氨氮对3种吸附态亚硝化单胞菌的抑制动力学

[背景] 氨氮浓度会明显影响亚硝化单胞菌的活性,但氨氮浓度对吸附态亚硝化单胞菌菌种的抑制动力学尚缺乏研究。[目的] 研究氨氮浓度对3种吸附态亚硝化单胞菌（Nitrosomonas eutropha CZ-4、Nitrosomonas halophila C-19和Nitrosomonas europaea SH-3）的影响。[方法] 以碳酸钙作为吸附基质,设定氨氮浓度为25-1 000 mg/L,测定3种亚硝化单胞菌（N.eutropha CZ-4、N. halophila C-19和N. europaea SH-3）的亚硝氮积累速率与最大比生长速率,并通过Edwares2模型建立氨氧化的抑制动力学方程。[结果] N. halophila C-19在初始氨氮浓度为50-100 mg/L时的亚硝氮积累最快,N. europaea SH-3的亚硝氮积累则在初始氨氮浓度为50-200 mg/L时最快,而N. eutropha CZ-4则适于在初始氨氮浓度为50-400 mg/L时积累亚硝氮;N. eutropha CZ-4的最大比生长速率出现在初始氨氮浓度为50-400 mg/L时,明显高于N. halophila C-19（25-100 mg/L）,而N. europaea SH-3的生长速度在初始氨氮浓度为50-800 mg/L区间内无显著差异;N. europaea SH-3的K_I（922.76 mg/L）显著高于N. eutropha CZ-4（597.88 mg/L）,而CZ-4的K_I又显著高于N. halophila C-19（186.24 mg/L）,N. europaea SH-3的K_m（72.06 mg/L）显著高于N. halophila C-19（23.23 mg/L）。[结论] 3种吸附态亚硝化单胞菌的生长和氨氧化对氨氮浓度变化的响应存在明显差异,对于认识不同亚硝化单胞菌在不同氨氮浓度污水中的功能并开发相应的工程技术具有重要意义。     

Role of nitrite reductase in the ammonia-oxidizing pathway of <Emphasis Type="Italic">Nitrosomonas europaea</Emphasis>

Metabolism of ammonia (NH₃) and hydroxylamine (NH₂OH) by wild-type and a nitrite reductase (nirK) deficient mutant of Nitrosomonas europaea was investigated to clarify the role of NirK in the NH₃ oxidation pathway. NirK-deficient N. europaea grew more slowly, consumed less NH₃, had a lower rate of nitrite (NO₂ ⁻) production, and a significantly higher rate of nitrous oxide (N₂O) production than the wild-type when incubated with NH₃ under high O₂ tension. In incubations with NH₃ under low O₂ tension, NirK-deficient N. europaea grew more slowly, but had only modest differences in NH₃ oxidation and product formation rates relative to the wild-type. In contrast, the nirK mutant oxidized NH₂OH to NO₂ ⁻ at consistently slower rates than the wild-type, especially under low O₂ tension, and lost a significant pool of NH₂OH–N to products other than NO₂ ⁻ and N₂O. The rate of N₂O production by the nirK mutant was ca. three times higher than the wild-type during hydrazine-dependent NO₂ ⁻ reduction under both high and low O₂ tension. Together, the results indicate that NirK activity supports growth of N. europaea by supporting the oxidation of NH₃ to NO₂ ⁻ via NH₂OH, and stimulation of hydrazine-dependent NO₂ ⁻ reduction by NirK-deficient N. europaea indicated the presence of an alternative, enzymatic pathway for N₂O production.     

