Comparing two hydrazine addition strategies to stabilize mainstream deammonification: Performance and microbial community analysis.

In this study, an expanded granular sludge blanket reactor (EGSB) was proposed to achieve stable mainstream deammonification process by adding hydrazine (N2H4). Two N2H4 addition methods consisted of constant concentration (strategy A) and variable concentration (strategy B) both can inhibit nitrite oxidizing bacteria. A efficient performance was achieved with higher total nitrogen removal efficiency (82 ±â€¯6%) and nitrogen removal rate (0.32 ±â€¯0.02 kg N/(m3·d)) under strategy B. For strategy A, anaerobic ammonia oxidizing bacteria (AnAOB) in-situ activity was decreased from 2.76 to 0.68 mg N/(g VSS·h) at 42 mg/L NH4+-N. Candidatus Brocadia abundance increase from 14.62% to 20.07% under the strategy may indicated the self-regulate mechanism of AnAOB. Aerobic ammonia oxidizing bacteria (AOB, mainly Nitrosomonas) and AnAOB (mainly Candidatus Brocadia) were always dominated under two strategies. Strategy B provided better environment for most microorganisms (mainly Chloroflexri, Planctomycetes, Proteobacteria and Chlorobi).

The drought and high wet soil condition impact on PAH (phenanthrene) toxicity towards nitrifying bacteria.

A few previous studies showed that the low soil moisture could interact with the toxic effect of the polycyclic aromatic hydrocarbons (PAHs) towards animals (mostly invertebrates). In the present research the impact of the soil moisture in the wide range (from the drought to high moisture conditions) in three different soil materials on toxic effect of the PAH (phenanthrene) towards soil microorganisms (nitrifying bacteria activity) was evaluated. The three dry soil materials were artificially contaminated with phenanthrene (0, 1, 10, 100 and 1000 mg kg-1 dry mass of soil) and moistened to the varied levels of the soil moisture (30% WHC (dry), 55% WHC (optimal) and 80% WHC (highly wet conditions)). After 7 days incubation, the nitrification potential was measured. The results of the proposed ANCOVA multiple regression model (adjusted R2 = 0.91), showed that the increase of soil moisture enhanced the toxicity of the phenanthrene towards nitrification potential and this combined moisture-phenanthrene effect was soil dependent. Therefore, the effect of the soil moisture in combination with the soil diversity should not be missed in the ecotoxicological risk assessment of the PAHs.

Dermal Microflora Restoration With Ammonia-Oxidizing Bacteria Nitrosomonas Eutropha in the Treatment of Keratosis Pilaris: A Randomized Clinical Trial.

Keratosis pilaris (KP) is a common skin finding that presents as follicular hyperkeratotic papules on the proximal extremities in patients with a propensity for atopy. Although often asymptomatic, the stippled appearance is cosmetically disturbing to patients and difficult to treat as current therapies are limited in availability and efficacy. Nitric oxide (NO) has been found to be essential in basic systemic and cutaneous physiologic function, specifically in terms of its anti-microbial and anti-inflammatory properties, which evolutionarily was maintained by ammonia-oxidizing bacteria (AOB). As modern hygiene practices have improved, there has been a gradual loss of cutaneous AOB and, therefore, the availability of an important source of human physiologic NO. We propose that restoring this dermal microflora with a purified strain of AOB, Nitrosomonas eutropha (D23), may reduce the overall cutaneous inflammatory state and, thus, be a potential therapeutic option for improving the cosmetic appearance of a skin condition such as KP which is often found in association with xerosis and atopic dermatitis. Clinical trial registry number: NCT03243617

J Drugs Dermatol. 2018;17(3):285-288.

Screening and optimizing of inhibitors for ammonia-oxidizing bacteria in sediments of malodorous river.

