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1.
Little information exists on the potential of NH3-oxidizing bacteria to cooxidize halogenated hydrocarbons in soil. A study was conducted to examine the cooxidation of methyl bromide (MeBr) by an NH3-oxidizing bacterium, Nitrosomonas europaea, under soil conditions. Soil and its water content modified the availability of NH4+ and MeBr and influenced the relative rates of substrate (NH3) and cosubstrate (MeBr) oxidations. These observations highlight the complexity associated with characterizing soil cooxidative activities when soil and water interact to differentially affect substrate and cosubstrate availabilities.  相似文献   

2.
Although cooxidative biodegradation of monohalogenated hydrocarbons has been well studied in the model NH3-oxidizing bacterium, Nitrosomonas europaea, virtually no information exists about cooxidation of these compounds by native populations of NH3-oxidizing bacteria. To address this subject, nitrifying activity was stimulated to 125–400 nmol NO3 produced g–1 soil h–1 by first incubating a Ca(OH)2-amended, silt loam soil (pH 7.0±0.2) at field capacity (270 g H2O kg–1 soil) with 10 μmol NH4 + g–1 soil for 14 days, followed by another 10 days of incubation in a shaken slurry (2:1 water:soil, v/w) with periodic pH adjustment and maintenance of 10 mM NH4 +. These slurries actively degraded both methyl bromide (MeBr) and ethyl chloride (EtCl) at maximum rates of 20–30 nmol ml–1 h–1 that could be sustained for approximately 12 h. Although the MeBr degradation rates were linear for the first 10–12 h of incubation, they could not be sustained regardless of NH4 + level and declined to zero over 20 h of incubation. The transformation capacity of the slurry enrichments (~1 μmol MeBr ml–1 soil slurry) was similar to the value measured previously in cell suspensions of N. europaea with similar NH3-oxidizing activity. Several MeBr-degrading characteristics of the nitrifying enrichments were found to be similar to those documented in the literature for MeBr-degrading methanotrophs and facultatively methylotrophic bacteria. Electronic Publication  相似文献   

3.
We examined the rates and sustainability of methyl bromide (MeBr) oxidation in moderately low density cell suspensions ( approximately 6 x 10(7) cells ml(-1)) of the NH(3)-oxidizing bacterium Nitrosomonas europaea. In the presence of 10 mM NH(4)(+) and 0.44, 0. 22, and 0.11 mM MeBr, the initial rates of MeBr oxidation were sustained for 12, 12, and 24 h, respectively, despite the fact that only 10% of the NH(4)(+), 18% of the NH(4)(+), and 35% of the NH(4)(+), respectively, were consumed. Although the duration of active MeBr oxidation generally decreased as the MeBr concentration increased, similar amounts of MeBr were oxidized with a large number of the NH(4)(+)-MeBr combinations examined (10 to 20 micromol mg [dry weight] of cells(-1)). Approximately 90% of the NH(3)-dependent O(2) uptake activity and the NO(2)(-)-producing activity were lost after N. europaea was exposed to 0.44 mM MeBr for 24 h. After MeBr was removed and the cells were resuspended in fresh growth medium, NO(2)(-) production increased exponentially, and 48 to 60 h was required to reach the level of activity observed initially in control cells that were not exposed to MeBr. It is not clear what percentage of the cells were capable of cell division after MeBr oxidation because NO(2)(-) accumulated more slowly in the exposed cells than in the unexposed cells despite the fact that the latter were diluted 10-fold to create inocula which exhibited equal initial activities. The decreases in NO(2)(-)-producing and MeBr-oxidizing activities could not be attributed directly to NH(4)(+) or NH(3) limitation, to a decrease in the pH, to the composition of the incubation medium, or to toxic effects caused by accumulation of the end products of oxidation (NO(2)(-) and formaldehyde) in the medium. Additional cooxidation-related studies of N. europaea are needed to identify the mechanism(s) responsible for the MeBr-induced loss of cell activity and/or viability, to determine what percentages of cells damaged by cooxidative activities are culturable, and to determine if cooxidative activity interferes with the regulation of NH(3)-oxidizing activity.  相似文献   

