitrification) accompanied by a significant reduction (> 80 %) of Nitrosomonas oligotropha in relative abundance. 50 mg/L of APAP had no significant effects on ammonia oxidation but interrupted nitrite oxidation (the second step of nitrification) with more than 90 % reduction of Candidatus Nitrospira defluvii. APAP was removed in nitrifying activated sludge via largely the biotransformation route. Both nitrifiers and heterotrophic microorganisms contributed to overall APAP removal. An AS bioreactor was acclimated to 100 mg/L APAP as the sole source of carbon, nitrogen, and energy to enrich the microbial community with APAP-metabolizing heterotrophs. During acclimation, dynamic changes in community phenotypes occurred with significant reduction in species richness and diversity. Community acclimation significantly increased APAP biotransformation rates. 16S rRNA gene-based community profiling showed selective enrichment for Pseudomonas and Sphingomonas, both with demonstrated APAP metabolic capacity.

Responses of AOA and AOB activity and DNA/cDNA community structure to allylthiourea exposure in the water level fluctuation zone soil.

Ammonia oxidation is mainly performed by ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB). Allylthiourea (ATU) has been found to specifically inhibit ammonia oxidation. However, the effect of ATU on AOA and AOB transcription has been infrequently studied. In the present study, we examined the responses of AOA and AOB activity and DNA/cDNA community structure to ATU exposure. The ammonia oxidation activity in the 100-mg/L ATU group was 4.3% ofthat in the control group after 7days. When exposed to ATU, the gene abundance of AOA was favored compared with that of AOB, and there were no statistically significant differences in the abundance of AOB amoA in DNA and cDNA between the two groups. Compared with the control group, the gene abundance of AOA significantly increased by 5.23 times, while the transcription of AOA significantly decreased by 0.70 times. Moreover, the transcriptional ratio of AOA in the ATU group was only 0.05 times as highasthat in the control group. ATU selectively affected AOB and completely inhibited Nitrosomonas europaea and Bacterium amoA.22.HaldeII.kultur at the genetic level. Under ATU exposure, all AOA clusters were transcribed, but three AOB clusters were not transcribed. Our results indicated that the ammonia oxidation potential of the soil of water level fluctuation areas, based on ATU inhibition, was associated mainly with AOA amoA gene abundance and AOB community shifts in DNA and cDNA.

Influence of metronidazole on activated sludge activity

ABSTRACT Metronidazole is potentially carcinogenic to humans and it has been detected in wastewaters. The Wastewater Treatment Plants using biological processes have been highly impacted by the emergent compounds of recalcitrant type, and the knowledge about that issue is quite relevant. Therefore, this paper was focused on how metronidazole influences the kinetics and metabolic behaviour of nitrification and heterotrophic activity on activated sludge in batch cultures. Eight concentrations of metronidazole in the range of 5–100 mg/L were evaluated, in the presence of 2109 ± 129 mg VSS/L. The increment of initial metronidazole concentration caused a decline on COD and ammonium removal efficiencies, nitrate production yields, as well as in the substrate-specific consumption rates. Metronidazole (MDZ) had a greater impact on heterotrophic activity than nitrifying activity; also, it had a greater inhibitory effect on nitrite oxidation than ammonium oxidation. The activated sludge was not able to biotransform metronidazole; however, the azole compound significantly affected the physiology of it. The inhibition of ammonium oxidation was non-competitive (q max = 120 mg NH4 +-N consumed/gVSS-d, and Ki = 41.5 mg MDZ/L) and the initial metronidazole concentration that inhibited 50% of nitrifying activity (IC50) was 43 mg MDZ/L.

K+ deactivation of V2O5-WO3/TiO2 catalyst during selective catalytic reduction of NO with NH3: Effect of vanadium content

The effect of vanadium content on the resistance to K+ deactivation of V2O5-WO3/TiO2 SCR catalyst in biomass-fired flue gas was investigated. Catalytic activity and ammonia oxidation were measured, and the properties of fresh and K+-deactivated catalysts were characterized by XRD, N2 physisorption, H2-TPR, NH3-TPD and NH3-DRIFT. The BET surface area decreases with increased vanadium content for both fresh and K+-poisoned samples, but it is not responsible for catalyst deactivation. Ammonia oxidation starts from 300 °C and becomes more important with increasing vanadium content and at higher temperature. K+ can inhibit ammonia oxidation, but inactivates the newly generated NO2 to be reduced. The increase of vanadium content reduces mainly the Lewis acid sites, while the amount of BrØnsted acid sites increases. Monomeric and polymeric vanadium are the dominant species on the TiO2 support, and the amount of isolated vanadyl (V = O) species decreases with V2O5 content while the amount of V-OH species in polymeric vanadia increases. Isolated vanadyl species are advantageous to high-temperature catalytic activity while polymeric vanadia species increase ammonia and K+ adsorption. The (3 wt% V2O5)-WO3/TiO2 catalyst shows the best performance for both NO reduction and K+ resistance due to it containing both monomeric and polymeric vanadia species (or V = O and V-OH). Catalysts with 3 wt% V2O5 are preferable for flue gases with high alkali metal contents. Finally, the mechanism of reaction for different vanadium contents and corresponding K+-poisoning are also discussed.