Inhibition of Chemoautotrophic Nitrification by Sodium Chlorate and Sodium Chlorite: a Reexamination

The oxidation of NH₄⁺ by Nitrosomonas europaea was insensitive to 10 mM NaClO₃ (sodium chlorate) but was strongly inhibited by NaClO₂ (sodium chlorite; K_i, 2 μM). The oxidation of NO₂⁻ by Nitrobacter winogradskyi was inhibited by both ClO₃⁻ and ClO₂⁻ (K_i for ClO₂⁻, 100 μM). N. winogradskyi reduced ClO₃⁻ to ClO₂⁻ under both aerobic and anaerobic conditions, and as much as 0.25 mM ClO₂⁻ was detected in the culture filtrate. In mixed N. europaea-N. winogradskyi cell suspensions, the oxidation of both NH₄⁺ and NO₂⁻ was inhibited in the presence of 10 mM ClO₃⁻ after a 2-h lag period, despite the fact that, under these conditions, ClO₂⁻ was not detected in the filtrate. The data are consistent with the hypothesis that, in mixed culture, NH₄⁺ oxidation is inhibited by ClO₂⁻ produced by reduction of ClO₃⁻ by the NO₂⁻ oxidizer. The use of ClO₃⁻ inhibition of NO₂⁻ oxidation in assays of nitrification by mixed populations necessitates cautious interpretation unless it can be shown that the oxidation of NH₄⁺ is not affected.     

Selective Inhibition of Ammonium Oxidation and Nitrification-Linked N2O Formation by Methyl Fluoride and Dimethyl Ether

Methyl fluoride (CH₃F) and dimethyl ether (DME) inhibited nitrification in washed-cell suspensions of Nitrosomonas europaea and in a variety of oxygenated soils and sediments. Headspace additions of CH₃F (10% [vol/vol]) and DME (25% [vol/vol]) fully inhibited NO₂^- and N₂O production from NH₄⁺ in incubations of N. europaea, while lower concentrations of these gases resulted in partial inhibition. Oxidation of hydroxylamine (NH₂OH) by N. europaea and oxidation of NO₂^- by a Nitrobacter sp. were unaffected by CH₃F or DME. In nitrifying soils, CH₃F and DME inhibited N₂O production. In field experiments with surface flux chambers and intact cores, CH₃F reduced the release of N₂O from soils to the atmosphere by 20- to 30-fold. Inhibition by CH₃F also resulted in decreased NO₃^- + NO₂^- levels and increased NH₄⁺ levels in soils. CH₃F did not affect patterns of dissimilatory nitrate reduction to ammonia in cell suspensions of a nitrate-respiring bacterium, nor did it affect N₂O metabolism in denitrifying soils. CH₃F and DME will be useful in discriminating N₂O production via nitrification and denitrification when both processes occur and in decoupling these processes by blocking NO₂^- and NO₃^- production.     

Chemoorganoheterotrophic Growth of <Emphasis Type="Italic">Nitrosomonas europaea</Emphasis> and <Emphasis Type="Italic">Nitrosomonas eutropha</Emphasis>

The ammonia oxidizers Nitrosomonas europaea and Nitrosomonas eutropha are able to grow chemoorganotrophically under anoxic conditions with pyruvate, lactate, acetate, serine, succinate, α-ketoglutarate, or fructose as substrate and nitrite as terminal electron acceptor. The growth yield of both bacteria is about 3.5 mg protein (mmol pyruvate)⁻¹ and the maximum growth rates of N. europaea and N. eutropha are 0.094 d⁻¹ and 0.175 d⁻¹, respectively. In the presence of pyruvate and CO₂ about 80% of the incorporated carbon derives from pyruvate and about 20% from CO₂. Pyruvate is used as energy and only carbon source in the absence of CO₂ (chemoorganoheterotrophic growth). CO₂ stimulates the chemoorganotrophic growth of both ammonia oxidizers and the expression of ribulose bisphosphate carboxylase/oxygenase is down-regulated at increasing CO₂ concentration. Ammonium, although required as nitrogen source, is inhibitory for the chemoorganotrophic metabolism of N. europaea and N. eutropha. In the presence of ammonium pyruvate consumption and the expression of the genes aceE, ppc, gltA, odhA, and ppsA (energy conservation) as well as nirK, norB, and nsc (denitrification) are reduced.     