4.
The oxidation of [(sup14)C]methyl bromide ([(sup14)C]MeBr) to (sup14)CO(inf2) was measured in field experiments with soils collected from two strawberry plots fumigated with mixtures of MeBr and chloropicrin (CCl(inf3)NO(inf2)). Although these fumigants are considered potent biocides, we found that the highest rates of MeBr oxidation occurred 1 to 2 days after injection when the fields were tarped, rather than before or several days after injection. No oxidation of MeBr occurred in heat-killed soils, indicating that microbes were the causative agents of the oxidation. Degradation of MeBr by chemical and/or biological processes accounted for 20 to 50% of the loss of MeBr during fumigation, with evasion to the atmosphere inferred to comprise the remainder. In laboratory incubations, complete removal of [(sup14)C]MeBr occurred within a few days, with 47 to 67% of the added MeBr oxidized to (sup14)CO(inf2) and the remainder of counts associated with the solid phase. Chloropicrin inhibited the oxidation of MeBr, implying that use of this substance constrains the extent of microbial degradation of MeBr during fumigation. Oxidation was by direct bacterial attack of MeBr and not of methanol, a product of the chemical hydrolysis of MeBr. Neither nitrifying nor methane-oxidizing bacteria were sufficiently active in these soils to account for the observed oxidation of MeBr, nor could the microbial degradation of MeBr be linked to cooxidation with exogenously supplied electron donors. However, repeated addition of MeBr to live soils resulted in higher rates of its removal, suggesting that soil bacteria used MeBr as an electron donor for growth. To support this interpretation, we isolated a gram-negative, aerobic bacterium from these soils which grew with MeBr as a sole source of carbon and energy.  相似文献   

5.

Background and Aims

Soil treatment by anaerobic soil disinfestation (ASD) combined with soil solarization can effectively control soilborne plant pathogens and plant-parasitic nematodes in specialty crop production systems. At the same time, research is limited on the impact of soil treatment by ASD?+?solarization on soil fertility, crop performance and plant nutrition. Our objectives were to evaluate the response of 1) soil nutrients and 2) vegetable crop performance to ASD?+?solarization with differing levels of irrigation, molasses amendment, and partially-composted poultry litter amendment (CPL) compared to an untreated control and a methyl bromide (MeBr)?+?chloropicrin-fumigated control.

Methods

A 2-year field study was established in 2008 at the USDA-ARS U.S. Horticultural Research Lab in Fort Pierce, Florida, USA to determine the effectiveness of ASD as an alternative to MeBr fumigation for a bell pepper (Capsicum annum L.)-eggplant (Solanum melongena L.) double crop system. A complete factorial combination of treatments in a split-split plot was established to evaluate three levels of initial irrigation [10, 5, or 0 cm], two levels of CPL (amended or unamended), and two levels of molasses (amended or unamended) in combination with solarization. Untreated and MeBr controls were established for comparison to ASD treatments.

Conclusions

Results suggest that ASD treatment using molasses as the carbon source paired with solarization can be an effective strategy to maintain crop yields in the absence of soil fumigants. For both bell pepper and eggplant crops, ASD treatments with molasses as the carbon source had equivalent or greater marketable yields than the MeBr control. The application of organic amendments in ASD treatment (molasses or molasses?+?CPL) caused differences in soil nutrients and plant nutrition compared to the MeBr control that must be effectively managed in order to implement ASD on a commercial scale as a MeBr replacement.  相似文献   

