Mmol Zn Research Articles

Antibiotic Resistance in Metal-Tolerant Microorganisms from Treatment Facilities.

The study examines the antibiotic resistance of metal-tolerant bacteria isolated from the wastewater treatment plant of a large city to six antibiotics belonging to the β-lactam antibiotics, aminoglycosides and amphenicols. Resistance of bacteria from sewage sludge multitolerant to heavy metals to 18 antibiotics of the β-lactam antibiotics, tetracyclines, aminoglycosides, diaminopyrimidines, amphenicols and ansamycins was studied also. Out of 10, the metal-tolerant microorganisms isolated from wastewater treatment facilities only the Klebsiella pneumonia strain (tolerant to 3 mM Cu) from the sludge of a secondary settling tank did not show resistance to the studied antibiotics at the concentrations considered. Resistance to the maximum amount of antibiotics was typical for strains Serratia fonticola SS0-1, isolated from fresh sewage sludge and resistant to 5 mmol Cu and 3 mmol Pb, or Stenotrophomonas maltophilia SS0-5, also isolated from fresh sludge and resistant to 3 mmol Zn and Cu. It is possible that bacterial resistance to antibiotics develops not only as a result of the use of antibiotics themselves, but also as a result of environmental pollution with heavy metals, and vice versa.

Extracellular Zn Detoxication by <i>Penicillium chrysogenum</i> and <i>Aspergillus niger</i>

Microfungi are promising objects for bioremediation due to their ability to convert metals into forms that are less mobile and accessible to organisms. The study showed that the fungi Penicillium chrysogenum and Aspergillus niger have physiological mechanisms of extracellular Zn detoxification at its initial concentration of 250 µmol – 2 mmol in the medium. At concentrations of 250–500 µmol Zn promotes the accumulation of biomass and abundant sporulation of A. niger and Penicillium chrysogenum, and at concentrations of 1–2 mmol inhibits the growth of fungi. Extracellular Zn detoxification in Aspergillus niger culture occurs by the formation of dihydrate zinc oxalate katsarosite, due to the active biosynthesis of oxalic acid in A. niger. The main mechanism of zinc detoxification in Penicillium chrysogenum culture was the formation of zinc phosphate hopeite. The formation of zinc phosphate hopeite under the action of the fungus was established for the first time. The key factors that determine the direction of the processes of formation of extracellular mineral phases are the amounts of EPS and oxalic acid produced by fungi, as well as changes in the pH of the medium during the growth of cultures.

Biosorbtion, localization and bioavailability of zinc in Spirulina platensis culture

A study of Spirulina platensis taking up Zn was conducted in modified culture media with various Zn salts. Spirulina platensis cultured for 7 days in modified Zarrouk’s medium 1.6 µmol Zn of ZnSO4 (MZS) contained the highest Zn content. The MZS were 69.55±2.70 µg/g dry weight for Zn concentration, 63.11±1.40 %db for protein content, 0.46 day-1 for specific growth rate (µ), 0.55 g/L.day dry weight cell for productivity (Px), and 1.50 days for doubling time (td). The majority of Zn deposit was in the cytoplasm which was 2.7- fold higher than that in the cell walls. Fourier transform infrared (FTIR) spectra showed the decreasing of peak intensity for the amide groups after Zn uptake which indicated their important role in the binding of Zn. The simulated digestion in gastric and intestinal tracks of S. platensis cells cultured in MZS cells was the accessibility of 55.20±0.57 and 63.55±0.21%, respectively. After sequentially gastric and intestinal digestion, the Zn availability was 34.63±0.95%.

Recovery of Heavy Metal Ions Using Magnetic Glycine-Modified Chitosan—Application to Aqueous Solutions and Tailing Leachate

The necessity of decontaminating effluents for the dual purpose of environmental beneficiation and valorization of low-grade resources is driving the development of new sorbents. The functionalization of biopolymers is a promising strategy for improving sorption performance. Incorporating magnetic micro-particles offers an opportunity for the facilitated recovery of spent micron-size sorbent. Combining magnetic facilities and biopolymer functionalization represents a winning strategy. Magnetic glycine-grafted chitosan (G@MChs) was synthesized for the sorption of Ni(II), Zn(II), and Hg(II) before being applied to the removal of hazardous and strategic metals from tailing leachates. The sorbent was characterized using Fourier transform infrared spectroscopy and scanning electron microscopy, before and after metal sorption. The acid–base properties of functionalized sorbent were also determined (pHPZC). Uptake kinetics were studied in mono- and multi-component solutions using different equations for kinetic modeling at optimized pH (i.e., pH0: 5.5). Langmuir and Sips equations were applied to model sorption isotherms in single-component solutions. In addition, sorption isotherms in multi-component solutions were used to evaluate the preference for selected metals. Maximum sorption capacities were 0.35 mmol Hg g−1, 0.47 mmol Zn g−1, and 0.50 mmol Ni g−1. Acidified urea solution (pH 2.7) successfully desorbs metal ions from G@MChs (desorption > 90%). The sorbent was tested for the recovery of hazardous and strategic metal ions from acidic leachates of tailings. This study demonstrates the promising performance of G@MChs for the treatment of complex metal-bearing solutions.