Phosphate requirements of the nitrifying bacteria

The influence of the phosphate concentration on the specific growth rate and the duration of lag has been studied inNitrobacter winogradskyi andNitrosomonas europaea.The optimum phosphate concentration range for the specific growth rate was 10 to 30mm forNitrobacter and 10 to 100mm forNitrosomonas. In this range the lag was least. Depletion of the cell-P does not affect the relation between specific growth rate and phosphate concentration while the lag seems to increase as cell-P depletion proceeds.     

Kinetics of Nitrosomonas europaea at extreme substrate,product and salt concentrations

Summary Nitrification of ammonia in concentrated waste streams is gaining a lot of attention nowadays. Nitrosomonas europaea is the predominant ammonia-oxidizing species in these environments. Prediction of the behaviour of a pure culture of N. europaea (ATCC 19718) under conditions prevailing in concentrated waste streams was the aim of this study. The initial oxygen consumption rate of a concentrated cell suspension was used as a rapid assay to measure the effects on N. europaea under various conditions. Several relationships, based on Michaelis-Menten kinetics, were derived. They describe the behaviour of N. europaea at substrate (NH₄ ⁺), product (NO₂ ^– and K⁺, Na⁺, SO _inf4 ^sup2– , NO₃ ^–, Cl^–) concentrations up to 500 mol/m³ and pHs ranging from 6.5 to 8.5. High concentrations of ions inhibited N. europaea but specific substrate inhibition was not observed. Product inhibition was strongly pH-dependent and severe inhibition was found at pH 6.5. Correspondence to: J. H. Hunik     

Mn2+对A/O-BAF系统处理效能和细菌群落多样性的影响

【目的】通过考察Mn²⁺对A/O-BAF系统处理效能及微生物群落多样性的影响,研究了15℃下不同浓度Mn²⁺对A/O-BAF系统处理效能的影响,并通过高通量测序考察了细菌群落多样性的变化情况。【方法】在温度15°C、水力负荷0.50 m³/(m²·h)、气水比10:1的条件下,在进水中投加Mn²⁺,考察反应器处理效能的变化情况,并通过高通量测序对BAF生物膜样品进行分析。【结果】2.0 mg/L Mn²⁺作用下A/O-BAF系统的COD、NH₄⁺-N、TN、TP去除率分别提高3.51%、2.21%、6.26%和12.13%;4.0 mg/L Mn²⁺作用下COD、NH₄⁺-N、TN、TP去除率分别提高了4.24%、1.92%、7.75%和10.73%;Mn²⁺作用下A/O-BAF系统内反硝化细菌和亚硝酸菌的数量明显增加,硝酸菌...     

Maintenance Energy Demand and Starvation Recovery Dynamics of Nitrosomonas europaea and Nitrobacter winogradskyi Cultivated in a Retentostat with Complete Biomass Retention

Nitrosomonas europaea and Nitrobacter winogradskyi (strain “Engel”) were grown in ammonia-limited and nitrite-limited conditions, respectively, in a retentostat with complete biomass retention at 25°C and pH 8. Fitting the retentostat biomass and oxygen consumption data of N. europaea and N. winogradskyi to the linear equation for substrate utilization resulted in up to eight-times-lower maintenance requirements compared to the maintenance energy demand (m) calculated from chemostat experiments. Independent of the growth rate at different stages of such a retention culture, the maximum specific oxygen consumption rate measured by mass spectrometric analysis of inlet and outlet gas oxygen content always amounted to approximately 45 μmol of O₂ mg⁻¹ of biomass-C · h⁻¹ for both N. europaea and N. winogradskyi. When bacteria were starved for different time periods (up to 3 months), the spontaneous respiratory activity after an ammonia or nitrite pulse decreased with increasing duration of the previous starvation time period, but the observed decrease was many times faster for N. winogradskyi than for N. europaea. Likewise, the velocity of resuscitation decreased with extended time periods of starvation. The increase in oxygen consumption rates during resuscitation referred to the reviving population only, since in parallel no significant increase in the cell concentrations was detectable. N. europaea more readily recovers from starvation than N. winogradskyi, explaining the occasionally observed nitrite accumulation in the environment after ammonia becomes available. From chloramphenicol (100 μg · ml⁻¹) inhibition experiments with N. winogradskyi, it has been concluded that energy-starved cells must have a lower protein turnover rate than nonstarved cells. As pointed out by Stein and Arp (L. Y. Stein and D. J. Arp, Appl. Environ. Microbiol. 64:1514–1521, 1998), nitrifying bacteria in soil have to cope with extremely low nutrient concentrations. Therefore, a chemostat is probably not a suitable tool for studying their physiological properties during a long-lasting nutrient shortage. In comparison with chemostats, retentostats offer a more realistic approach with respect to substrate provision and availability.     