The proper use of selective ammonia-oxidizing archaea (AOA) and/or ammonia-oxidizing bacteria (AOB) inhibitors is critical to distinguish AOA and AOB contribution. In this research, three inhibitors including ampicillin, dicyandiamide (DCD), and allylthiourea (ATU) were examined mainly focusing on inhibiting dosage, adaptability, and effects. The results showed that the optimized inhibitory dosage of ampicillin, DCD, and ATU was separately 1.5 g L-1, 1 mM, and 25 µM. Among the three inhibitors, ATU exhibited the strongest and persistent inhibition effects and resulted in up to 90% inhibition in the AOB-enriched culture. The seemingly weakening inhibiting effects of ATU in the simulated river systems can be attributed to the involved role of AOA, the uneven spatial distribution of ATU, and protection by sediment structure in complex malodorous rivers. The high-throughput pyrosequencing analysis showed the AOB-related genus Nitrosomonas and Nitrosococcus were mostly affected by ATU in the enrichments and the river systems, respectively. The inhibition of ATU was realized mainly by reducing the abundance and activity of AOB. The decrease of the ratio of AOB/AOA amoA gene copy numbers after addition of ATU further confirmed the inhibiting effectiveness of ATU in complex microbial community of malodorous rivers.

Use of functional gene expression and respirometry to study wastewater nitrification activity after exposure to low doses of copper.

Autotrophic nitrification in biological nitrogen removal systems has been shown to be sensitive to the presence of heavy metals in wastewater treatment plants. Using transcriptase-quantitative polymerase chain reaction (RT-qPCR) data, we examined the effect of copper on the relative expression of functional genes (i.e., amoA, hao, nirK, and norB) involved in redox nitrogen transformation in batch enrichment cultures obtained from a nitrifying bioreactor operated as a continuous reactor (24-h hydraulic retention time). 16S ribosomal RNA (rRNA) gene next-generation sequencing showed that Nitrosomonas-like populations represented 60-70% of the bacterial community, while other nitrifiers represented <5%. We observed a strong correspondence between the relative expression of amoA and hao and ammonia removal in the bioreactor. There were no considerable changes in the transcript levels of amoA, hao, nirK, and norB for nitrifying samples exposed to copper dosages ranging from 0.01 to 10 mg/L for a period of 12 h. Similar results were obtained when ammonia oxidation activity was measured via specific oxygen uptake rate (sOUR). The lack of nitrification inhibition by copper at doses lower than 10 mg/L may be attributed to the role of copper as cofactor for ammonia monooxygenase or to the sub-inhibitory concentrations of copper used in this study. Overall, these results demonstrate the use of molecular methods combined with conventional respirometry assays to better understand the response of wastewater nitrifying systems to the presence of copper.

Development and Quality Control of Nanohexaconazole as an Effective Fungicide and Its Biosafety Studies on Soil Nitifiers.

The study was aimed to develop a nano form of an existing fungicide for improving plant protection and reducing crop losses caused by fungal pathogens. The protocol for the preparation and estimation of nanohexaconazole was developed. Technically pure hexaconazole was converted into its nanoform using polyethyleneglycol-400 (PEG) as the surface stabilizing agent. Nanohexaconazole was characterized using Scanning Electron Microscopy (SEM) and Dynamic Light Scattering (DLS) studies. The average particle size of nanohexaconazole was about 100 nm. An analytical method was also developed for quality control of the nanofungicide by GLC fitted with flame ionization detector. Its limit of detection was 2.5 ppm. Fungicidal potential of nanohexaconazole was better in comparison to that of conventional hexaconazole. Hydrolytic and thermal stability studies confirmed its stability at par with the conventional formulation of fungicide. Impact of nanohexaconazole on soil nitrifiers was tested in vitro and there were no significant adverse effect in their numbers observed as compared to conventional registered formulation, proving the safety of the nanofungicide.

Influence of p-cresol on the proteome of the autotrophic nitrifying bacterium Nitrosomonas eutropha C91.

In this study, the effect of the organic micropollutant and known inhibitor of nitrification, p-cresol, was investigated on the metabolism of the ammonia oxidizing bacteria (AOB) Nitrosomonas eutropha C91 using MS-based quantitative proteomics. Several studies have demonstrated that AOB are capable of biotransforming a wide variety of aromatic compounds making them suitable candidates for bioremediation, yet the underlying molecular mechanisms are poorly described. The effect of two different concentrations of the aromatic micropollutant p-cresol (1 and 10 mg L(-1)) on the metabolism of N. eutropha C91, relative to a p-cresol absent control, was investigated. Though the rate of nitrification in N. eutropha C91 appeared essentially unaffected at both concentrations of p-cresol relative to the control, the expressional pattern of the proteins of N. eutropha C91 changed significantly. The presence of p-cresol resulted in the repressed expression of several key proteins related to N-metabolism, seemingly impairing energy production in N. eutropha C91, contradicting the observed unaltered rates of nitrification. However, the expression of proteins of the TCA cycle and proteins related to xenobiotic degradation, including a p-cresol dehydrogenase, was found to be stimulated by the presence of p-cresol. This indicates that N. eutropha C91 is capable of degrading p-cresol and that it assimilates degradation intermediates into the TCA cycle. The results reveal a pathway for p-cresol degradation and subsequent entry point in the TCA cycle in N. eutropha C91. The obtained data indicate that mixotrophy, rather than cometabolism, is the major mechanism behind p-cresol degradation in N. eutropha C91.