Changes in nutrient concentration and oxidative metabolism in pecan leaflets at different doses of zinc

Zinc deficiency limits pecan nut production. The objective of this study was to evaluate changes in nutrient concentration and oxidative metabolism in pecan leaflets in response to the application at different doses of zinc. The foliar concentration of nutrients, leaflet area, total chlorophyll, dry weight (leaflets and root), superoxide dismutase (SOD), hydrogen peroxide, catalase (CAT), guaiacol peroxidase (GP) and antioxidant capacity were evaluated. Statistical analysis indicates that the application of 200 µmol Zn<sup>2+</sup> affected the foliar concentration of N-total (24.50 ± 2.51 g/kg), P (10.34 ± 2.53 g/kg), Fe<sup>2+</sup> (153.33 ± 6.27 mg/kg) and Zn<sup>2+</sup> (42.00 ± 2.84 mg/kg), showing a greater area of the leaflet, total chlorophyll content and dry weight (leaflets and root). Plants treated with 50 µmol Zn<sup>2+</sup> showed a higher level of SOD activity (1.38 ± 0.016 units/min/g), GP (5.56 ± 0.229 nmol glutathione/min/g), and the production of hydrogen peroxide, without exceeding the control. On the other hand, Zn treatments caused a significant decrease in CAT activity. Zn is an essential micronutrient for the growth and development of pecan, which promotes the accumulation of other nutrients. Therefore, its absence affects the generation of oxidative stress with the subsequent activation of the antioxidant defense enzyme system.

Alleviation of zinc deficiency in wheat inoculated with root endophytic fungus Piriformospora indica and rhizobacterium Pseudomonas putida

Plant growth, grain yield, and the nutritional grain quality reduce in Zinc (Zn)-deficient soils. As a practical approach, the utilization of plant growth-promoting microorganisms offers a promising approach to cope with Zinc deficiency. This study was carried out to determine the effects of endophytic fungus Piriformospora indica and rhizobacterium Pseudomonas putida on growth, nutrient status, and antioxidant enzyme activities of two wheat (Triticum aestivum L.) genotypes under Zn deficiency. The experiment was conducted using a completely randomized factorial design with three replications and three treatments: wheat genotypes [Zn-efficient (H-Zn) genotype, Azadi and Zn-inefficient (L-Zn) genotype, Niknejad]), Zn concentrations (0 and 2 μmol Zn L−1) and microbial inoculations (P. indica and P. putida). Under Zn deficiency, P. indica, P. putida, and dual inoculation of P. indica with P. putida significantly increased the shoot dry weight of L-Zn genotype up to 10.3%, 25.6% and, 15.4% compared to control plants, respectively. Inoculation of P. indica led to an increase in thinner and highly-branched roots and subsequently increased Zn uptake in the shoot of L-Zn genotype under Zn deficiency. Under such condition, individual inoculation of L-Zn genotype with P. putida and P. indica increased peroxidase activity by 85.7% and 40% compared with non-inoculated plants. Malondialdehyde content of both wheat genotypes inoculated with P. indica was significantly decreased compared to the non-inoculated plants. Generally, depending on the wheat genotype, P. indica and P. putida can increase plant tolerance by affecting only part of the tolerance mechanisms (including antioxidant and non-antioxidant mechanisms) under Zn deficiency stress. In summary, P. indica and P. putida could improve the fitness of the L-Zn genotype under Zn-deficiency stress.

Coordination Polymers Integrating Metalloimmunology with Immune Modulation to Elicit Robust Cancer Chemoimmunotherapy

Nanoscale coordination polymers (NCPs) constructed by metal ions and organic ligands via metalligand bonds have attracted great attention for their biomedical application. Herein, a new type of NCP...

