Inhibition Of Ammonia Oxidation Research Articles

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 Caffeic acid, curcumin, methylene blue hydrate and trolox were tested onNitrosopumilus maritimus, two unpublished AOA representatives (AOA-6f and AOA-G6) as well as the AOB representative Nitrosomonas europaea All four scavengers inhibited ammonia oxidation by AOA at lower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.

Kinetic Constants for Biological Ammonium and Nitrite Oxidation Processes Under Sulfide Inhibition.

Inhibition of nitrification by sulfide was assessed using sludge obtained from a steady-state nitrifying reactor. Independent batch activity assays were performed with ammonium and nitrite as substrate, in order to discriminate the effect of sulfide on ammonium and nitrite oxidation. In the absence of sulfide, substrate affinity constants (K S,NH4 = 2.41 ± 0.11mg N/L; K s, NO2 = 0.74 ± 0.03mg N/L) and maximum specific rates (q max,NH4 = 0.086 ± 0.008mg N/mg microbial proteinh; q max,NO2 = 0.124 ± 0.001mg N/mg microbial proteinh) were determined. Inhibition of ammonium oxidation was no-competitive (inhibition constant (K i , NH4 ) of 2.54 ± 0.12mg HS(-)-S/L) while inhibition of nitrite oxidation was mixed (competitive inhibition constant (K' i , NO2 ) of 0.22 ± 0.03mg HS(-)-S/L and no-competitive inhibition constant (K i , NO2 ) of 1.03 ± 0.06mg HS(-)-S/L). Sulfide has greater inhibitory effect on nitrite oxidation than ammonium oxidation, and its presence in nitrification systems should be avoided to prevent accumulation of nitrite. By simulating the effect of sulfide addition in a continuous nitrifying reactor under steady-state operation, it was shown that the maximum sulfide concentration that the sludge can tolerate without affecting the ammonium consumption efficiency and nitrate yield is 1mg HS(-)-S/L.

Removal of triclosan in nitrifying activated sludge: Effects of ammonia amendment and bioaugmentation

This study investigated two possible strategies, increasing ammonia oxidation activity and bioaugmenting with triclosan-degrader Sphingopyxis strain KCY1, to enhance triclosan removal in nitrifying activated sludge (NAS). Triclosan (2mgL−1) was removed within 96-h in NAS bioreactors amended with 5, 25 and 75mgL−1 of ammonium (NH4–N). The fastest triclosan removal was observed in 25mg NH4–NL−1 amended-bioreactors where high ammonia oxidation occurred. Inhibition of ammonia oxidation and slower triclosan removal were observed in 75mg NH4–NL−1 amended-bioreactors. Triclosan removal was correlated to the molar ratio of the amount of nitrate produced to the amount of ammonium removed. Bioaugmentation with strain KCY1 did not enhance triclosan removal in the bioreactors with active ammonia oxidation. Approximately 36–42% and 59% of triclosan added were removed within 24-h by ammonia-oxidizing bacteria and unknown triclosan-degrading heterotrophs, respectively. The results suggested that increasing ammonia oxidation activity can be an effective strategy to enhance triclosan removal in NAS.

Short-Term Effects of Biogas Digestates and Pig Slurry Application on Soil Microbial Activity

The effect of four biogas digestates (BD-A, BD-B, BD-C, and BD-D) and pig slurry (PS) on soil microbial functions was assessed at application rates corresponding to 0–1120 kgNH4+-N ha−1. At dose corresponding to 140 kgNH4+-N ha−1, 30.9–32.5% of the carbon applied in BD-A, BD-C, and PS was utilized during 12 days, while for BD-B and BD-D corresponding utilization was 19.0 and 16.9%, respectively. All BDs resulted in net nitrogen assimilation at low rates (17.5–140 kgNH4+-N ha−1) but net mineralization dominated at higher rates. PS resulted in net mineralization at all application rates. All residues inhibited potential ammonium oxidation (PAO), with EC50-values ranging between 45 and 302 kgNH4+-N ha−1. Low rates of BDs appeared to weakly stimulate potential denitrification activity (PDA), while higher rates resulted in logarithmic decrease. The EC50-values for PDA were between 238 and 347 kgNH4+-N ha−1. No inhibition of PDA was observed after amendment with PS. In conclusion, biogas digestates inhibited ammonia oxidation and denitrification, which could be an early warning of potential hazardous substances in the digestates. However, this effect can also be regarded as positive, since it may reduce nitrogen losses.