6.
We examined the rates and sustainability of methyl bromide (MeBr) oxidation in moderately low density cell suspensions (~6 × 107 cells ml−1) of the NH3-oxidizing bacterium Nitrosomonas europaea. In the presence of 10 mM NH4+ and 0.44, 0.22, and 0.11 mM MeBr, the initial rates of MeBr oxidation were sustained for 12, 12, and 24 h, respectively, despite the fact that only 10% of the NH4+, 18% of the NH4+, and 35% of the NH4+, respectively, were consumed. Although the duration of active MeBr oxidation generally decreased as the MeBr concentration increased, similar amounts of MeBr were oxidized with a large number of the NH4+-MeBr combinations examined (10 to 20 μmol mg [dry weight] of cells−1). Approximately 90% of the NH3-dependent O2 uptake activity and the NO2-producing activity were lost after N. europaea was exposed to 0.44 mM MeBr for 24 h. After MeBr was removed and the cells were resuspended in fresh growth medium, NO2 production increased exponentially, and 48 to 60 h was required to reach the level of activity observed initially in control cells that were not exposed to MeBr. It is not clear what percentage of the cells were capable of cell division after MeBr oxidation because NO2 accumulated more slowly in the exposed cells than in the unexposed cells despite the fact that the latter were diluted 10-fold to create inocula which exhibited equal initial activities. The decreases in NO2-producing and MeBr-oxidizing activities could not be attributed directly to NH4+ or NH3 limitation, to a decrease in the pH, to the composition of the incubation medium, or to toxic effects caused by accumulation of the end products of oxidation (NO2 and formaldehyde) in the medium. Additional cooxidation-related studies of N. europaea are needed to identify the mechanism(s) responsible for the MeBr-induced loss of cell activity and/or viability, to determine what percentages of cells damaged by cooxidative activities are culturable, and to determine if cooxidative activity interferes with the regulation of NH3-oxidizing activity.  相似文献   

7.
Moisture may limit microbial activity in a wide range of environments including salt water, food, wood, biofilms, and soils. Low water availability can inhibit microbial activity by lowering intracellular water potential and thus reducing hydration and activity of enzymes. In solid matrices, low water content may also reduce microbial activity by restricting substrate supply. As pores within solid matrices drain and water films coating surfaces become thinner, diffusion path lengths become more tortuous, and the rate of substrate diffusion to microbial cells declines. We used two independent techniques to evaluate the relative importance of cytoplasmic dehydration versus diffusional limitations in controlling rates of nitrification in soil. Nitrification rates in shaken soil slurries, in which NH(inf4)(sup+) was maintained at high concentrations and osmotic potential was controlled by the addition of K(inf2)SO(inf4), were compared with rates in moist soil incubations, in which substrate supply was controlled by the addition of NH(inf3) gas. Comparison of results from these techniques demonstrated that diffusional limitation of substrate supply and adverse physiologic effects associated with cell dehydration can explain all of the decline in activity of nitrifying bacteria at low soil water content. However, the relative importance of substrate limitation and dehydration changes at different water potentials. For the soil-microbial system we worked with, substrate limitation was the major inhibiting factor when soil water potentials were greater than -0.6 MPa, whereas adverse physiological effects associated with cell dehydration were more inhibiting at water potentials of less than -0.6 MPa.  相似文献   

8.
A dynamic dilution system for producing low mixing ratios of methyl bromide (MeBr) and a sensitive analytical technique were used to study the uptake of MeBr by various soils. MeBr was removed within minutes from vials incubated with soils and ~10 parts per billion by volume of MeBr. Killed controls did not consume MeBr, and a mixture of the broad-spectrum antibiotics chloramphenicol and tetracycline inhibited MeBr uptake by 98%, indicating that all of the uptake of MeBr was biological and by bacteria. Temperature optima for MeBr uptake suggested a biological sink, yet soil moisture and temperature optima varied for different soils, implying that MeBr consumption activity by soil bacteria is diverse. The eucaryotic antibiotic cycloheximide had no effect on MeBr uptake, indicating that soil fungi were not involved in MeBr removal. MeBr consumption did not occur anaerobically. A dynamic flowthrough vial system was used to incubate soils at MeBr mixing ratios as low as those found in the remote atmosphere (5 to 15 parts per trillion by volume [pptv]). Soils consumed MeBr at all mixing ratios tested. Temperate forest and grassy lawn soils consumed MeBr most rapidly (rate constant [k] = 0.5 min−1), yet sandy temperate, boreal, and tropical forest soils also readily consumed MeBr. Amendments of CH4 up to 5% had no effect on MeBr uptake even at CH4:MeBr ratios of 107, and depth profiles of MeBr and CH4 consumption exhibited very different vertical rate optima, suggesting that methanotrophic bacteria, like those presently in culture, do not utilize MeBr when it is at atmospheric mixing ratios. Data acquired with gas flux chambers in the field demonstrated the very rapid in situ consumption of MeBr by soils. Uptake of MeBr at mixing ratios found in the remote atmosphere occurs via aerobic bacterial activity, displays first-order kinetics at mixing ratios from 5 pptv to ~1 part per million per volume, and is rapid enough to account for 25% of the global annual loss of atmospheric MeBr.  相似文献   