Successive phytoextraction alters ammonia oxidation and associated microbial communities in heavy metal contaminated agricultural soils

Phytoextraction is an attractive strategy for remediation of soils contaminated by heavy metal (HM), yet the effects of this practice on biochemical processes involved in soil nutrient cycling remain unknown. Here we investigated the impact of successive phytoextraction with a Cd/Zn co-hyperaccumulator Sedum alfredii (Crassulaceae) on potential nitrification rates (PNRs), abundance and composition of nitrifying communities and functional genes associated with nitrification using archaeal and bacterial 16S rRNA gene profiling and quantitative real-time PCR. The PNRs in rhizosphere were significantly (P < 0.05) lower than in the unplanted soils, and decreased markedly with planting time. The decrease of PNR was more paralleled by changes in numbers of copy and transcript of archaeal amoA gene than the bacterial counterpart. Phylogenetic analysis revealed that phytoextraction induced shifts in community structure of soil group 1.1b lineage-dominated ammonia-oxidizing archaea (AOA), Nitrosospira cluster 3-like ammonia-oxidizing bacteria (AOB) and Nitrospira-like nitrite-oxidizing bacteria (NOB). A strong positive correlation was observed between amoA gene transcript numbers and PNRs, whereas root exudates showed negative effect on PNR. This effect was further corroborated by incubation test with the concentrated root exudates of S. alfredii. Partial least squares path model demonstrated that PNR was predominantly controlled by number of AOA amoA gene transcripts which were strongly influenced by root exudation and HM level in soil. Our result reveals that successive phytoextraction of agricultural soil contaminated by HMs using S. alfredii could inhibit ammonia oxidation and thereby reduce nitrogen loss.

Denitrification- and anammox-dominant simultaneous nitrification, anammox and denitrification (SNAD) process in subsurface flow constructed wetlands

Simultaneous nitrification, anammox and denitrification (SNAD) process was developed in subsurface flow constructed wetlands (SFCWs) to treat polluted surface water. The effects of vegetation, hydraulic retention time (HRT), C/N, and influent nitrogen forms on nitrogen removal and microbial communities were investigated. Results showed that denitrification- and anammox-dominant SNAD corresponded to nitrate- and ammonia-dominant influent, respectively, and both could achieve more efficient nitrogen removal in planted SFCWs than the unplated. These higher efficiencies were due to the microbial growth, organic carbon release, oxygen supply and plant uptake promoted by vegetation. The electron donors accelerated denitrification but inhibited ammonia oxidation with deficient oxygen. Anammox contributed to nitrogen removal of 27.34% under oxygen-limited conditions without vegetation. Anammox combined with denitrification and plant uptake were over 90% in planted SFCWs. For the investigated factors, the ammonia, nitrate and C/N were the most significant ones influencing the microbial communities, further nitrogen removal pathways and performances.

Commentary on 'Inhibition by acetylene of ammonia oxidation in Nitrosomonas europaea'.

The discovery that acetylene inhibited ammonia oxidation by Nitrosomonas europeae came about by studying linkages of microbial Nitrogen cycling processes in laboratory soil columns. Continuous addition of ammonium chloride (25μgN/ml) to the soil columns resulted in steady state nitrification of all of the ammonia to nitrate. Acetylene (10.1kPa) was introduced to detect nitrous oxide, a product of denitrification of nitrate, however, nitrification was inhibited and washout and disappearance of nitrate in the column effluent was detected. Concomitantly, ammonium levels in the effluent rose. Nitrification of nitrite was not affected by acetylene. Summary Acetylene inhibited the metalloenzyme, ammonia monooxygenase in the obligate chemoautotrophic nitrifier, Nitrosomonas europeae.

Inhibition effect of zinc in wastewater on the N2O emission from coastal loam soils

The effects of zinc (Zn) on nitrous oxide (N2O) fluxes from coastal loam soil and the abundances of soil nitrifier and denitrifier were studied in a tidal microcosm receiving livestock wastewater with different Zn levels. Soil N2O emission significantly increased due to discharge of wastewater rich in ammonia (NH4+-N) while the continuous measurements of gas flux showed a durative reduction in N2O flux by high Zn input (40mgL−1) during the low tide period. Soil inorganic nitrogen concentrations increased at the end of the experiment and even more soil NH4+-N was measured in the high-Zn-level treatment, indicating an inhibition of ammonia oxidation by Zn input. Quantitative PCR of soil amoA, narG and nirK genes encoding ammonia monooxygenase, nitrate reductase and nitrite reductase, respectively, showed that the microbial abundances involved in these metabolisms were neither affected by wastewater discharge nor Zn contamination.

Nitric oxide scavengers differentially inhibit ammonia oxidation in ammonia-oxidizing archaea and bacteria.

Differential inhibitors are important for measuring the relative contributions of microbial groups, such as ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA), to biogeochemical processes in environmental samples. In particular, 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide (PTIO) represents a nitric oxide scavenger used for the specific inhibition of AOA, implicating nitric oxide as an intermediate of thaumarchaeotal ammonia oxidation. This study investigated four alternative nitric oxide scavengers for their ability to differentially inhibit AOA and AOB in comparison to PTIO. Caffeic acid, curcumin, methylene blue hydrate and trolox were tested onNitrosopumilus maritimus, two unpublished AOA representatives (AOA-6f and AOA-G6) as well as the AOB representative Nitrosomonas europaea All four scavengers inhibited ammonia oxidation by AOA at lower concentrations than for AOB. In particular, differential inhibition of AOA and AOB by caffeic acid (100 μM) and methylene blue hydrate (3 μM) was comparable to carboxy-PTIO (100 μM) in pure and enrichment culture incubations. However, when added to aquarium sponge biofilm microcosms, both scavengers were unable to inhibit ammonia oxidation consistently, likely due to degradation of the inhibitors themselves. This study provides evidence that a variety of nitric oxide scavengers result in differential inhibition of ammonia oxidation in AOA and AOB, and provides support to the proposed role of nitric oxide as a key intermediate in the thaumarchaeotal ammonia oxidation pathway.