Impact of TiO₂ and ZnO nanoparticles at predicted environmentally relevant concentrations on ammonia-oxidizing bacteria cultures under ammonia oxidation.

Increased application of titanium dioxide and zinc oxide nanoparticles (nano-TiO2 and nano-ZnO) raises concerns related to their environmental impacts. The effects that such nanoparticles have on environmental processes and the bacteria that carry them out are largely unknown. In this study, ammonia-oxidizing bacteria (AOB) enrichment cultures, grown from surface sediments taken from an estuary wetland in Fujian Province, China, were spiked with nano-TiO2 and nano-ZnO (with an average size of 32 and 43 nm, respectively) at predicted environmentally relevant concentrations (≤2 mg L(-1)) to determine their impacts on ammonia oxidation and the mechanisms involved. Results showed that higher nano-TiO2 concentrations significantly inhibited ammonia oxidation in enrichment cultures. It is noteworthy that the average ammonia oxidation rate was significantly correlated to the Shannon index, the Simpson's index, and AOB abundance. This suggested that ammonia oxidation inhibition primarily resulted from a reduction of AOB biodiversity and abundance. However, AOB biodiversity and abundance as well as the average ammonia oxidation rate were not inhibited by nano-ZnO at predicted environmentally relevant concentrations. Accordingly, an insignificant correlation was established between biodiversity and abundance of the AOB amoA gene and the average ammonia oxidation rate under nano-ZnO treatments. AOB present in samples belonged to the β-Proteobacteria class with an affinity close to Nitrosospira and Nitrosomonas genera. This suggested that identified impacts of nano-TiO2 and nano-ZnO on ammonia oxidation processes can be extrapolated to some extent to natural aquatic environments. Complex impacts on AOB may result from different nanomaterials present in aquatic environments at various ambient conditions. Further investigation on how and to what extent different nanomaterials influence AOB diversity and abundance and their subsequent ammonia oxidation processes is therefore required.

A Mesophilic, Autotrophic, Ammonia-Oxidizing Archaeon of Thaumarchaeal Group I.1a Cultivated from a Deep Oligotrophic Soil Horizon

Soil nitrification plays an important role in the reduction of soil fertility and in nitrate enrichment of groundwater. Various ammonia-oxidizing archaea (AOA) are considered to be members of the pool of ammonia-oxidizing microorganisms in soil. This study reports the discovery of a chemolithoautotrophic ammonia oxidizer that belongs to a distinct clade of nonmarine thaumarchaeal group I.1a, which is widespread in terrestrial environments. The archaeal strain MY2 was cultivated from a deep oligotrophic soil horizon. The similarity of the 16S rRNA gene sequence of strain MY2 to those of other cultivated group I.1a thaumarchaeota members, i.e., Nitrosopumilus maritimus and "Candidatus Nitrosoarchaeum koreensis," is 92.9% for both species. Extensive growth assays showed that strain MY2 is chemolithoautotrophic, mesophilic (optimum temperature, 30°C), and neutrophilic (optimum pH, 7 to 7.5). The accumulation of nitrite above 1 mM inhibited ammonia oxidation, while ammonia oxidation itself was not inhibited in the presence of up to 5mM ammonia. The genome size of strain MY2 was 1.76 Mb, similar to those of N. maritimus and "Ca. Nitrosoarchaeum koreensis," and the repertoire of genes required for ammonia oxidation and carbon fixation in thaumarchaeal group I.1a was conserved. A high level of representation of conserved orthologous genes for signal transduction and motility in the noncore genome might be implicated in niche adaptation by strain MY2. On the basis of phenotypic, phylogenetic, and genomic characteristics, we propose the name "Candidatus Nitrosotenuis chungbukensis" for the ammonia-oxidizing archaeal strain MY2.