9.
Methane oxidation by Nitrosomonas europaea.   总被引:19,自引:0,他引:19       下载免费PDF全文
Methane inhibited NH4+ utilization by Nitrosomonas europaea with a Ki of 2mM. O2 consumption was not inhibited. In the absence of NH4+, or with hydrazine as reductant, methane caused nearly a doubling in the rate of O2 uptake. The stimulation was abolished by allylthiourea, a sensitive inhibitor of the oxidation of NH4+. Analysis revealed that methanol was being formed in these experiments, with yields approaching 1 mol of methanol per mol of O2 consumed under certain conditions. When cells were incubated with NH4+ under an atmosphere of 50% methane, 50 microM-methanol was generated in 1 h. It is concluded that methane is an alternative substrate for the NH3-oxidizing enzyme (ammonia mono-oxygenase),m albeit with a much lower affinity than for methane mono-oxygenase of methanotrophs.  相似文献   

10.
Cell suspensions of Methylococcus capsulatus mineralized methyl bromide (MeBr), as evidence by its removal from the gas phase, the quantitative recovery of Br- in the spent medium, and the production of 14CO2 from [14C]MeBr. Methyl fluoride fluoride (MeF) inhibited oxidation of methane as well as that of [14C]MeBr. The rate of MeBr consumption by cells varied inversely with the supply of methane, which suggested a competitive relationship between these two substrates. However, MeBr did not support growth of the methanotroph. In soils exposed to high levels (10,000 ppm) of MeBr, methane oxidation was completely inhibited. At this concentration, MeBr removal rates were equivalent in killed and live controls, which indicated a chemical rather than biological removal reaction. At lower concentration (1,000 ppm) of MeBr, methanotrophs were active and MeBr consumption rates were 10-fold higher in live controls than in killed controls. Soils exposed to trace levels (10 ppm) of MeBr demonstrated complete consumption within 5 h of incubation, while controls inhibited with MeF or incubated without O2 had 50% lower removal rates. Aerobic soils oxidized [14C]MeBr to 14CO2, and MeF inhibited oxidation by 72%. Field experiments demonstrated slightly lower MeBr removal rates in chambers containing MeF than in chambers lacking MeF. Collectively, these results show that soil methanotrophic bacteria, as well as other microbes, can degrade MeBr present in the environment.  相似文献   