A potentiometric flow biosensor based on ammonia-oxidizing bacteria for the detection of toxicity in water.

A flow biosensor for the detection of toxicity in water using the ammonia-oxidizing bacterium (AOB) Nitrosomonas europaea as a bioreceptor and a polymeric membrane ammonium-selective electrode as a transducer is described. The system is based on the inhibition effects of toxicants on the activity of AOB, which can be evaluated by measuring the ammonium consumption rates with the ammonium-selective membrane electrode. The AOB cells are immobilized on polyethersulfone membranes packed in a holder, while the membrane electrode is placed downstream in the flow cell. Two specific inhibitors of the ammonia oxidation-allylthiourea and thioacetamide-have been tested. The IC50 values defined as the concentration of an inhibitor causing a 50% reduction in the ammonia oxidation activity have been measured as 0.17 µM and 0.46 µM for allylthiourea and thioacetamide, respectively. The proposed sensor offers advantages of simplicity, speed and high sensitivity for measuring toxicity in water.

Effects of nitrogen dioxide and anoxia on global gene and protein expression in long-term continuous cultures of Nitrosomonas eutropha C91.

Nitrosomonas eutropha is an ammonia-oxidizing betaproteobacterium found in environments with high ammonium levels, such as wastewater treatment plants. The effects of NO(2) on gene and protein expression under oxic and anoxic conditions were determined by maintaining N. eutropha strain C91 in a chemostat fed with ammonium under oxic, oxic-plus-NO(2), and anoxic-plus-NO(2) culture conditions. Cells remained viable but ceased growing under anoxia; hence, the chemostat was switched from continuous to batch cultivation to retain biomass. After several weeks under each condition, biomass was harvested for total mRNA and protein isolation. Exposure of N. eutropha C91 to NO(2) under either oxic or anoxic conditions led to a decrease in proteins involved in N and C assimilation and storage and an increase in proteins involved in energy conservation, including ammonia monooxygenase (AmoCAB). Exposure to anoxia plus NO(2) resulted in increased representation of proteins and transcripts reflective of an energy-deprived state. Several proteins implicated in N-oxide metabolism were expressed and remained unchanged throughout the experiment, except for NorCB nitric oxide reductase, which was not detected in the proteome. Rather, NorY nitric oxide reductase was expressed under oxic-plus-NO(2) and anoxic-plus-NO(2) conditions. The results indicate that exposure to NO(2) results in an energy-deprived state of N. eutropha C91 and that anaerobic growth could not be supported with NO(2) as an oxidant.

Effect of Metasystox-R on marine Nitrosomonas sp. as a nitrification inhibitor.

Metasystox-R is a systemic soluble liquid insecticide for the control of aphids on brassica vegetable crops, cotton and lupins and it is possible enter to the marine environment and may be have a hazard effects for the marine organisms and nitrification processes. Effect of Metasystox-R on ammonia oxidizing activity by marine Nitrosomonas sp. was investigated by determining nitrification inhibitor assay in the cell suspension. Results showed that 8 microg mL(-1) of Metasystox-R with PI50 = 4.48 significantly inhibited nitrite production by marine Nitrosomonas sp. These results suggested marine Nitrosomonas sp. may be one of the target bacteria which was inhibitor and decreasing nitrification in the marine environment.

[Ammonia oxidation kinetics of ammonia oxidizer mixed culture under the conditions of O2 and trace NO2 mixed gasses].