Inhibition of ammonia oxidation in the thermophilic ammonia‐oxidizing archaeon Nitrosocaldus yellowstonii (617.6)

Molecular data suggests ammonia‐oxidizing archaea (AOA) play an important role in nitrogen cycling. In particular, organisms related to the thermophilic archaeon Nitrosocaldus yellowstonii appear to be the only nitrifiers present in terrestrial hydrothermal systems at temperatures above 65°C. To better understand the contribution these thermophilic archaeal nitrifiers have in geochemical processes such as nitrogen cycling, we need tools to manipulate their metabolic activity in laboratory cultures and in situ. Nitrosocaldus yellowstonii HL72 is a thermophilic AOA cultivated from a hot spring in Yellowstone National Park (Wyoming, USA). To understand how N. yellowstonii contributes to nitrogen cycling in hot springs, we must first characterize the effect of nitrification inhibitors on its metabolic function. In this study, we have focused on allylthiourea (ATU), a commonly‐used inhibitor of nitrification. We have shown that ATU is stable at temperatures up to 72°C, the growth temperature of N. yellowstonii. Ammonia oxidation in N. yellowstonii is completely inhibited by ATU concentrations of 1 mM, concentrations that are approximately 100 times higher than those required to inhibit ammonia oxidation in bacteria. Addition of 1 mM ATU decreased the rate of nitrite production of N. yellowstonii from 0.039 ± 0.001 mM/h‐1 prior to addition, to 0.001 ± 0.001 mM/h‐1. Because nitrite is used for a proxy of growth, quantitative PCR was used to measure cell numbers for the single copy gene amoA per cell. There was no change in amoA copy number after the addition of 1 mM ATU while the unamended control continued to exponentially increase, further confirming that the addition of 1 mM ATU halted cell growth completely. RT‐qPCR showed amoA expression decreased 23.5% and 47.9% compared to the unamended control, two and four hours after ATU addition. We conclude that ATU is an effective inhibitor of AOA cell growth and is will be a useful tool to manipulate metabolic activity of AOA in hot spring systems.Grant Funding Source: Supported by NIH MARC T34‐GM00857

Transcriptional responses of bacterial amoA gene to dimethyl sulfide inhibition in complex microbial communities

This study presented an approach by combining the real-time reverse transcription polymerase chain reaction with the terminal restriction fragment length polymorphism (T-RFLP) to investigate transcriptional responses of ammonia-oxidizing bacteria (AOB) to dimethyl sulfide (DMS) inhibition. Batch experiments with added ammonium and DMS were conducted using three activated sludges and Nitrosomonas europaea, and the transcriptional responses of the amo subunit A (amoA) mRNA were evaluated. It was found that DMS inhibited ammonium oxidation and amoA mRNA expression in all batch experiments but the inhibition degree observed was different for different sludges examined. It is likely that the different inhibitory effects of DMS on ammonium oxidation and amoA mRNA expression stemmed from different dominant AOB populations in the sludges. The T-RFLP results for amoA mRNA suggested that inhibition of ammonium oxidation by DMS to Nm. europaea-like AOB with T-RF 219/270 is relatively minor compared to other AOB populations in the examined sludges, such as Nm. europaea-like AOB with T-RF 491/491.

Responses of the terrestrial ammonia-oxidizing archaeonCa. Nitrososphaera viennensis and the ammonia-oxidizing bacteriumNitrosospira multiformisto nitrification inhibitors

Nitrification inhibitors have been used for decades to improve nitrogen fertilizer utilization in farmland. However, their effect on ammonia-oxidizing Archaea (AOA) in soil is little explored. Here, we compared the impact of diverse inhibitors on nitrification activity of the soil archaeon Ca. Nitrososphaera viennensis EN76 and compared it to that of the ammonia-oxidizing bacterium (AOB) Nitrosospira multiformis. Allylthiourea, amidinothiourea, and dicyandiamide (DCD) inhibited ammonia oxidation in cultures of both N. multiformis and N. viennensis, but the effect on N. viennensis was markedly lower. In particular, the effective concentration 50 (EC50) of allylthiourea was 1000 times higher for the AOA culture. Among the tested nitrification inhibitors, DCD was the least potent against N. viennensis. Nitrapyrin had at the maximal soluble concentration only a very weak inhibitory effect on the AOB N. multiformis, but showed a moderate effect on the AOA. The antibiotic sulfathiazole inhibited the bacterium, but barely affected the archaeon. Only the NO-scavenger carboxy-PTIO had a strong inhibitory effect on the archaeon, but had little effect on the bacterium in the concentrations tested. Our results reflect the fundamental metabolic and cellular differences of AOA and AOB and will be useful for future applications of inhibitors aimed at distinguishing activities of AOA and AOB in soil environments.