11.
12.
A facultatively methylotrophic bacterium, strain IMB-1, that has been isolated from agricultural soil grows on methyl bromide (MeBr), methyl iodide, methyl chloride, and methylated amines, as well as on glucose, pyruvate, or acetate. Phylogenetic analysis of its 16S rRNA gene sequence indicates that strain IMB-1 classes in the alpha subgroup of the class Proteobacteria and is closely related to members of the genus Rhizobium. The ability of strain IMB-1 to oxidize MeBr to CO2 is constitutive in cells regardless of the growth substrate. Addition of cell suspensions of strain IMB-1 to soils greatly accelerates the oxidation of MeBr, as does pretreatment of soils with low concentrations of methyl iodide. These results suggest that soil treatment strategies can be devised whereby bacteria can effectively consume MeBr during field fumigations, which would diminish or eliminate the outward flux of MeBr to the atmosphere.Methyl bromide (MeBr) is a fumigant used in the cultivation of selected fruits, vegetables, and flowers and in the preservation of stored grains and structures. Use of MeBr as a pesticide increases the yield and quality of crops without leaving behind toxic residues characteristic of more complex organopesticides. However, because bromine released from MeBr destroys stratospheric ozone (18, 22, 29, 33), its use will be eliminated in the United States and elsewhere under the auspices of the Clean Air Act and the Montreal Protocol unless effective mechanisms which prevent its escape to the atmosphere can be found (36). Currently, much uncertainty exists with regard to the tropospheric residence time (τ) of MeBr, a factor which is used to calculate its ozone degradation potential (2). Estimates of τ range from ∼1.7 years when only oxidation by tropospheric OH radicals is considered (22) to less than 1.2 years when oceanic sinks are factored in (20). The discovery that soil bacteria oxidize MeBr from the atmosphere, when quantified and combined with the two preceding sinks, lowers τ to ∼0.8 years (32). Chemical destruction of MeBr occurs by hydrolysis, exchange with other halides, and reaction with organic matter (8, 9, 12), but its destruction by microorganisms has been noted in soils and aquatic environments (3, 16, 17a, 19, 23, 27, 28, 32). In aerobic environments, MeBr is oxidized to CO2 and Br (3, 16, 23, 27).Bacterial oxidation of MeBr in soils has been reported both at very low (∼5 to 15 parts per trillion) ambient atmospheric mixing ratios (17a) and at the very high concentrations employed for field fumigation (23). The relative contributions that chemical reactions and bacterial oxidation make to the destruction of MeBr during agricultural fumigation are not yet known, but their combined effect will constrain the emissions of MeBr from soils. Reported destruction of MeBr within the soil matrix, as evidenced by the accumulation of Br, can be substantial and account for as much as 39 to 70% of the applied MeBr in some cases (39, 40). Physical manipulations (e.g., soil compaction and deeper injection of MeBr) have been proposed to increase the retention time of MeBr within the soil matrix, thereby allowing for its more extensive degradation and subsequent decrease in its outward flux to the atmosphere (13). In addition, use of thicker, impermeable covering tarps has been proposed to reduce losses (14, 37), as has the substitution of methyl iodide for MeBr (11, 25). However, enhancement of microbial degradation of MeBr while it is present in the soil matrix may also be a means to eliminate emissions. This could be achieved by exploiting the ability of certain soil bacteria that use MeBr as a carbon and energy source (23). Here, we report further details on the characteristics of such an isolate (23), which we designate strain IMB-1. We demonstrate how the properties of IMB-1 can be used to greatly accelerate the oxidation of MeBr in fumigated soils. Because agricultural field fumigation represents the largest source of anthropogenic emissions of MeBr to the atmosphere, it is at least possible in theory that the overall goal of eliminating most human-derived emission of MeBr could be achieved by in situ biodegradation of this substance.  相似文献   

13.
Impact of fumigants on soil microbial communities.   总被引:12,自引:0,他引:12  
Agricultural soils are typically fumigated to provide effective control of nematodes, soilborne pathogens, and weeds in preparation for planting of high-value cash crops. The ability of soil microbial communities to recover after treatment with fumigants was examined using culture-dependent (Biolog) and culture-independent (phospholipid fatty acid [PLFA] analysis and denaturing gradient gel electrophoresis [DGGE] of 16S ribosomal DNA [rDNA] fragments amplified directly from soil DNA) approaches. Changes in soil microbial community structure were examined in a microcosm experiment following the application of methyl bromide (MeBr), methyl isothiocyanate, 1,3-dichloropropene (1,3-D), and chloropicrin. Variations among Biolog fingerprints showed that the effect of MeBr on heterotrophic microbial activities was most severe in the first week and that thereafter the effects of MeBr and the other fumigants were expressed at much lower levels. The results of PLFA analysis demonstrated a community shift in all treatments to a community dominated by gram-positive bacterial biomass. Different 16S rDNA profiles from fumigated soils were quantified by analyzing the DGGE band patterns. The Shannon-Weaver index of diversity, H, was calculated for each fumigated soil sample. High diversity indices were maintained between the control soil and the fumigant-treated soils, except for MeBr (H decreased from 1.14 to 0.13). After 12 weeks of incubation, H increased to 0.73 in the MeBr-treated samples. Sequence analysis of clones generated from unique bands showed the presence of taxonomically unique clones that had emerged from the MeBr-treated samples and were dominated by clones closely related to Bacillus spp. and Heliothrix oregonensis. Variations in the data were much higher in the Biolog assay than in the PLFA and DGGE assays, suggesting a high sensitivity of PLFA analysis and DGGE in monitoring the effects of fumigants on soil community composition and structure. Our results indicate that MeBr has the greatest impact on soil microbial communities and that 1,3-D has the least impact.  相似文献   

