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1.
Enhanced nitrogen (N) availability is one of the main drivers of biodiversity loss and degradation of ecosystem functions. However, in very nutrient-poor ecosystems, enhanced N input can, in the short-term, promote diversity. Mediterranean Basin ecosystems are nutrient-limited biodiversity hotspots, but no information is available on their medium- or long-term responses to enhanced N input. Since 2007, we have been manipulating the form and dose of available N in a Mediterranean Basin maquis in south-western Europe that has low ambient N deposition (<4 kg N ha −1 yr −1) and low soil N content (0.1%). N availability was modified by the addition of 40 kg N ha −1 yr −1 as a 1∶1 NH 4Cl to (NH 4) 2SO 4 mixture, and 40 and 80 kg N ha −1 yr −1 as NH 4NO 3. Over the following 5 years, the impacts on plant composition and diversity (richness and evenness) and some ecosystem characteristics (soil extractable N and organic matter, aboveground biomass and % of bare soil) were assessed. Plant species richness increased with enhanced N input and was more related to ammonium than to nitrate. Exposure to 40 kg NH 4
+-N ha −1 yr −1 (alone and with nitrate) enhanced plant richness, but did not increase aboveground biomass; soil extractable N even increased under 80 kg NH 4NO 3-N ha −1 yr −1 and the % of bare soil increased under 40 kg NH 4
+-N ha −1 yr −1. The treatment containing less ammonium, 40 kg NH 4NO 3-N ha −1 yr −1, did not enhance plant diversity but promoted aboveground biomass and reduced the % of bare soil. Data suggest that enhanced NH y availability affects the structure of the maquis, which may promote soil erosion and N leakage, whereas enhanced NO x availability leads to biomass accumulation which may increase the fire risk. These observations are relevant for land use management in biodiverse and fragmented ecosystems such as the maquis, especially in conservation areas. 相似文献
2.
In short-term water culture experiments with different 15N labeled ammonium or nitrate concentrations, citrus seedlings absorbed NH 4
+ at a higher rate than NO 3
–. Maximum NO 3
– uptake by the whole plant occurred at 120 mg L –1 NO 3
–-N, whereas NH 4
+ absorption was saturated at 240 mg L –1 NH 4
+-N. 15NH 4
+ accumulated in roots and to a lesser degree in both leaves and stems. However, 15NO 3
– was mostly partitioned between leaves and roots.Adding increasing amounts of unlabeled NH 4
+ (15–60 mg L –1 N) to nutrient solutions containing 120 mg L –1 N as 15N labeled nitrate reduced 15NO 3
– uptake. Maximum inhibition of 15NO 3
– uptake was about 55% at 2.14 m M NH 4
+ (30 mg L –1 NH 4
+-N) and it did not increase any further at higher NH 4
+ proportions.In a long-term experiment, the effects of concentration and source of added N (NO 3
– or NH 4
+) on nutrient concentrations in leaves from plants grown in sand were evaluated. Leaf concentration of N, P, Mg, Fe and Cu were increased by NH 4
+ versus NO 3
– nutrition, whereas the reverse was true for Ca, K, Zn and Mn.The effects of different NO 3
–-N:NH 4
+-N ratios (100:0, 75:25, 50:50, 25:75 and 0:100) at 120 mg L –1 total N on leaf nutrient concentrations, fruit yield and fruit characteristics were investigated in another long-term experiment with plants grown in sand cultures. Nitrogen concentrations in leaves were highest when plants were provided with either NO 3
– or NH 4
+ as a sole source of N. Lowest N concentration in leaves was found with a 75:25 NO 3
–-N/NH 4
+-N ratio. With increasing proportions of NH 4
+ in the N supply, leaf nutrients such as P, Mg, Fe and Cu increased, whereas Ca, K, Mn and Zn decreased. Yield in number of fruits per tree was increased significantly by supplying all N as NH 4
+, although fruit weight was reduced. The number of fruits per tree was lowest with the 75:25 NO 3
–-N:NH 4
+-N ratio, but in this treatment fruits reached their highest weight. Rind thickness, juice acidity, and colour index of fruits decreased with increasing NH 4
+ in the N supply, whereas the % pulp and maturity index increased. Percent of juice in fruits and total soluble solids were only slightly affected by NO 3
–:NH 4
+ ratio. 相似文献
3.