The kinetics of the NO2-dependent ammonia oxidation was developed for ammonia oxidizer mixed culture when there was no molecular oxygen in the batch tests. The kinetics parameters were determined, where the half saturate coefficient of NO2 was 0.821 micromol x L(-1), inhibition coefficient of NO2 concentration was 1.721 micromol x L(-1), and the maximum ammonia oxidation rate were 0.144 mg x (mg x h)(-1). After adding the volume fraction of O2 was 2% to trace NO2, the ammonia oxidation rates increased obviously. The maximum ammonia oxidation rate, 0.198 mg x (mg x h)(-1) occurred under the condition of the mixed gasses containing the volume fraction of O2 was 2% and 50 x 10(-6) NO2. Under the condition of mixed gasses containing the volume fraction of O2 was 21% to trace NO2, the ammonia oxidation rates further increased greatly. The maximum ammonia oxidation rate, 0.477 mg x (mg x h)(-1) occurred when the volume fraction of O2 was 21% and 100 x 10(-6) NO2 in the mixed gas, which is 3 times higher than the general aerobic ammonia oxidation rate. The function for NO2 apparently to enhance ammonia oxidation was suggested. The kinetics model of ammonia oxidation under the conditions of O2 and trace NO2 mixed gasses was developed. The model was validated by the results of ammonia oxidation experiments under the conditions of the mixed gasses containing 2% O2 and trace NO2. The mechanism for NO2 to enhance ammonia oxidation under the conditions of O2 and trace NO2 mixed gasses was discussed.

Free ammonia and free nitrous acid inhibition on the anabolic and catabolic processes of Nitrosomonas and Nitrobacter.

The inhibitory effects of free ammonia (FA) and free nitrous acid (FNA) on the catabolic and anabolic processes of Nitrosomonas and Nitrobacter were investigated using a method that allows decoupling the growth and energy generation processes. Lab-scale sequencing batch reactors (SBRs) were operated for the enrichment of Nitrosomonas and Nitrobacter. Fluorescent In-Situ Hybridization (FISH) analysis showed that the reactors were 82% and 73% enriched with Nitrosomonas and Nitrobacter, respectively. Batch tests were carried out to measure the oxygen uptake rate (OUR) by the enriched cultures at various FA and FNA levels, in the presence (OUR with CO2 ) or absence (OUR without CO2) of inorganic carbon (CO2, HCO*3 and CO 2*3). FA up to 16.0 mgNH3-N.L(-1) was not found to have any inhibitory effect on either the catabolic or anabolic processes of the Nitrosomonas culture, but both these processes were inhibited by FNA. While an FNA level of 0.40-0.63 mgHNO2-N.L(-1) inhibited the energy production capability of Nitrosomonas by 50%, the growth process of the culture was completely inhibited by FNA at a concentration of 0.40 mgHNO2-N.L(-1). Both FA and FNA were found to have strong inhibition on the anabolic processes of Nitrobacter, but with limited inhibitory effects on the catabolism of this culture. The biosynthesis of Nitrobacter was totally inhibited at an FA level of 6.0 mgNH3-N.L(-1) (or above) or an FNA level of 0.02 mgHNO2-N.L(-1) (or above). At the same level of FA, the energy production capability of Nitrobacter was only inhibited by 12%, whereas an FNA level of up to 0.024 mgHNO2-N.L(-1) did not show any inhibition on the energy production of Nitrobacter. Further, these inhibitory effects appears to be much stronger on Nitrobacter than on Nitrosomonas, supporting that FA and FNA inhibition may play a major role in the elimination of nitrite oxidizing bacteria in processes treating wastewater containing a high level of nitrogen.

Pilot-scale studies on biological treatment of hypersaline wastewater at low temperature.

In order to investigate the feasibility of biological treatment of hypersaline wastewater produced from toilet flushing with seawater at low temperature, pilot-scale studies were established with plug-flow activated sludge process at low temperature (5-9 degrees C) based on bench-scale experiments. The critical salinity concentration of 30 g/L, which resulted from the cooperation results of the non-halophilic bacteria and the halophilic bacteria, was drawn in bench-scale experiments. Pilot-scale studies showed that high COD removal efficiency, higher than 80%, was obtained at low temperature when 30 percent seawater was introduced. The salinity improved the settleability of activated sludge, and average sludge value dropped down from 38% to 22.5% after adding seawater. Seawater salinity had a strong negative effect on notronomonas and nitrobacter growth, but much more on the nitrobacter. The nitrification action was mainly accomplished by nitrosomonas. Bench-scale experiments using two SBRs were carried out for further investigation under different conditions of salinities, ammonia loadings and temperatures. Biological nitrogen removal via nitrite pathway from wastewater containing 30 percent seawater was achieved, but the ammonia removal efficiency was strongly related not only to the influent ammonia loading at different salinities but also to temperature. When the ratio of seawater to wastewater was 30 percent, and the ammonia loading was below the critical value of 0.15 kgNH4+-N/(kgMLSS.d), the ammonia removal efficiency via nitrite pathway was above 90%. The critical level of ammonia loading was 0.15, 0.08 and 0.03 kgNH4+-N/(kgMLSS.d) respectively at the different temperature 30 degrees C, 25 degrees C and 20 degrees C when the influent ammonia concentration was 60-80 mg/L and pH was 7.5-8.0.