14.
Rates of nitrification and organic C production were determined in batch and chemostat cultures of marine nitrifying bacteria; two NH 4 + -oxidizing species and one NO 2 -oxidizing spezies. With increasing age in batch cultures and with decreasing flow rates in chemostats, cellular organic C and N concentrations declined while the intracellular ratio of C:N remained constant. With decreasing flow rates in chemostats, there was a reduction in (a) carboxylating enzyme activity per unit of cellular organic C (the potential for chemoautotrophic CO2 fixation), and (b) the yield of organic C. For both NH 4 + and NO 2 oxidizers, rates of nitrification and C yield were lowest at very slow chemostat growth rates, when compared with optimal growth rates in batch cultures. For both NH 4 + and NO 2 -oxidizing species, the stoichiometric relationship between nitrification and organic C production did not remain constant and appeared to be dependent on the availability of the inorganic N substrate. The organic C yield from NH 4 + oxidation and hence the free energy efficiency declined with increasing age in batch cultures and with decreasing flow rates in chemostats. The C yield from NO 2 oxidation and the free energy efficiency at slow chemostat growth rates was also lower than that at the optimal growth rate in batch culture.  相似文献   

15.
Although nitrification has been well studied in coniferous forests of Western North America, communities of NH(3)-oxidizing bacteria in these forests have not been characterized. Studies were conducted along meadow-to-forest transects at two sites (Lookout and Carpenter) in the H. J. Andrews Experimental Forest, located in the Cascade Mountains of Oregon. Soil samples taken at 10- or 20-m intervals along the transects showed that several soil properties, including net nitrogen mineralization and nitrification potential rates changed significantly between vegetation zones. Nonetheless, terminal restriction fragment length polymorphism (T-RFLP) analysis of the PCR-amplified NH(3) monooxygenase subunit A gene (amoA) showed the same DNA fragments (TaqI [283 bp], CfoI [66 bp], and AluI [392 bp]) to dominate >/=45 of 47 soil samples recovered from both sites. Two fragments (491-bp AluI [AluI491] and CfoI135) were found more frequently in meadow and transition zone soil samples than in forest samples at both sites. At the Lookout site the combination AluI491-CfoI135 was found primarily in meadow samples expressing the highest N mineralization rates. Four unique amoA sequences were identified among 15 isolates recovered into pure culture from various transect locations. Six isolates possessed the most common T-RFLP amoA fingerprint of the soil samples (TaqI283-AluI392-CfoI66), and their amoA sequences shared 99.8% similarity with a cultured species, Nitrosospira sp. strain Ka4 (cluster 4). The other three amoA sequences were most similar to sequences of Nitrosospira sp. strain Nsp1 and Nitrosospira briensis (cluster 3). 16S ribosomal DNA sequence analysis confirmed the affiliation of these isolates with Nitrosospira clusters 3 and 4. Two amoA clone sequences matched T-RFLP fingerprints found in soil, but they were not found among the isolates.  相似文献   