Proton Electrochemical Gradients in Washed Cells of Nitrosomonas europaea and Nitrobacter agilis

The components of the proton motive force (Δp), namely, membrane potential (Δψ) and transmembrane pH gradient (ΔpH), were determined in the nitrifying bacteria Nitrosomonas europaea and Nitrobacter agilis. In these bacteria both Δψ and ΔpH were dependent on external pH. Thus at pH 8.0, Nitrosomonas europaea and Nitrobacter agilis had Δψ values of 173 mV and 125 mV (inside negative), respectively, as determined by the distribution of the lipophilic cation [³H]tetraphenyl phosphonium. Intracellular pH was determined by the distribution of two weak acids, ¹⁴C-benzoic and ¹⁴C-acetyl salicylic, and the weak base [¹⁴C]methylamine. Nitrosomonas europaea accumulated ¹⁴C-benzoic acid and ¹⁴C-acetyl salicylic acid when the external pH was below 7.0 and [¹⁴C]methylamine at alkaline pH. Similarly, Nitrobacter agilis accumulated the two weak acids below an external pH of about 7.5 and [¹⁴C]methylamine above this pH. As these bacteria grow best between pH 7.5 and 8.0, they do not appear to have a ΔpH (inside alkaline). Thus, above pH 7.0 for Nitrosomonas europaea and pH 7.5 for Nitrobacter agilis, Δψ only contributed to Δp. In Nitrosomonas europaea the total Δp remained almost constant (145 to 135 mV) when the external pH was varied from 6 to 8.5. In Nitrobacter agilis, Δp decreased from 178 mV (inside negative) at pH 6.0 to 95 mV at pH 8.5. Intracellular pH in Nitrosomonas europaea varied from 6.3 at an external pH of 6.0 to 7.8 at external pH 8.5. In Nitrobacter agilis, however, intracellular pH was relatively constant (7.3 to 7.8) over an external pH range of 6 to 8.5. In Nitrosomonas europaea, Δp and its components (Δψ and ΔpH) remained constant in cells at various stages of growth, so that the metabolic state of cells did not affect Δp. Such an experiment was not possible with Nitrobacter agilis because of low cell yields. The effects of protonophores and ATPase inhibitors on ΔpH and Δψ in the two nitrifying bacteria are considered.     

Efficient itaconic acid production by Aspergillus terreus: Overcoming the strong inhibitory effect of manganese

Itaconic acid (IA), a building block platform chemical, is produced industrially by Aspergillus terreus utilizing glucose. Lignocellulosic biomass can serve as a low cost source of sugars for IA production. However, the fungus could not produce IA from dilute acid pretreated and enzymatically saccharified wheat straw hydrolyzate even at 100-fold dilution. Furfural, hydroxymethyl furfural and acetic acid were inhibitory, as is typical, but Mn²⁺ was particularly problematic for IA production. It was present in the hydrolyzate at a level that was 230 times over the inhibitory limit (50 ppb). Recently, it was found that PO₄³⁻ limitation decreased the inhibitory effect of Mn²⁺ on IA production. In the present study, a novel medium was developed for production of IA by varying PO₄³⁻, Fe³⁺ and Cu²⁺ concentrations using response surface methodology, which alleviated the strong inhibitory effect of Mn²⁺. The new medium contained 0.08 g KH₂PO₄, 3 g NH₄NO₃, 1 g MgSO₄·7H₂O, 5 g CaCl₂·2 H₂O, 0.83 mg FeCl₃·6H₂O, 8 mg ZnSO₄·7H₂O, and 45 mg CuSO₄·5H₂O per liter. The fungus was able to produce IA very well in the presence of Mn²⁺ up to 100 ppm in the medium. This medium will be extremely useful for IA production in the presence of Mn²⁺. This is the first report on the development of Mn²⁺ tolerant medium for IA production by A. terreus.     