Selective Inhibition of Ammonia Oxidation and Nitrite Oxidation Linked to N2O Emission with Activated Sludge and Enriched Nitrifiers

Nitrification in wastewater treatment emits a significant amount of nitrous oxide (N2O), which is one of the major greenhouse gases. However, the actual mechanism or metabolic pathway is still largely unknown. Selective nitrification inhibitors were used to determine the nitrification steps responsible for N2O emission with activated sludge and enriched nitrifiers. Allylthiourea (86 microM) completely inhibited ammonia oxidation and N2O emission both in activated sludge and enriched nitrifiers. Sodium azide (24 microM) selectively inhibited nitrite oxidation and it led to more N2O emission than the control experiment both in activated sludge and enriched nitrifiers. The inhibition tests showed that N2O emission was mainly related to the activity of ammonia oxidizers in aerobic condition, and the inhibition of ammonia monooxygenase completely blocked N2O emission. On the other hand, N2O emission increased significantly as the nitrogen flux from nitrite to nitrate was blocked by the selective inhibition of nitrite oxidation.

Inhibition of ammonium oxidation by a liquid formulation of 3,4-Dimethylpyrazole phosphate (DMPP) compared with a dicyandiamide (DCD) solution in six new Zealand grazed grassland soils

Purpose The oxidation of ammonium (NH4+) to nitrate (NO3−) in the soil is an important biogeochemical process, which has major environmental implications as it can contribute to NO3− leaching and nitrous oxide (N2O) emissions. Nitrification inhibitors have been used to slow down this process to reduce NO3− leaching and N2O emissions from agricultural land. The objective of this study was to determine the effectiveness of a liquid formulation of 3,4-Dimethylpyrazole phosphate (DMPP) compared with a dicyandiamide (DCD) solution in inhibiting the growth of ammonium-oxidizing bacteria (AOB) and ammonium oxidizing archaea (AOA) and slowing down the rate of NH4+ oxidation in soil.

Nitrogen removal from wastewater with a low COD/N ratio at a low oxygen concentration

The goal of the study was to determine the effectiveness of nitrification and denitrification and the kinetics of ammonia removal from a mixture of wastewater and anaerobic sludge digester supernatant in an SBR at limited oxygen concentration. In addition, the COD removal efficiency and sludge production were assessed.In the SBR cycle alternating aerobic and anaerobic phases occurred; in the aeration phase the dissolved oxygen (DO) concentration was below 0.7mg O2/L. The low DO concentration did not inhibit ammonia oxidation–nitrification and the efficiency was ca. 96–98%. However, a relatively high COD concentration in the effluent was detected. The values of Km and Vmax, calculated from the Michaelis–Menten equation, were 43mg N–NH4/L and 15.64mg N–NH4/Lh, respectively. Activated sludge production was almost stable (0.62–0.66g MLVSS/g COD). A high net biomass production resulted from a low specific biomass decay rate of 0.0015d−1.

Inhibition of Nitrification of Ammonia-Rich Wastewater in Immobilized Nitrifiers System

Inhibition of ammonia oxidation and nitrite oxidation was studied in an immobilized biomass system. Ammonia oxidizing bacteria (AOB) and nitrite oxidizing bacteria (NOB) suffered different inhibition in ammonia-rich wastewater. NOB started to be inhibited when FA (free ammonia) was 2mg/L, and was totally inhibited when FA was 8mg/L. AOB started to be inhibited when FA was 22mg/L, and tended to lose activity when FA was higher than 170mg/L. Inhibition kinetics of ammonia oxidation and nitrite oxidation could be described by Haldane model. Immobilization alleviated inhibition of FA to nitrifiers, maintaining high activity even at high strength ammonia solution. Partial nitrification could be achieved by varying degrees of inhibition of FA to AOB and NOB.