Effects of seawater salinity on nitrite accumulation in short-range nitrification to nitrite as end product.

The effect of seawater salinity on nitrite accumulation in short-range nitrification to nitrite as the end product was studied by using a SBR. Experimental results indicated that the growth of nitrobacteria was inhibited and very high levels of nitrite accumulation at different salinities were achieved under the conditions of 25-28 degrees C, pH 7.5-8.0, and the influent ammonia nitrogen of 40-70 mg/L when seawater flow used to flush toilet was less than 35% (salinity 12393 mg/L, Cl- 6778 mg/L) of total domestic wastewater flow, which is mainly ascribed to much high chlorine concentration of seawater. Results showed that high seawater salinity is available for short-range nitrification to nitrite as the end product. When the seawater flow used to flush toilet accounting for above 70% of the total domestic wastewater flow, the removal efficiency of ammonia was still above 80% despite the removal of organics declined obviously (less than 60%). It was found that the effect of seawater salinity on the removal of organics was negative rather than positive one as shown for ammonia removal.

Susceptibility of ammonia-oxidizing bacteria to nitrification inhibitors.

Activity of nitrification inhibitors to several typical ammonia-oxidizing bacteria isolated recently, i. e. Nitrosococcus, Nitrosolobus, Nitrosomonas, Nitrosospira and Nitrosovibrio species was assayed using 2-amino-4-methyl-trichloromethyl-1,3,5-triazine (MAST), 2-amino-4-tribromomethyl-6-trichloromethyl-1,3,5-triazine (Br-MAST), 2-chloro-6-trichloromethylpyridine (nitrapyrin) and others, and compared to confirm the adequate control of ammonia-oxidizing bacteria by the inhibitors. The order of activity of the inhibitors to 13 species of ammonia-oxidizing bacteria examined was approximately summarized as Br-MAST > or = nitrapyrin > or = MAST > other inhibitors. Two Nitrosomonas strains, N. europaea ATCC25978 and N. sp. B2, were extremely susceptible to Br-MAST, exhibiting a pI50 > or = 6.40. These values are the position logarithms of the molar half-inhibition concentration. The 16S rRNA gene sequence similarity for the highly susceptible 4 strains of genus Nitrosomonas was 94% to 100% of Nitrosomonas europaea, although those of the less susceptible 3 strains of ammonia-oxidizing bacteria, Nitrosococcus oceanus C-107 ATCC19707, Nitrosolobus sp. PJA1 and Nitrosolobus multiformis ATCC25196, were 77.85, 91.53 and 90.29, respectively. However, no clear correlation has been found yet between pI50-values and percent similarity of 16S rRNA gene sequence among ammonia-oxidizing bacteria.

Excision and integration of cassettes by an integron integrase of Nitrosomonas europaea.

We found in the environmental strain Nitrosomonas europaea a chromosomal integron-like structure with an integrase gene, intI(Neu). We have tested the capacity of the IntINeu integrase to excise and integrate several resistance gene cassettes. The results allow us to consider IntINeu a new functional integron integrase.

Ammonia oxidation by Nitrosomonas eutropha with NO(2) as oxidant is not inhibited by acetylene.

The effect of acetylene ((14)C(2)H(2)) on aerobic and anaerobic ammonia oxidation by Nitrosomonas eutropha was investigated. Ammonia monooxygenase (AMO) was inhibited and a 27 kDa polypeptide (AmoA) was labelled during aerobic ammonia oxidation. In contrast, anaerobic, NO(2)-dependent ammonia oxidation (NO(2)/N(2)O(4) as oxidant) was not affected by acetylene. Further studies gave evidence that the inhibition as well as the labelling reaction were O(2)-dependent. Cells pretreated with acetylene under oxic conditions were unable to oxidize ammonia with O(2) as oxidant. After these cell suspensions were supplemented with gaseous NO(2), ammonia oxidation activity of about 140 micromol NH(4)(+) (g protein)(-1) h(-1) was detectable under both oxic and anoxic conditions. A significantly reduced acetylene inhibition of the ammonia oxidation activity was observed for cells incubated in the presence of NO. This suggests that NO and acetylene compete for the same binding site on AMO. On the basis of these results a new hypothetical model of ammonia oxidation by N. eutropha was developed.