Use of quantitative real-time PCR to monitor population dynamics of ammonia-oxidizing bacteria in batch process

A quantitative real-time PCR (QPCR) assay with the TaqMan system was used to quantify 16S rRNA genes of β-proteobacterial ammonia-oxidizing bacteria (AOB) in a batch nitrification bioreactor. Five different sets of primers, together with a TaqMan probe, were used to quantify the 16S rRNA genes of β-proteobacterial AOB belonging to the Nitrosomonas europaea, Nitrosococcus mobilis, Nitrosomonas nitrosa, and Nitrosomonas cryotolerans clusters, and the genus Nitrosospira. We also used PCR followed by denaturing gradient gel electrophoresis (DGGE), cloning, and sequencing of their 16S rRNA genes to identify the AOB species. Seed sludge from an industrial wastewater treatment process controlling high-strength nitrogen wastewater (500 mg/L NH₄ ⁺–N) was used as the inoculum for subsequent batch experiment. The Nitrosomonas nitrosa cluster was the predominant AOB (2.3 × 10⁵ copies/mL) in the start-up period of the batch experiment. However, from the exponential growth period, the Nitrosomonas europaea cluster was the most abundant AOB, and its 16S rRNA gene copy number increased to 8.9 × 10⁶ copies/mL. The competitive dominance between the two AOB clusters is consistent with observed differences in ammonia tolerance and substrate affinity. Analysis of the DGGE results indicated the presence of Nitrosomonas europaea ATCC19718 and Nitrosomonas nitrosa Nm90, consistent with the QPCR results.     

Mechanisms of Oxidation of Inorganic Electron Donors in Autotrophic Bacteria

The enzymatic reactions involved in the oxidation of sulfide,sulfite, thiosulfate, ferrous ions, ammonia, and nitrite arereviewed for Chlorobium limicola f. thiosulfato philum, Thiobacillusnovellus, Thiobacillus ferrooxidans, Nitrosomonas europaea,and Nitrobacter winogradskyi. The properties of the purifiedredox enzymes and of proteins that participate in the oxidationof the inorganic compounds in these autotrophic bacteria aresummarized, and the mechanisms of the oxidation of the inorganiccompounds are discussed on the basis of the interactions betweenthe redox enzymes and carriers. (Received December 21, 1995; Accepted May 29, 1996)     

Growth,chemical composition,and nitrate reductase activity of Rumex species in relation to form and level of N supply

Growth, chemical composition, and nitrate reductase activity (NRA) of hydroponically cultured Rumex crispus, R. palustris, R. acetosa, and R. maritimus were studied in relation to form (NH₄ ⁺, NO₃ ^-, or both) and level of N supply (4 mM N, and zero-N following a period of 4mM N). A distinct preference for either NH₄ ⁺ or NO₃ ^- could not be established. All species were characterized by a very efficient uptake and utilization of N, irrespective of N source, as evident from high concentrations of organic N in the tissues and concurrent excessive accumulations of free NO₃ ^- and free NH₄ ⁺. Especially the accumulation of free NH₄ ⁺ was unusually large. Generally, relative growth rate (RGR) was highest with a combination of NH₄ ⁺ and NO₃ ^-. Compared to mixed N supply, RGR of NO₃ ^-- and NH₄ ⁺-grown plants declined on average 3% and 9%, respectively. Lowest RGR with NH₄ ⁺ supply probably resulted from direct or indirect toxicity effects associated with high NH₄ ⁺ and/or low Ca²⁺ contents of tissues. NRA in NO₃ ^- and NH₄NO₃ plants was very similar with maxima in the leaves of ca 40 μmol NO₂ ^- g^-1 DW h^-1. ‘Basal’ NRA levels in shoot tissues of NH₄ ⁺ plants appeared relatively high with maxima in the leaves of ca 20 μmol NO₂ ^- g^-1 DW h^-1. Carboxylate to organic N ratios, (C-A)/N_org, on a whole plant basis varied from 0.2 in NH₄ ⁺ plants to 0.9 in NO₃ ^- plants. After withdrawal of N, all accumulated NO₃ ^- and NH₄ ⁺ was assimilated into organic N and the organic N redistributed on a large scale. NRA rapidly declined to similar low levels, irrespective of previous N source. Shoot/root ratios of -N plants were 50–80% lower than those from +N plants. In comparison with +N, RGR of -N plants did not decline to a large extent, decreasing by only 15% in -NH₄ ⁺ plants due to very high initial organic-N contents. N-deprived plants all exhibited an excess cation over anion uptake (net proton efflux), and whole-plant (C-A)/N_org ratios increased to values around unity. Possible difficulties in interpreting the (C-A)/N_org ratio and NRA of plants in their natural habitats are briefly discussed.     