Arid areas play a significant role in the global nitrogen cycle. Dry and wet deposition of inorganic nitrogen (N) species were monitored at one urban (SDS) and one suburban (TFS) site at Urumqi in a semi-arid region of central Asia. Atmospheric concentrations of NH 3, NO 2, HNO 3, particulate ammonium and nitrate (pNH 4
+ and pNO 3
−) concentrations and NH 4-N and NO 3-N concentrations in precipitation showed large monthly variations and averaged 7.1, 26.6, 2.4, 6.6, 2.7 µg N m −3 and 1.3, 1.0 mg N L −1 at both SDS and TFS. Nitrogen dry deposition fluxes were 40.7 and 36.0 kg N ha −1 yr −1 while wet deposition of N fluxes were 6.0 and 8.8 kg N ha −1 yr −1 at SDS and TFS, respectively. Total N deposition averaged 45.8 kg N ha −1 yr −1at both sites. Our results indicate that N dry deposition has been a major part of total N deposition (83.8% on average) in an arid region of central Asia. Such high N deposition implies heavy environmental pollution and an important nutrient resource in arid regions. 相似文献
4.
To date, few studies are conducted to quantify the effects of reduced ammonium (NH 4
+) and oxidized nitrate (NO 3
−) on soil CH 4 uptake and N 2O emission in the subtropical forests. In this study, NH 4Cl and NaNO 3 fertilizers were applied at three rates: 0, 40 and 120 kg N ha −1 yr −1. Soil CH 4 and N 2O fluxes were determined twice a week using the static chamber technique and gas chromatography. Soil temperature and moisture were simultaneously measured. Soil dissolved N concentration in 0–20 cm depth was measured weekly to examine the regulation to soil CH 4 and N 2O fluxes. Our results showed that one year of N addition did not affect soil temperature, soil moisture, soil total dissolved N (TDN) and NH 4
+-N concentrations, but high levels of applied NH 4Cl and NaNO 3 fertilizers significantly increased soil NO 3
−-N concentration by 124% and 157%, respectively. Nitrogen addition tended to inhibit soil CH 4 uptake, but significantly promoted soil N 2O emission by 403% to 762%. Furthermore, NH 4
+-N fertilizer application had a stronger inhibition to soil CH 4 uptake and a stronger promotion to soil N 2O emission than NO 3
−-N application. Also, both soil CH 4 and N 2O fluxes were driven by soil temperature and moisture, but soil inorganic N availability was a key integrator of soil CH 4 uptake and N 2O emission. These results suggest that the subtropical plantation soil sensitively responses to atmospheric N deposition, and inorganic N rather than organic N is the regulator to soil CH 4 uptake and N 2O emission. 相似文献
5.
The effect of nitrogen form (NH 4-N, NH 4-N + NO 3−, NO 3−) on nitrate reductase activity in roots and shoots of maize ( Zea mays L. cv INRA 508) seedlings was studied. Nitrate reductase activity in leaves was consistent with the well known fact that NO 3− increases, and NH 4+ and amide-N decrease, nitrate reductase activity. Nitrate reductase activity in the roots, however, could not be explained by the root content of NO 3−, NH 4-N, and amide-N. In roots, nitrate reductase activity in vitro was correlated with the rate of nitrate reduction in vivo. Inasmuch as nitrate reduction results in the production of OH − and stimulates the synthesis of organic anions, it was postulated that nitrate reductase activity of roots is stimulated by the released OH − or by the synthesized organic anions rather than by nitrate itself. Addition of HCO 3− to nutrient solution of maize seedlings resulted in a significant increase of the nitrate reductase activity in the roots. As HCO 3−, like OH −, increases pH and promotes the synthesis of organic anions, this provides circumstantial evidence that alkaline conditions and/or organic anions have a more direct impact on nitrate reductase activity than do NO 3−, NH 4-N, and amide-N. 相似文献
6.