Effects of gaseous NO2 on cells of Nitrosomonas eutropha previously incapable of using ammonia as an energy source.

Cells of Nitrosomonas eutropha grown under anoxic conditions with hydrogen as electron donor and nitrite as electron acceptor were initially unable to oxidize ammonia (ammonium) and hydroxylamine when transferred to oxic conditions. Recovery of ammonia and hydroxylamine oxidation activity was dependent on the presence of NO2. Under oxic conditions, without addition of NO2, ammonia consumption started after 8 - 9 days, and small amounts of NO and NO2 were detectable in the gas atmosphere. Removing these nitrogen oxides by intensive aeration, ammonia oxidation activity decreased and broke off after 15 days. Addition of gaseous NO2 (25 ppm1) led to a fast recovery of ammonia oxidation (3 days). Simultaneously, the arrangement of intracytoplasmic membranes (ICM) changed from circular to flattened vesicles, the protein pattern revealed an increase in the concentration of a 27 and a 30 kDa polypeptide, and the cytochrome c content increased significantly.

Toxic effects of linear alkylbenzene sulfonate on metabolic activity, growth rate, and microcolony formation of Nitrosomonas and Nitrosospira strains.

Strong inhibitory effects of the anionic surfactant linear alkylbenzene sulfonate (LAS) on four strains of autotrophic ammonia-oxidizing bacteria (AOB) are reported. Two Nitrosospira strains were considerably more sensitive to LAS than two Nitrosomonas strains were. Interestingly, the two Nitrosospira strains showed a weak capacity to remove LAS from the medium. This could not be attributed to adsorption or any other known physical or chemical process, suggesting that biodegradation of LAS took place. In each strain, the metabolic activity (50% effective concentration [EC(50)], 6 to 38 mg liter(-1)) was affected much less by LAS than the growth rate and viability (EC(50), 3 to 14 mg liter(-1)) were. However, at LAS levels that inhibited growth, metabolic activity took place only for 1 to 5 days, after which metabolic activity also ceased. The potential for adaptation to LAS exposure was investigated with Nitrosomonas europaea grown at a sublethal LAS level (10 mg liter(-1)); compared to control cells, preexposed cells showed severely affected cell functions (cessation of growth, loss of viability, and reduced NH(4)(+) oxidation activity), demonstrating that long-term incubation at sublethal LAS levels was also detrimental. Our data strongly suggest that AOB are more sensitive to LAS than most heterotrophic bacteria are, and we hypothesize that thermodynamic constraints make AOB more susceptible to surfactant-induced stress than heterotrophic bacteria are. We further suggest that AOB may comprise a sensitive indicator group which can be used to determine the impact of LAS on microbial communities.

A bioluminescence assay using Nitrosomonas europaea for rapid and sensitive detection of nitrification inhibitors.

An expression vector for the luxAB genes, derived from Vibrio harveyi, was introduced into Nitrosomonas europaea. Although the recombinant strain produced bioluminescence due to the expression of the luxAB genes under normal growing conditions, the intensity of the light emission decreased immediately, in a time-and dose-dependent manner, with the addition of ammonia monooxygenase inhibitors, such as allylthiourea, phenol, and nitrapyrin. When whole cells were challenged with several nitrification inhibitors and toxic compounds, a close relationship was found between the change in the intensity of the light emission and the level of ammonia-oxidizing activity. The response of bioluminescence to the addition of allylthiourea was considerably faster than the change in the ammonia-oxidizing rate, measured as both the O2 uptake and NO2- production rates. The bioluminescence of cells inactivated by ammonia monooxygenase inhibitor was recovered rapidly by the addition of certain substrates for hydroxylamine oxidoreductase. These results suggested that the inhibition of bioluminescence was caused by the immediate decrease of reducing power in the cell due to the inactivation of ammonia monooxygenase, as well as by the destruction of other cellular metabolic pathways. We conclude that the assay system using luminous Nitrosomonas can be applied as a rapid and sensitive detection test for nitrification inhibitors, and it will be used to monitor the nitrification process in wastewater treatment plants.