Use of liquid chromatography to measure chlorite production by Nitrobacter winogradskyi NRCC26538

A high-pressure liquid chromatography (HPLC) technique, previously developed for nitrite (NO⁻₂) and nitrate (NO⁻₃) measurements [3], was used to measure chlorite (ClO⁻₂) production by Nitrobacter winogradskyi. The determination of ClO⁻₂ by HPLC involves monitoring the column effluent with a UV detector at 214 or 254 nm. Although the absorbance of ClO⁻₂ at 214 nm was about 5 times greater than at 254 nm, interference from other compounds in the culture filtrates of N. winogradskyi contributed to an unstable detector signal. The detection limit at 254 nm for ClO⁻₂ in deionized water was about 1 μM.The measurement of ClO⁻₂ in N. winogradskyi culture filtrates was done with detection at 254 nm. The maximum concentration of ClO⁻₂ produced by anaerobically incubated cell suspensions of N. winogradskyi was about 80 μM.     

Influence of nitrogen and sulphur form on manganese acquisition by barley (<Emphasis Type="Italic">shape Hordeum vulgare</Emphasis>)

The influence of various nitrogen (N) and sulphur (S) forms on the uptake of manganese (Mn) in young spring barley (Hordeum vulgare L cv Golf) plants was examined in both a hydroponic system and in a soil-based system. The soil was a typical Danish Mn-deficient soil viz. a sandy loam soil developed on old marine sediments. Plants growing in solution culture with NO₃^– as the only N source had a higher Mn uptake than plants receiving mixtures of NO₃^– and NH₄⁺. These findings were opposite to the results obtained in the soil-based experiments, where plants fertilized with NO₃^– as the only N source accumulated much less Mn than plants fertilized with NH₄⁺. Combining the results of these experiments confirmed that NH₄⁺ acted as a powerful antagonist to Mn²⁺ during uptake but that this antagonistic effect was more than compensated for by the influence of NH₄⁺ in reducing plant-unavailable Mn(IV) to plant-available Mn(II) in the soil. Furthermore the soil experiments showed that fertilizers containing sulphur in the form of reduced S (thiosulphate) had a strong mobilizing effect on Mn, and enabled the plants to accumulate large amounts of Mn in the biomass compared with oxidized S (sulphate). Thus, fertilization with thiosulphate may be very effective in alleviating Mn-deficiency in soils developed on old marine sediments where Mn availability is limiting plant growth.     

The effect of copper toxicity on the contents of nitrogen compounds in Silene vulgaris (Moench) Garcke

The effect of copper on the uptake of nitrogen and the tissue contents of inorganic nitrogen, amino acids and proteins were studied in cooper-sensitive Silene vulgaris (Moench) Garcke, grown at different nitrogen sources (NH₄ ⁺ or NO₃ ^-). All the toxic copper levels tested, i.e. 4, 8, 16 M Cu²⁺, strongly inhibited the uptake of nitrogen, especially of NO₃ ^-, and decreased the content of NO₃ ^-, amino acids and proteins. Especially at 4 and 8 M Cu²⁺, NH₄ ⁺ accumulated in the plants, suggesting that the conversion of NH₄ ^- into amino acids was inhibited.     