We compared growth kinetics of Prorocentrum donghaiense cultures on different nitrogen (N) compounds including nitrate (NO 3
−), ammonium (NH 4
+), urea, glutamic acid (glu), dialanine (diala) and cyanate. P. donghaiense exhibited standard Monod-type growth kinetics over a range of N concentraions (0.5–500 μmol N L −1 for NO 3
− and NH 4
+, 0.5–50 μmol N L −1 for urea, 0.5–100 μmol N L −1 for glu and cyanate, and 0.5–200 μmol N L −1 for diala) for all of the N compounds tested. Cultures grown on glu and urea had the highest maximum growth rates (μ m, 1.51±0.06 d −1 and 1.50±0.05 d −1, respectively). However, cultures grown on cyanate, NO 3
−, and NH 4
+ had lower half saturation constants (K μ, 0.28–0.51 μmol N L −1). N uptake kinetics were measured in NO 3
−-deplete and -replete batch cultures of P. donghaiense. In NO 3
−-deplete batch cultures, P. donghaiense exhibited Michaelis-Menten type uptake kinetics for NO 3
−, NH 4
+, urea and algal amino acids; uptake was saturated at or below 50 μmol N L −1. In NO 3
−-replete batch cultures, NH 4
+, urea, and algal amino acid uptake kinetics were similar to those measured in NO 3
−-deplete batch cultures. Together, our results demonstrate that P. donghaiense can grow well on a variety of N sources, and exhibits similar uptake kinetics under both nutrient replete and deplete conditions. This may be an important factor facilitating their growth during bloom initiation and development in N-enriched estuaries where many algae compete for bioavailable N and the nutrient environment changes as a result of algal growth. 相似文献
7.
The present lab-scale research reveals the potential of implementation of an oxygen-limited autotrophic nitrification-denitrification (OLAND) system with normal nitrifying sludge as the biocatalyst for the removal of nitrogen from nitrogen-rich wastewater in one step. In a sequential batch reactor, synthetic wastewater containing 1 g of NH 4+-N liter −1 and minerals was treated. Oxygen supply to the reactor was double-controlled with a pH controller and a timer. At a volumetric loading rate ( Bv) of 0.13 g of NH 4+-N liter −1 day −1, about 22% of the fed NH 4+-N was converted to NO 2−-N or NO 3−-N, 38% remained as NH 4+-N, and the other 40% was removed mainly as N 2. The specific removal rate of nitrogen was on the order of 50 mg of N liter −1 day −1, corresponding to 16 mg of N g of volatile suspended solids −1 day −1. The microorganisms which catalyzed the OLAND process are assumed to be normal nitrifiers dominated by ammonium oxidizers. The loss of nitrogen in the OLAND system is presumed to occur via the oxidation of NH 4+ to N 2 with NO 2− as the electron acceptor. Hydroxylamine stimulated the removal of NH 4+ and NO 2−. Hydroxylamine oxidoreductase (HAO) or an HAO-related enzyme might be responsible for the loss of nitrogen. 相似文献
8.
The influence of NH 4+, in the external medium, on fluxes of NO 3− and K + were investigated using barley ( Hordeum vulgare cv Betzes) plants. NH 4+ was without effect on NO 3− ( 36ClO 3−) influx whereas inhibition of net uptake appeared to be a function of previous NO 3− provision. Plants grown at 10 micromolar NO 3− were sensitive to external NH 4+ when uptake was measured in 100 micromolar NO 3−. By contrast, NO 3− uptake (from 100 micromolar NO 3−) by plants previously grown at this concentration was not reduced by NH 4+ treatment. Plants pretreated for 2 days with 5 millimolar NO 3− showed net efflux of NO 3− when roots were transferred to 100 micromolar NO 3−. This efflux was stimulated in the presence of NH 4+. NH 4+ also stimulated NO 3− efflux from plants pretreated with relatively low nitrate concentrations. It is proposed that short term effects on net uptake of NO 3− occur via effects upon efflux. By contrast to the situation for NO 3−, net K + uptake and influx of 36Rb +-labeled K + was inhibited by NH 4+ regardless of the nutrient history of the plants. Inhibition of net K + uptake reached its maximum value within 2 minutes of NH 4+ addition. It is concluded that the latter ion exerts a direct effect upon K + influx. 相似文献
9.