Impact of Bulk ZnO, ZnO Nanoparticles and Dissolved Zn on Early Growth Stages of Barley-A Pot Experiment.

Zinc is among the most in-demand metals in the world which also means that a considerable amount of this element is released to the environment each year as a result of human activities. A pot experiment was conducted to study the impact of low- and high-dose zinc amendments on plant growth and biomass yield, with Calcic Chernozem as a growing medium and barley (Hordeum vulgare L.) as a model plant. The distribution of zinc in various plant parts was also investigated. Zn (II) was added in powder as bulk ZnO and in solution as ZnO nanoparticles and ZnSO4 in two dosages (3 and 30 mmol kg−1 soil) prior to planting. The plants were harvested after 10 days of growth. The three sets of data were taken under identical experimental conditions. The application of zinc in aqueous solution and in particulate form (having particle sizes in the range of <100 nm to >500 nm) at concentration of 3 and 30 mmol Zn kg−1 to the soil resulted in decreased growth (root length, shoot length) and biomass yield; the only exception was the addition of 30 mmol Zn kg−1 in the form of bulk ZnO, which had a positive effect on the root growth. The dry weight reduction (sprout biomass) was lowest in plants grown in soil treated with dissolved zinc. There were no statistically significant changes in the content of chlorophyll a, chlorophyll b, and total chlorophyll, although flame atomic absorption spectrometry (FAAS) analysis indicated that plants bioaccumulated the zinc applied. This implies that the transport of zinc into the above-ground plant parts is controlled by the presence of effective mechanical and physiological barriers in roots. Crop performance under zinc stress in relation to biomass production and the growth of roots and shoots is also partly a reflection of the effects of soil properties. Our findings emphasize the importance of considering plant-soil interactions in research of potential toxicity and bioavailability of zinc in the environment.

Controllable Evolution of Dual Defect Zni and VO Associate-Rich ZnO Nanodishes with (0001) Exposed Facet and Its Multiple Sensitization Effect for Ethanol Detection.

Building an effective way for finding the role of surface defects in gas sensing property remains a big challenge. In the present work, we synthesized the ZnO nanodishes (NDs) and first explored the formation process of rich electron donor surface defects by means of studying mechanism for the ZnO NDs synthesis. The test results revealed that ZnO-6, added by 6 mmol Zn powder, had the best gas-sensing properties with the excellent selectivity to ethanol than the others. Specially, the ZnO-6 sensor exhibited the best response (about 49) to 100 ppm ethanol at 230 °C among four as-synthesized samples, while noncustomized ZnO was only 28. It was mainly caused by the following two reasons: the exposure of target (0001) crystal facet and rich electron donor surface defects zinc interstitial (Zni) and oxygen vacancy (VO). As a guide, the formation process of surface defects was revealed by an ideal defect model. By the small-angle XRD and TEM patterns, we could conclude that ZnO NDs, changing stoichiometric ratio, increased the content of Zni by adding Zn powder, while excessive Zn powder promoted the growth of c axis of ZnO NDs in the self-assembly engineering. Besides, a depletion model has been provided to explain how the surface defects work on the sensors and the complex mechanism of gas sensing performance. These findings will develop the application of ZnO-based gas sensor in health and security.

Functionalization of Magnetic Chitosan Particles for the Sorption of U(VI), Cu(II) and Zn(II)-Hydrazide Derivative of Glycine-Grafted Chitosan.

A new magnetic functionalized derivative of chitosan is synthesized and characterized for the sorption of metal ions (environmental applications and metal valorization). The chemical modification of the glycine derivative of chitosan consists of: activation of the magnetic support with epichlorohydrin, followed by reaction with either glycine to produce the reference material (i.e., Gly sorbent) or glycine ester hydrochloride, followed by hydrazinolysis to synthesize the hydrazide functionalized sorbent (i.e., HGly sorbent). The materials are characterized by titration, elemental analysis, FTIR analysis (Fourrier-transform infrared spectrometry), TGA analysis (thermogravimetric analysis) and with SEM-EDX (scanning electron microscopy coupled to energy dispersive X-ray analysis). The sorption performances for U(VI), Cu(II), and Zn(II) are tested in batch systems. The sorption performances are compared for Gly and HGly taking into account the effect of pH, the uptake kinetics (fitted by the pseudo-second order rate equation), and the sorption isotherms (described by the Langmuir and the Sips equations). The sorption capacities of the modified sorbent reach up to 1.14 mmol U g−1, 1.69 mmol Cu g−1, and 0.85 mmol Zn g−1. In multi-metal solutions of equimolar concentration, the chemical modification changes the preferences for given metal ions. Metal ions are desorbed using 0.2 M HCl solutions and the sorbents are re-used for five cycles of sorption/desorption without significant loss in performances.