Aerobic cometabolic degradation of trichloroethene by methane and ammonia oxidizing microorganisms naturally associated with Carex comosa roots

The degradation potential of trichloroethene by the aerobic methane- and ammonia-oxidizing microorganisms naturally associated with wetland plant (Carex comosa) roots was examined in this study. In bench-scale microcosm experiments with washed (soil free) Carex comosa roots, the activity of root-associated methane- and ammonia-oxidizing microorganisms, which were naturally present on the root surface and/or embedded within the roots, was investigated. Significant methane and ammonia oxidation were observed reproducibly in batch reactors with washed roots incubated in growth media, where methane oxidation developed faster (2 weeks) compared to ammonia oxidation (4 weeks) in live microcosms. After enrichment, the methane oxidizers demonstrated their ability to degrade 150 μg l(-1) TCE effectively at 1.9 mg l(-1) of aqueous CH(4). In contrast, ammonia oxidizers showed a rapid and complete inhibition of ammonia oxidation with 150 μg l(-1) TCE at 20 mg l(-1) of NH(4)(+)-N, which may be attributed to greater sensitivity of ammonia oxidizers to TCE or its degradation product. No such inhibitory effect of TCE degradation was detected on methane oxidation at the above experimental conditions. The results presented here suggest that microorganisms associated with wetland plant roots can assist in the natural attenuation of TCE in contaminated aquatic environments.

Diversity of nitrifying bacteria in a full-scale petroleum refinery wastewater treatment plant experiencing unstable nitrification

We have investigated bacterial populations relevant to nitrification in a full-scale activated sludge plant receiving wastewater from a petroleum refinery showing unstable nitrification. Inhibition of ammonia oxidation was related to phenol concentration according to a model of non-competitive inhibition. While the number of ammonia-oxidizing bacteria (AOB) did not correlate with nitrification performance, the total number of nitrite-oxidizing bacteria (NOB) dropped considerably during periods of nitrite accumulation or no nitrification. Diversity of nitrifiers in the sludge of the full-scale facility was examined at a time of full nitrification with the construction of clone libraries of ammonia monooxygenase ( amoA) gene and of the 16S rRNA gene of NOB. Nucleotide sequences of amoA gene belonged to one dominant population, associated with Nitrosomonas europaea, and to a minor population related to the Nitrosomonas nitrosa lineage. The majority of sequences retrieved in the NOB-like clone library also clustered within a single operational taxonomic unit. The high dominance of Nitrobacter over Nitrospira and the low diversity of nitrifying bacteria observed in this wastewater treatment plant might account for the increased risk of failure in the presence of disturbances.

Modeling kinetics of ammonium oxidation and nitrite oxidation under simultaneous inhibition by free ammonia and free nitrous acid

Inhibition of ammonium oxidation and nitrite oxidation by free ammonia (FA) and free nitrous acid (FNA) was studied using three different sludges. An uncompetitive inhibition model fit the experimental data well when the reactions were under FA inhibition, whereas a noncompetitive model fit well under FNA inhibition. The estimates of the inhibition constant ( K I) of nitrite oxidation were 46 μM for FA and 1.7–6.8 μM for FNA, each of which was significantly smaller than that of ammonium oxidation, which were 290–1600 μM for FA and 12 μM for FNA. The much smaller values of K I for nitrite oxidation reflected the susceptibility of that reaction to inhibition by FA and FNA, which could lead to accumulation of nitrite during nitrification. A kinetic model for simultaneous inhibition by FA and FNA was derived. The model predicted that nitrite oxidation should be affected more seriously than ammonium oxidation by the simultaneous inhibition, which would accelerate the accumulation of nitrite in a strong nitrogenous wastewater treatment. It also indicated that a complete removal of ammonia could be achieved with high accumulation of nitrite in a sequencing batch reactor, which is impossible in a continuous-flow reactor.

Physiological and Transcriptional Responses of Nitrosomonas europaea to Toluene and Benzene Inhibition

Ammonia oxidizing bacteria (AOB) are inhibited by many compounds found in wastewater treatment plant (WWTP) influent, including aromatic hydrocarbons. The detection of "sentinel genes" to identify the presence of aromatic hydrocarbons could be useful to WWTP operators. In this study, the transcriptomic responses of Nitrosomonas europaea during the cometabolism of benzene to phenol and toluene to benzyl alcohol and benzaldehyde were evaluated using whole genome Affymetrix microarrays and qRT-PCR. Benzyl alcohol and benzaldehyde were found not to inhibit N. europaea. However, phenol concentrations as low as 5 microM directly inhibited ammonia oxidation. Surprisingly, there were no significant up- or down-regulation of genes in N. europaea cells exposed to 20 microM toluene, which caused 50% inhibition of ammonia oxidation. Exposing N. europaea to 40 microM benzene, which caused a similar degree of inhibition, resulted in the up-regulation of seven adjacent genes, including NE 1545 (a putative pirin protein) and NE 1546 (a putative membrane protein), that appear to be involved with fatty-acid metabolism, lipid biosynthesis, and membrane protein synthesis. qRT-PCR analysis revealed that NE 1545 and NE 1546 were significantly up-regulated upon exposure to benzene and phenol, but not upon exposure to toluene. Transmission electron microscope images revealed a shift in outer cell structure in response to benzene exposure.