Ricinus communis L. plants were grown in nutrient solutions in which N was supplied as NO 3− or NH 4+, the solutions being maintained at pH 5.5. In NO 3−-fed plants excess nutrient anion over cation uptake was equivalent to net OH − efflux, and the total charge from NO 3− and SO 42− reduction equated to the sum of organic anion accumulation plus net OH − efflux. In NH 4+-fed plants a large H + efflux was recorded in close agreement with excess cation over anion uptake. This H + efflux equated to the sum of net cation (NH 4+ minus SO 42−) assimilation plus organic anion accumulation. In vivo nitrate reductase assays revealed that the roots may have the capacity to reduce just under half of the total NO 3− that is taken up and reduced in NO 3−-fed plants. Organic anion concentration in these plants was much higher in the shoots than in the roots. In NH 4+-fed plants absorbed NH 4+ was almost exclusively assimilated in the roots. These plants were considerably lower in organic anions than NO 3−-fed plants, but had equal concentrations in shoots and roots. Xylem and phloem saps were collected from plants exposed to both N sources and analyzed for all major contributing ionic and nitrogenous compounds. The results obtained were used to assist in interpreting the ion uptake, assimilation, and accumulation data in terms of shoot/root pH regulation and cycling of nutrients. 相似文献
10.
Net uptakes of K + and NO 3− were monitored simultaneously and continuously for two barley ( Hordeum vulgare) cultivars, Prato and Olli. The cultivars had similar rates of net K + and NO 3− uptake in the absence of NH 4+ or Cl −. Long-term exposure (over 6 hours) to media which contained equimolar mixtures of NH 4+, K +, Cl −, or NO 3− affected the cultivars very differently: (a) the presence of NH 4+ as NH 4Cl stimulated net NO 3− uptake in Prato barley but inhibited net NO 3− uptake in Olli barley; (b) Cl − inhibited net NO 3− uptake in Prato but had little effect in Olli; and (c) NH 4+ as (NH 4) 2SO 4 inhibited net K + uptake in Prato but had little effect in Olli. Moreover, the immediate response to the addition of an ion often varied significantly from the long-term response; for example, the addition of Cl − initially inhibited net K + uptake in Olli barley but, after a 4 hour exposure, it was stimulatory. For both cultivars, net NH 4+ and Cl − uptake did not change significantly with time after these ions were added to the nutrient medium. These data indicate that, even within one species, there is a high degree of genotypic variation in the control of nutrient absorption. 相似文献
11.
Nitrogen-14 and nitrogen-15 nuclear magnetic resonance (NMR) spectra were recorded for freshly dissected buds of Picea glauca and for buds grown for 3, 6 and 9 weeks on shoot-forming medium. Resonances for Glu (and other αNH 2 groups), Pro, Ala, and the side chain groups in Gln, Arg, Orn, and γ-aminobutyric acid could be detected in in vivo15N NMR spectra. Peaks for α-amino groups, Pro, NO 3− and NH 4+ could also be identified in 14N NMR spectra. Perfusion experiments performed for up to 20 hours in the NMR spectrometer showed that 15N-labeled NH 4+ and NO 3− are first incorporated into the amide group of Gln and then in the αNH 2 pool. Subsequently, it also emerges in Ala and Arg. These data suggest that the glutamine synthetase/ glutamate synthase pathway functions under these conditions. The assimilation of NH 4+ is much faster than that of NO 3−. Consequently after 10 days of growth more than 70% of the newly synthesized internal free amino acid pool derives its nitrogen from NH 4+ rather than NO 3−. If NH 4+ is omitted from the medium, no NO 3− is taken up during 9 weeks and the buds support limited growth by utilizing their endogenous amino acid pools. It is concluded that NH 4+ and NO 3− are both required for the induction of nitrate- and nitrite reductase. 相似文献
12.