Can biofortification of zinc improve the antioxidant capacity and nutritional quality of beans?

The aim of this study was to evaluate the effect of zinc biofortification on the antioxidant capacity and nutritional quality of green bean (Phaseolus vulgaris L.) under greenhouse conditions. Zinc was applied using two forms (ZnSO4 and Zn-DTPA) at four doses of application (0, 25, 50 and 100 µmol) added under a hydroponic system, and were tested over a period of 40 days. The antioxidant capacity was assessed in seeds, as well as the nutritional quality. The results indicate that biofortification with Zn improves antioxidant capacity and nutritional quality of beans, considered the main attributes bean quality and consumer health. The highest antioxidant capacity was obtained with the dose of 25 µmol Zn chelate compared to the form of sulphate; while in the nutritional value of beans both forms of Zn chelate and sulfate content had similar doses of 25 to 50 µmol. Finally, we conclude that it is feasible to implement a program biofortification Zn bean, as this nutrient is more concentrated in the seeds (edible part of the plant), allowing improve its nutritional quality and antioxidant capacity.

Genotypic Variations in Phenolic, Flavonoids and Their Antioxidant Activities in Maize Plants Treated with Zn (II) HEDTA Grown in Salinized Media

Zinc (Zn (II) HEDTA) was used to determine their effect on salt-induced damages in maize plants. The aim of this study was to investigate the antioxidant capacity and the levels of enhanced total phenolic (TPC), total flavonoid (TFC) contents and their antioxidant activity in leaves of two maize cultivars Single cross 10 (SC10) and Single cross 162 (SC162) grown in two levels of salinity 0.00 and 100 mmol in response to 20 μmol Zn (II) HEDTA foliar spray treatments. Significant differences (P ≤ 0.05) in amounts of TPC ranged from (2.55 to 4.62 mg/gdw as Gallic) in Single cross 10 (SC10) and from (2.53 to 4.38 mg/gdw as Gallic) in Single cross 162 (SC162), TFC (ranged 1.53 to 2.41 mg/gdw as qurestien) in Single cross 10 (SC10) and from (1.28 to 2.41 mg/gdw as qurestien) in Single cross 162 (SC162) among all treated plants were observed. The levels of their compounds increase related to foliar spraying of Zn (II) HEDTA. A significant positive correlation between TPC, TFC and DPPH scavenging activity and iron chelating activity was observed which shows that phenolic compounds were involved in the mechanism of salt tolerance of the two cultivars by showing enhanced antioxidant activity which resulted in reduced membrane damage and hence improved growth. According to the results obtained, the adverse effects of salt stress on maize plants can partly be alleviated with application of Zn (II)-HEDTA chelates. It is concluded that the application of Zn (II) HEDTA to maize plants grown in salt conditions leads to the increase of antioxidant compounds and maize tolerance.

Rates and Mechanisms of Zn2+ Adsorption on a Meat and Bonemeal Biochar

Biochars produced from meat and bonemeal (MBM) waste materials contain large amounts of calcium phosphate and are potentially useful sorbents for the remediation of metals. Because the reactivity of biochars depends strongly upon the conditions used in their production, the objective of this study was to evaluate the rates and mechanisms of Zn sorption onto a commercially supplied MBM biochar prior to its application in a field-scale revegetation project. Sorption experiments varying pH, time, and Zn concentration found that, above pH 6.1, Zn adsorbed to MBM biochar quickly (within 5 h) with a maximum adsorption capacity of 0.65 mmol Zn g(-1). Synchrotron-based Zn K-edge extended X-ray absorption fine structure spectroscopy was consistent with a tetrahedral Zn bound to phosphate groups in a monodentate inner-sphere surface complex for all conditions tested. With an acidification pretreatment and at more acidic pH, MBM biochar causes precipitation of a ZnPO4 phase. On the basis of these results, this MBM biochar has a high capacity to rapidly adsorb Zn in adsorption experiments and can be considered a promising sorbent for Zn remediation of contaminated soils.