16.
Ash (Fraxinus spp.) logs, infested with fully developed, cold-acclimated larval and prepupal emerald ash borer, Agrilus planipennis Fairmaire (Coleoptera: Buprestidae), were fumigated with methyl bromide (MeBr) at 4.4 and 10.0 degrees C for 24 h. Concentrations X time dosages of MeBr obtained were 1579 and 1273 g-h/m3 (24-h exposure) at 4.4 and 10.0 degrees C after applied doses of 112 and 96 g/m3, respectively. MeBr concentrations were simultaneously measured with a ContainIR infrared monitor and Fumiscope thermal conductivity meter calibrated for MeBr to measure the effect of CO2 on Fumiscope concentration readings compared with the infrared (IR) instrument. The presence of CO2 caused false high MeBr readings. With the thermal conductivity meter, CO2 measured 11.36 g/m3 MeBr per 1% CO2 in clean air, whereas the gas-specific infrared ContainIR instrument measured 9.55% CO2 as 4.2 g/m3 MeBr (0.44 g/m3 per 1% CO2). The IR instrument was 0.4% as sensitive to CO2 as the thermal conductivity meter. After aeration, fumigated and control logs were held for 8 wk to capture emerging beetles. No A. planipennis adults emerged from any of the fumigated logs, whereas 262 emerged from control logs (139 and 123/m2 at 4.4 and 10.0 degrees C, respectively). An effective fumigation dose and minimum periodic MeBr concentrations are proposed. The use of a CO2 scrubber in conjunction with nonspecific thermal conductivity instruments is necessary to more accurately measure MeBr concentrations.  相似文献   

17.
Maleylacetate reductase (EC 1.3.1.32) plays a major role in the degradation of chloroaromatic compounds by channeling maleylacetate and some of its substituted derivatives into the 3-oxoadipate pathway. The enzyme was purified to apparent homogeneity from an extract of 2,4-dichlorophenoxyacetate (2,4-D)-grown cells of Alcaligenes eutrophus JMP134. Maleylacetate reductase appears to be a dimer of two identical subunits of 35 kDa. The pI was determined to be at pH 5.4. There was no indication of a flavin prosthetic group. The enzyme was inactivated by p-chloromercuribenzoate but not by EDTA, 1,10-phenanthroline, or dithiothreitol. Maleylacetate and 2-chloromaleylacetate were converted with similar efficiencies (with NADH as cosubstrate, Km = 31 microM for each substrate and kcat = 8,785 and 7,280/min, respectively). NADH was preferred to NADPH as the cosubstrate. Upon reduction of 2-chloramaleylacetate by the purified enzyme, chloride was liberated and the resulting maleylacetate was further reduced by a second NADH. These results and the kinetic parameters suggest that the maleylacetate reductase is sufficient to channel the 2,4-D degradation intermediate 2-chloromaleylacetate into the 3-oxoadipate pathway. In a data base search the NH2-terminal sequence of maleylacetate reductase was found to be most similar to that of TfdF, a pJP4-encoded protein of as-yet-unknown function in 2,4-D degradation.  相似文献   

18.
McCalley CK  Sparks JP 《Oecologia》2008,156(4):871-881
Emissions of reactive N compounds produced during terrestrial N cycling can be an important N loss pathway from ecosystems. Most measurements of this process focus on NO and N(2)O efflux; however, in alkaline soils such as those in the Mojave Desert, NH(3) production can be an important component of N gas loss. We investigated patterns of NO and NH(3) emissions in the Mojave Desert and identified seasonal changes in temperature, precipitation and spatial heterogeneity in soil nutrients as primary controllers of soil efflux. Across all seasons, NH(3) dominated reactive N gas emissions with fluxes ranging from 0.9 to 10 ng N m(-2) s(-1) as compared to NO fluxes of 0.08-1.9 ng N m(-2) s(-1). Fluxes were higher in April and July than in October; however, a fall precipitation event yielded large increases in both NO and NH(3) efflux. To explore the mechanisms driving field observations, we combined NO and NH(3) soil flux measurements with laboratory manipulations of temperature, water and nutrient conditions. These experiments showed a large transient NH(3) pulse (~70-100 ng N m(-2) s(-1)) following water addition, presumably driven by an increase in soil NH(4) (+) concentrations. This was followed by an increase in NO production, with maximum NO flux rates of 34 ng N m(-2) s(-1). Our study suggests that immediately following water addition NH(3) volatilization proceeds at high rates due to the absence of microbial competition for NH(4) (+); during this period N gas loss is insensitive to changes in temperature and soil nutrients. Subsequently, NO emission increases and rates of both NO and NH(3) emission are sensitive to temperature and nutrient constraints on microbial activity. Addition of labile C reduces gaseous N losses, presumably by increasing microbial immobilization, whereas addition of NO(3) (-) stimulates NO and NH(3) efflux.  相似文献   