Assimilation of NO 3− and NH 4+ by perennial ryegrass ( Lolium perenne L.) turf, previously deprived of N for 7 days, was examined. Nitrogen uptake rate was increased up to four- to five-fold for both forms of N by N-deprivation as compared to N-sufficient controls, with the deficiency-enhanced N absorption persisting through a 48 hour uptake period. Nitrate, but not NH 4+, accumulated in the roots and to a lesser degree in shoots. By 48 hours, 53% of the absorbed NO 3− had been reduced, whereas 97% of the NH 4+ had been assimilated. During the early stages (0 to 8 hours) of NO 3− uptake by N-deficient turf, reduction occurred primarily in the roots. Between 8 and 16 hours, however, the site of reduction shifted to the shoots. Nitrogen form did not affect partitioning of the absorbed N between roots (40%) and shoots (60%) but did affect growth. Compared to NO 3−, NH 4+ uptake inhibited root, but not shoot, growth. Total soluble carbohydrates decreased in both roots and shoots during the uptake period, principally the result of fructan metabolism. Ammonium uptake resulted in greater total depletion of soluble carbohydrates in the root compared to NO 3− uptake. The data indicate that N assimilation by ryegrass turf utilizes stored sugars but is also dependent on current photosynthate. 相似文献
13.
The effect of exogenous NH 4+ on NO 3− uptake and in vivo NO 3− reductase activity (NRA) in roots of Phaseolus vulgaris L. cv Witte Krombek was studied before, during, and after the apparent induction of root NRA and NO 3− uptake. Pretreatment with NH 4Cl (0.15-50 millimolar) affected neither the time pattern nor the steady state rate of NO 3− uptake. When NH4+ was given at the start of NO3− nutrition, the time pattern of NO3− uptake was the same as in plants receiving no NH4+. After 6 hours, however, the NO3− uptake rate (NUR) and root NRA were inhibited by NH4+ to a maximum of 45% and 60%, respectively. The response of the NUR of NO3−-induced plants depended on the NH4Cl concentration. Below 1 millimolar NH4+, the NUR declined immediately and some restoration occurred in the second hour. In the third hour, the NUR became constant. In contrast, NH4+ at 2 millimolar and above caused a rapid and transient stimulation of NO3− uptake, followed again by a decrease in the first, a recovery in the second, and a steady state in the third hour. Maximal inhibition of steady state NUR was 50%. With NO3−-induced plants, root NRA responded less and more slowly to NH4+ than did NUR. Methionine sulfoximine and azaserine, inhibitors of glutamine synthetase and glutamate synthase, respectively, relieved the NH4+ inhibition of the NUR of NO3−-induced plants. We conclude that repression of the NUR by NH4+ depends on NH4+ assimilation. The repression by NH4+ was least at the lowest and highest NH4+ levels tested (0.04 and 25 millimolar). 相似文献
14.