Investigation of Nitrification and Nitrogen Removal from Centrate in a Submerged Attached-Growth Bioreactor

The purpose of this study was to investigate the nitrification and nitrogen removal from centrate produced in the dewatering process of anaerobically digested sludge, using a single-unit, single-zone submerged attached-growth bioreactor. The nitrogen loading varied from 0.54 to 1.51 kg-N/m3 x d. Stable ammonia oxidation (nitritification) to nitrite was demonstrated. A nitritification efficiency of 98% was achieved, while the denitrification efficiency varied from 84 to 99% (with methanol). The average total nitrogen removal was 85%. Inhibition of nitrite oxidation by a limited penetration of dissolved oxygen into the biofilm and free ammonia resulted in the accumulation of nitrite, while inhibition of ammonia oxidation by free nitrous acid did not occur. The quantity of biomass, in terms of volatile solids, ranged from 10123 to 16034 mg-VS/L of media.

Kinetic Modeling of Inhibition of Ammonia Oxidation by Nitrite under Low Dissolved Oxygen Conditions

In recent years the application of partial nitrification techniques has been denoted as very promising. These methods are based on the oxidation of ammonia to nitrite and the inhibition of the nitratation using different strategies. In most cases, this inhibition causes an increase in the concentration of nitrite. However, the effect of high nitrite concentrations under low dissolved oxygen (DO) conditions on the nitrification process is not well understood. In this paper, the effect of ammonia, nitrite, and nitrate concentrations on the nitrification process under low dissolved oxygen concentrations were studied using respirometric techniques. Results showed that the specific oxygen uptake rate (SOUR) followed a Monod-type equation with respect to the DO concentration. The coefficient SOURm was constant with respect to the ammonia, nitrite, and nitrate within the tested concentrations; in addition, KO was constant with respect to ammonia and nitrate but it increased linearly with the nitrite concentratio...

Response of Nitrosomonas, Nitrobacter and Escherichia coli to drilling fluids

Inhibition of ammonia oxidation by Nitrosomonas, nitrite oxidation by Nitrobacterand carbon IV oxide evolution by Escherichia coli were employed as indices to determine the effect of four drilling fluids (Paradril, IMC0-W, IMC0-O and Gel/KCL/polymer to these bacteria. The percentage survival (determined by aerobic spread plate count) of these bacteria when exposed to these fluids was also used as toxicity index. Ammonia oxidation (nitrite accumulation) and nitrite oxidation (consumption) were determined by coupling of diazotised sulfanilic acid with a-napthyl-ethylene diamine dihydrochloride. Carbon IV oxide evolution was monitored by passing air from inoculated lactose broth into calcium hydroxide solution and titration with sulphuric acid. Toxicity indices were determined throughout the following exposure period (0, 1, 2, 3, 4, 8, 12, 24, 36 and 48 h) in static shake flask systems. Percentage survival decreased with increasing concentration of the drilling fluids and increasing exposure period of the three bacteria especially at high concentrations (10.0, 100 and 1000 mg/l). At lower concentrations (0.01, 0.1 and 1.0 mg/l) some fluids stimulated bacterial growth. Controls revealed bacterial growth through out the exposure period. Nitrite accumulation by Nitrosomonas, nitrite consumption by Nitrobacter and carbon IV oxide evolution by E. coli decreased with increase in concentration and exposure time of the four fluids. Controls revealed an increase in these indices with increase in exposure period. Results showed that the drilling fluids inhibited the two stages of the nitrification in the nitrogen cycle and respiration (by E. coli). This may affect the agricultural productivity of ecosystems in the Niger Delta where they are employed by causing dislocations in the nitrogen cycle where nitrifying and aerobic bacteria participate. Key words: Nitrite accumulation, nitrite consumption, respiration, drilling fluids.