Toxic Heavy Metal Capture Using a Novel Electronic Waste-Based Material—Mechanism, Modeling and Comparison

In the modern communication era, the disposal of printed circuit boards is ecologically of dire concern on a global scale. The two prevalent methods applied for the disposal of this waste are either incineration or landfilling both of which are viewed with skepticism due to their negative environmental impact. Activation of the nonmetallic fraction of this waste leads to the development of a mesoporous material with highly functional groups which can potentially be applied for heavy metal uptake. The removal of copper, lead, and zinc was studied employing a cost-effective novel adsorbent based on waste printed circuit boards. The results indicate that the modification of the original e-waste material has a considerable effect on its surface area enhancement. Adsorption experiments revealed that the modified novel material had uptake capacities of 2.9 mmol Cu, 3.4 mmol Pb, and 2.0 mmol Zn per each gram of the adsorbent which are significantly higher values than its commercial counterparts used in industry.

The analysis of selected microelements in neonatal umbilical cord blood

Microelements and trace elements play a vital role in the body. Low quantities of these elements are essential to ensure proper metabolic processes. The aim of the thesis was to determine levels of selected microelements (zinc, copper, iron, manganese, chromium and aluminium) in cord blood plasma, and to find out if the gestational age, gender and birth weight of a newborn affect these concentrations. The research was conducted on the cord blood of 71 newborns. Regarding the gestational age, three groups were distinguished: I – neonates born after 37 week of pregnancy (n=7), II – neonates born between 38 and 41 week (n=59), and III – neonates born after 42 week of pregnancy (n=5). Subsequently, another division concerned the gender: male (n=35) and female (n=36) neonates, as well as their weight: neonates with regular birth weight – 2.500-3.500 g (n=61), and neonates with high birth weight – over 3.500 g (n=10). The concentrations of zinc, copper, iron, manganese, chromium, and aluminium in cord blood plasma were determined on the basis of inductively coupled plasma atomic emission spectroscopy method. The results were subjected to statistical analysis using Statistica 10.0 software, with the assumption that the level of significance was p<0.05. The average concentrations of analysed elements in umbilical cord blood were the following: 18.67±3.05 μmol Zn dm–3, 16.60±2.64 μmol Cu dm–3, 23.32±3.29 μmol Fe dm–3, 0.96±0.21 μmol Mn dm–3, 2.81±0.14 μmol Cr dm–3 and 0.041±0.028 μmol Al dm–3, which is within the standard ranges, suggesting efficient regulating mechanisms of the developing foetuses. The research proved that the neonate’s gestational age, gender and birth weight

Uranium and other heavy metal resistance and accumulation in bacteria isolated from uranium mine wastes

Ten bacterial strains isolated from uranium mine wastes were characterized in terms of their uranium and other metal resistance and accumulation. 16S rRNA gene sequence analysis identified the strains as members of genera Bacillus, Serratia, and Arthrobacter. Strains were able to utilize various carbon sources, particularly aromatic hydrocarbons, grow at broad pH and temperature ranges and produce non specific acid phosphatase relevant for metal phosphate precipitation in contaminated environment. The isolates exhibited high uranium and other heavy metals (Ni, Co, Cu and Cd) resistance and accumulation capacities. Particularly, Arthrobacter sp. J001 and Bacillus sp. J003 were superior in terms of U resistance at low pH (pH 4.0) along with metals and actinides (U and Th) removal with maximum cell loading of 1088 μmol U, 1293 μmol Th, 425 μmol Cu, 305 μmol Cd, 377 μmol Zn, 250 μmol Ni g−1 cell dry wt. Genes encoding P1B-type ATPases (Cu-CPx and Zn-CPx) and ABC transporters (nik) as catalytic tools for maintaining cellular metal homeostasis were detected within several Bacillus spp., with possible incidence of horizontal gene transfer for the later gene showing phylogenetic lineage to α Proteobacteria members. The study provides evidence on intrinsic abilities of indigenous bacteria from U-mine suitable for survival and cleaning up of contaminated mine sites.

COMPARISON OF ZINC EFFICIENCY AMONG WINTER WHEAT GENOTYPES CULTURED HYDROPONICALLY IN CHELATOR-BUFFERED SOLUTIONS

Zinc (Zn) efficient genotypes grow and yield well in Zn deficient environments. The objective of this study was to compare Zn efficiency and seed Zn content among nine winter wheat (Triticum aestivum L.) genotypes grown in chelator-buffered nutrient solutions containing 0 μmol Zn L−1 (−Zn treatment) or 3 μmol Zn L−1 (+Zn treatment). The Zn efficiency of the genotypes ranged from 24% to 46%. Zinc efficiency was positively correlated with shoot dry weight, shoot Zn content, but there was no significant correlation between Zn efficiency and shoot Zn concentration, seed Zn concentration, or seed Zn content. The results suggested that variation in Zn efficiency among these nine wheat genotypes is genetically inherent. Differences in Zn efficiency among these wheat genotypes, which are widely grown in northern China, indicate the potential to breed for wheat genotypes with increased tolerance to soil Zn deficiency.