Macroalgae has bloomed in the brackish lake of Shenzhen Bay, China continuously from 2010 to 2014. Gracilaria tenuistipitata was identified as the causative macroalgal species. The aim of this study was to explore the outbreak mechanism of G. tenuistipitata, by studying the effects of salinity and nitrogen sources on growth, and the different nitrogen sources uptake characteristic. Our experimental design was based on environmental conditions observed in the bloom areas, and these main factors were simulated in the laboratory. Results showed that salinity 12 to 20 ‰ was suitable for G. tenuistipitata growth. When the nitrogen sources'' (NH 4
+, NO 3
−) concentrations reached 40 µM or above, the growth rate of G. tenuistipitata was significantly higher. Algal biomass was higher (approximately 1.4 times) when cultured with NH 4
+ than that with NO 3
− addition. Coincidentally, macroalgal bloom formed during times of moderate salinity (∼12 ‰) and high nitrogen conditions. The NH 4
+ and NO 3
− uptake characteristic was studied to understand the potential mechanism of G. tenuistipitata bloom. NH 4
+ uptake was best described by a linear, rate-unsaturated response, with the slope decreasing with time intervals. In contrast, NO 3
− uptake followed a rate-saturating mechanism best described by the Michaelis-Menten model, with kinetic parameters Vmax = 37.2 µM g −1 DM h −1 and Ks = 61.5 µM. Further, based on the isotope 15N tracer method, we found that 15N from NH 4
+ accumulated faster and reached an atom% twice than that of 15N from NO 3
−, suggesting when both NH 4
+ and NO 3
− were available, NH 4
+ was assimilated more rapidly. The results of the present study indicate that in the estuarine environment, the combination of moderate salinity with high ammonium may stimulate bloom formation. 相似文献
15.
At root temperature below 14 C the absorption of 15N from NH 4+ greatly exceeded that from NO 2− by tillers of Lolium multiflorum and Lolium perenne under conditions where pH, external concentration, plant N status, and pretreatment temperature were varied. There was a marked increase in the temperature sensitivity of NO 3− transport below 14 C, irrespective of the temperature at which plants were grown previously. A marked increase in the temperature sensitivity was also seen for NH 4+ transport, but this occurred at the lower temperature of 10 C. Pretreatment of roots at 8 C lowered this still further to 5 C. Above and below these transition temperatures the Q 10 values for NO 3− and NH 4+ transport were similar. Thus, the increased absorption of NH 4+ relative to NO 3− at low temperatures seems to be related primarily to the difference in transition temperatures. 相似文献
16.
NH 4
+-N can have inhibitory effects on plant growth. However, the mechanisms of these inhibitory effects are still poorly understood. In this study, effects of different N forms and a combination of ammonium + 6-benzylaminopurine (6-BA, a synthetic cytokinin) on growth, transpiration, uptake and flow of water and potassium in 88-days-old tobacco ( Nicotiana tabacum L. K 326) plants were studied over a period of 12 days. Plants were supplied with equal amounts of N in different forms: NO 3
–, NH 4NO 3, NH 4
+ or NH 4
++6-BA (foliar spraying every 2 days after onset of the treatments). For determining flows and partitioning upper, middle and lower strata of three leaves each were analysed. During the 12 days study period, 50% replacement of NO 3
–-N by NH 4
+-N (NH 4NO 3) did not change growth, transpiration, uptake and flow of water and K + compared with the NO 3
–-N treatment. However, NH 4
+-N as the sole N-source caused: (i) a substantial decrease in dry weight gain to 42% and 46% of the NO 3
–-N and NH 4NO 3 treatments, respectively; (ii) a marked reduction in transpiration rate, due to reduced stomatal conductance, illustrated by more negative leaf carbon-isotope discrimination ( 13C) compared with the NO 3
– treatment, especially in upper leaves; (iii) a strong reduction both in total water uptake, and in the rate of water uptake by roots, likely due to a decrease in root hydraulic conductivity; (iv) a marked reduction of K + uptake to 10%. Under NH 4
+ nutrition the middle leaves accumulated 143%, and together with upper leaves 206% and the stem 227% of the K + currently taken up, indicating massive mobilisation of K + from lower leaves and even the roots. Phloem retranslocation of K + from the shoot and cycling through the root contributed 67% to the xylem transport of K +, and this was 2.2 times more than concurrent uptake. Foliar 6-BA application could not suppress or reverse the inhibitory effects on growth, transpiration, uptake and flow of water and ions (K +) caused by NH 4
+-N treatment, although positive effects by 6-BA application were observed, even when 6-BA (10 –8
M) was supplied in nutrient solution daily with watering. Possible roles of cytokinin to regulate growth and development of NH 4
+-fed plants are discussed. 相似文献
17.