19.
Impact of Fumigants on Soil Microbial Communities   总被引:11,自引:1,他引:11       下载免费PDF全文
Agricultural soils are typically fumigated to provide effective control of nematodes, soilborne pathogens, and weeds in preparation for planting of high-value cash crops. The ability of soil microbial communities to recover after treatment with fumigants was examined using culture-dependent (Biolog) and culture-independent (phospholipid fatty acid [PLFA] analysis and denaturing gradient gel electrophoresis [DGGE] of 16S ribosomal DNA [rDNA] fragments amplified directly from soil DNA) approaches. Changes in soil microbial community structure were examined in a microcosm experiment following the application of methyl bromide (MeBr), methyl isothiocyanate, 1,3-dichloropropene (1,3-D), and chloropicrin. Variations among Biolog fingerprints showed that the effect of MeBr on heterotrophic microbial activities was most severe in the first week and that thereafter the effects of MeBr and the other fumigants were expressed at much lower levels. The results of PLFA analysis demonstrated a community shift in all treatments to a community dominated by gram-positive bacterial biomass. Different 16S rDNA profiles from fumigated soils were quantified by analyzing the DGGE band patterns. The Shannon-Weaver index of diversity, H, was calculated for each fumigated soil sample. High diversity indices were maintained between the control soil and the fumigant-treated soils, except for MeBr (H decreased from 1.14 to 0.13). After 12 weeks of incubation, H increased to 0.73 in the MeBr-treated samples. Sequence analysis of clones generated from unique bands showed the presence of taxonomically unique clones that had emerged from the MeBr-treated samples and were dominated by clones closely related to Bacillus spp. and Heliothrix oregonensis. Variations in the data were much higher in the Biolog assay than in the PLFA and DGGE assays, suggesting a high sensitivity of PLFA analysis and DGGE in monitoring the effects of fumigants on soil community composition and structure. Our results indicate that MeBr has the greatest impact on soil microbial communities and that 1,3-D has the least impact.  相似文献   

20.
Kokalis–Burelle  N.  Vavrina  C. S.  Rosskopf  E. N.  Shelby  R. A. 《Plant and Soil》2002,238(2):257-266
Field trials were performed in Florida to evaluate tomato and pepper transplants amended with formulations of several plant growth-promoting rhizobacteria (PGPR) in a production system that included soil solarization. Transplants grown in five different formulations of PGPR were planted into plots treated by soil solarization, MeBr fumigation, or untreated soil. Treatments were assessed for incidence of several naturally occurring tomato and pepper pathogens including root-knot nematode (Meloidogyne incognita) and species of Pythium, Phytophthora, and Fusarium. Highly significant increases in tomato and pepper transplant growth occurred in response to most formulations of PGPR tested. Transplant vigor and survival in the field were improved by PGPR treatments in both tomato and pepper. Diseases of tomato caused by root-knot nematodes, Fusarium, Phytophthora, and Pythium were not affected by PGPR treatments. PGPR formulation LS261 reduced numbers of root-knot nematode galls on pepper while pepper root condition was improved with formulations LS213, LS256 and LS261. Individual PGPR strains affected the number of Pythium colonies isolated from pepper roots, but did not affect isolation of Pythium from tomato roots. Greater numbers of colonies of Pythium were isolated from pepper roots in the MeBr treatment and fewest in the solarization treatment. Numbers of colony forming units of Fusarium were significantly higher in the untreated soil than in MeBr fumigated or solarized soil with no effect of PGPR on isolation of Fusarium from either crop. Incidence of wilt symptoms on tomato was significantly lower in MeBr treated plots and highest in the untreated plots. Yield of extra large tomato fruit and total yield increased with PGPR formulation LS256. Yield of pepper was increased with formulations LS255 and LS256. Solarization combined with LS256 on pepper produced yields comparable to MeBr.  相似文献   

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