New hybrid materials as Zn(II) sorbents in water samples

Mesoporous silicas have been chemically modified with 5-mercapto-1-methyltetrazole (MTTZ) obtaining hybrid materials denominated MTTZ-MSU-2 and MTTZ-HMS. These materials were employed as Zn(II) sorbents from aqueous media at room temperature. The effect of several variables (stirring time, pH, presence of other metals) has been studied using batch and column techniques. Flame atomic absorption spectrometry (FAAS) was used to determinate Zn(II) concentration in the filtrate or in the eluted solution after the adsorption process. The results indicate that under pH 8, the maximum adsorption value was 0.94 ± 0.01 and 0.72 ± 0.01 mmol Zn(II)/g for MTTZ-MSU-2 and MTTZ-HMS, respectively. In tap water samples, a preconcentration factor of 200 was obtained. On the basis of these results, it can be concluded that it is possible to modify chemically MSU-2 and HMS with 5-mercapto-1-methyltetrazole and to use the resulting modified mesoporous silica as an effective adsorbent for Zn(II) in aqueous media.

Biological hydrogen sulfide production in an ethanol–lactate fed fluidized-bed bioreactor

Sulfate-reducing fluidized-bed bioreactor (FBR) fed with ethanol–lactate mixture was operated at 35 °C for 540 days to assess mine wastewater treatment, biological hydrogen sulfide production capacity and acetate oxidation kinetics. During the mine wastewater treatment period with synthetic wastewater, the sulfate reduction rate was 62 mmol SO 4 2 - L −1 d −1 and Fe and Zn precipitation rates were 11 mmol Fe L −1 d −1 and 1 mmol Zn L −1 d −1. After this, the hydrogen sulfide production was optimized, resulting in sulfate reduction rate of 100 mmol SO 4 2 - L −1 d −1 and H 2S production rate of 73.2 mmol H 2S L −1 d −1. The limiting step in the H 2S production was the rate of acetate oxidation, being 50 mmol acetate L −1 d −1. Therefore, FBR batch assays were designed to determine the acetate oxidation kinetics, and following kinetic parameters were obtained: K m of 63 μmol L −1 and V max of 0.76 μmol acetate g VSS −1 min −1. The present study demonstrates high-rate hydrogen sulfide production and high-rate mine wastewater treatment with ethanol and lactate fed fluidized-bed bioreactor.

Bacteria, not archaea, restore nitrification in a zinc-contaminated soil

Biological ammonia oxidation had long been thought to be mediated solely by discrete clades of beta- and gamma-proteobacteria (ammonia-oxidizing bacteria; AOB). However, ammonia-oxidizing Crenarchaeota (ammonia-oxidizing archaea; AOA) have recently been identified and proposed to be the dominant agents of ammonia oxidation in soils. Nevertheless, the dynamics of AOB versus AOA, and their relative contribution to soil ammonia oxidation and ecosystem functioning on stress and environmental perturbation, remain unknown. Using a 3-year longitudinal field study and the amoA gene as a molecular marker, we demonstrate that AOB, but not AOA, mediate recovery of nitrification after zinc (Zn) contamination. Pristine soils showed approximately equal amoA gene copy numbers and transcript levels for AOB and AOA. At an intermediate Zn dose (33.7 mmol Zn per kg), ammonia oxidation was completely inhibited, and the numbers of AOB and AOA amoA gene copies and gene transcripts were reduced. After 2 years, ammonia oxidation in the field soils was fully restored to preexposure levels, and this restoration of function was concomitant with an increase of AOB amoA gene copy and gene transcript numbers. Analysis of the restored community revealed domination by a phylogenetically distinct Zn-tolerant Nitrosospira sp. community. In contrast, the numbers of AOA amoA gene copies and gene transcripts remained 3- and 10(4)-fold lower than recovered AOB values, respectively. Thus, although recent findings have emphasized a dominant role of archaea in soil-borne ammonia oxidation, we demonstrate that a phylogenetic shift within the AOB community drives recovery of nitrification from Zn contamination in this soil.