A potential for heterotrophic nitrification was identified in soil from a mature conifer forest and from a clear-cut site. Potential rates of NO 2− production were determined separately from those of NO 3− by using acetylene to block autotrophic NH 4+ oxidation and chlorate to block NO 2− oxidation to NO 3− in soil slurries. Rates of NO 2− production were similar in soil from the forest and the clear-cut site and were strongly inhibited by acetylene. The rate of NO 3− production was much greater than that of NO 2− production, and NO 3− production was not significantly affected by acetylene or chlorate. Nitrate production was partially inhibited by cycloheximide, but was not significantly reduced by streptomycin. Neither the addition of ammonium nor the addition of peptone stimulated NO 3− production. 15N labeling of the NH 4+ pool demonstrated that NO 3− was not coming from NH 4+. The potential for heterotrophic nitrification in these forest soils was greater than that for autotrophic nitrification. 相似文献
18.
The effect of phosphate (PO 4
+3) and pH in regulating nitrate (NO 3) and ammonia (NH 3
+) uptake by phytoplankton was investigated in two Oklahoma lakes using 15N tracers. Addition of PO 4
+3 above ambient concentrations had a negligible effect on the rate of uptake of NO 3
– or NH 3
+. Manipulation of pH of lake water had little effect on uptake of either NO 3
– or NH 3
+. A correlation analysis suggested that NO 3
– is not used by phytoplankton when NH 3
+ concentrations exceed about 210 µg NH 3
+-N(1) –1. 相似文献
19.
The inducibility and kinetics of the NO 3−, NO 2−, and NH 4+ transporters in roots of wheat seedlings ( Triticum aestivum cv Yercora Rojo) were characterized using precise methods approaching constant analysis of the substrate solutions. A microcomputer-controlled automated high performance liquid chromatography system was used to determine the depletion of each N species (initially at 1 millimolar) from complete nutrient solutions. Uptake rate analyses were performed using computerized curve-fitting techniques. More precise estimates were obtained for the time required for and the extent of the induction of each transporter. Up to 10 and 6 hours, respectively, were required to achieve apparent full induction of the NO 3− and NO 2− transporters. Evidence for substrate inducibility of the NH 4+ transporters requiring 5 hours is presented. The transport of NO 3− was mediated by a dual system (or dual phasic), whereas only single systems were found for transport of NO 2− and NH 4+. The Km values for NO 3−, NO 2−, and NH 4+ were, respectively, 0.027, 0.054, and 0.05 millimolar. The Km for mechanism II of NO 3− transport could not be defined in this study as it exhibited only apparent first order kinetics up to 1 millimolar. 相似文献
20.
Dissimilatory reduction of NO 2− to N 2O and NH 4+ by a soil Citrobacter sp. was studied in an attempt to elucidate the physiological and ecological significance of N 2O production by this mechanism. In batch cultures with defined media, NO 2− reduction to NH 4+ was favored by high glucose and low NO 3− concentrations. Nitrous oxide production was greatest at high glucose and intermediate NO 3− concentrations. With succinate as the energy source, little or no NO 2− was reduced to NH 4+ but N 2O was produced. Resting cell suspensions reduced NO 2− simultaneously to N 2O and free extracellular NH 4+. Chloramphenicol prevented the induction of N 2O-producing activity. The Km for NO 2− reduction to N 2O was estimated to be 0.9 mM NO 2−, yet the apparent Km for overall NO 2− reduction was considerably lower, no greater than 0.04 mM NO 2−. Activities for N 2O and NH 4+ production increased markedly after depletion of NO 3− from the media. Amendment with NO 3− inhibited N 2O and NH 4+ production by molybdate-grown cells but not by tungstate-grown cells. Sulfite inhibited production of NH 4+ but not of N 2O. In a related experiment, three Escherichia coli mutants lacking NADH-dependent nitrite reductase produced N 2O at rates equal to the wild type. These observations suggest that N 2O is produced enzymatically but not by the same enzyme system responsible for dissimilatory reduction of NO 2− to NH 4+. 相似文献
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