Competition for Ammonium between Nitrifying and Heterotrophic Bacteria in Dual Energy-Limited Chemostats

The absence of nitrification in soils rich in organic matter has often been reported. Therefore, competition for limiting amounts of ammonium between the chemolithotrophic ammonium-oxidizing species Nitrosomonas europaea and the heterotrophic species Arthrobacter globiformis was studied in the presence of Nitrobacter winogradskyi in continuous cultures at dilution rates of 0.004 and 0.01 h⁻¹. Ammonium limitation of A. globiformis was achieved by increasing the glucose concentration in the reservoir stepwise from 0 to 5 mM while maintaining the ammonium concentration at 2 mM. The numbers of N. europaea and N. winogradskyi cells decreased as the numbers of heterotrophic bacteria rose with increasing glucose concentrations for both dilution rates. Critical carbon-to-nitrogen ratios of 11.6 and 9.6 were determined for the dilution rates of 0.004 and 0.01 h⁻¹, respectively. Below these critical values, coexistence of the competing species was found in steady-state situations. Although the numbers were strongly reduced, the nitrifying bacteria were not fully outcompeted by the heterotrophic bacteria above the critical carbon-to-nitrogen ratios. Nitrifying bacteria could probably maintain themselves in the system above the critical carbon-to-nitrogen ratios because they are attached to the glass wall of the culture vessels. The numbers of N. europaea decreased more than did those of N. winogradskyi. This was assumed to be due to heterotrophic growth of the latter species on organic substrates excreted by the heterotrophic bacteria.     

Iron nutrition and physiological responses to iron stress in Nitrosomonas europaea

Nitrosomonas europaea, as an ammonia-oxidizing bacterium, has a high Fe requirement and has 90 genes dedicated to Fe acquisition. Under Fe-limiting conditions (0.2 μM Fe), N. europaea was able to assimilate up to 70% of the available Fe in the medium even though it is unable to produce siderophores. Addition of exogenous siderophores to Fe-limited medium increased growth (final cell mass). Fe-limited cells had lower heme and cellular Fe contents, reduced membrane layers, and lower NH₃- and NH₂OH-dependent O₂ consumption activities than Fe-replete cells. Fe acquisition-related proteins, such as a number of TonB-dependent Fe-siderophore receptors for ferrichrome and enterobactin and diffusion protein OmpC, were expressed to higher levels under Fe limitation, providing biochemical evidence for adaptation of N. europaea to Fe-limited conditions.     

Competition for limiting amounts of oxygen between Nitrosomonas europaea and Nitrobacter winogradskyi grown in mixed continuous cultures

Chemolithotrophic nitrifying bacteria are dependent on the presence of oxygen for the oxidation of ammonium via nitrite to nitrate. The success of nitrification in oxygen-limited environments such as waterlogged soils, will largely depend on the oxygen sequestering abilities of both ammonium- and nitrite-oxidizing bacteria. In this paper the oxygen consumption kinetics of Nitrosomonas europaea and Nitrobacter winogradskyi serotype agilis were determined with cells grown in mixed culture in chemostats at different growth rates and oxygen tensions.Reduction of oxygen tension in the culture repressed the oxidation of nitrite before the oxidation of ammonium was affected and hence nitrite accumulated. K _m values found were within the range of 1–15 and 22–166 M O₂ for the ammonium- and nitrite-oxidizing cells, respectively, always with the lowest values for the N. europaea cells. Reduction of the oxygen tension in the culture lowered the half saturation constant K _m for oxygen of both species. On the other hand, the maximal oxygen consumption rates were reduced at lower oxygen levels especially at 0 kPa. The specific affinity for oxygen indicated by the V _max/K _m ratio, was higher for cells of N. europaea than for N. winogradskyi under all conditions studied. Possible consequences of the observed differences in specific affinities for oxygen of ammonium-and nitrite-oxidizing bacteria are discussed with respect to the behaviour of these organisms in oxygen-limited environments.