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1.
We test the hypothesis that pyranopterin (PPT) coordination plays a critical role in defining molybdenum active site redox chemistry and reactivity in the mononuclear molybdoenzymes. The molybdenum atom of Escherichia coli nitrate reductase A (NarGHI) is coordinated by two PPT-dithiolene chelates that are defined as proximal and distal based on their proximity to a [4Fe-4S] cluster known as FS0. We examined variants of two sets of residues involved in PPT coordination: (i) those interacting directly or indirectly with the pyran oxygen of the bicyclic distal PPT (NarG-Ser719, NarG-His1163, and NarG-His1184); and (ii) those involved in bridging the two PPTs and stabilizing the oxidation state of the proximal PPT (NarG-His1092 and NarG-His1098). A S719A variant has essentially no effect on the overall Mo(VI/IV) reduction potential, whereas the H1163A and H1184A variants elicit large effects (ΔEm values of −88 and −36 mV, respectively). Ala variants of His1092 and His1098 also elicit large ΔEm values of −143 and −101 mV, respectively. An Arg variant of His1092 elicits a small ΔEm of +18 mV on the Mo(VI/IV) reduction potential. There is a linear correlation between the molybdenum Em value and both enzyme activity and the ability to support anaerobic respiratory growth on nitrate. These data support a non-innocent role for the PPT moieties in controlling active site metal redox chemistry and catalysis.  相似文献   
2.
尽管国内外对养殖污水处理已提出了一系列的工艺技术,但因经济原因养殖污水直接灌溉农田在我国农村地区仍较为常见.为了解长期畜禽养殖污水直灌对稻田土壤质量的影响,在浙江省绍兴市柯桥区选择了养殖污水直灌不同年限(0、4、7、13年)的稻田,比较研究了养殖污水直接灌溉对稻田土壤不同深度土层中各形态氮和磷含量的影响,探讨了长期养殖污水灌溉对氮、磷在土壤剖面中垂直迁移的影响.结果表明: 长期养殖污水直接灌溉可显著提高稻田土壤中氮和磷的积累,积累量随灌溉年限的增长而增加,其中磷素的增幅高于氮素.养殖污水灌溉4、7和13年后,土壤表层中全P和全N分别比对照增加了43.6%、95.2%、1484%和7.7%、17.0%、28.4%.土壤氮素的变化为NH4+-N、NO3--N>酸解有机氮>非酸解有机氮;土壤有效磷的变化明显大于全磷.长期养殖污水灌溉可促进氮、磷在土壤剖面中的垂直迁移,增加对地下水的污染风险.
  相似文献   
3.
AxenicTrentepohlia odorata was cultured at three different NH4Cl levels (3.5 × 10–2, 3.5 × 10–3, 3.5 × 10–4 M) and three different light intensities (48, 76, 122 µmol m–2 s–1). Chloride had no effect on growth over this range of concentration. High light intensity and high NH4Cl concentration enhanced the specific growth rate. The carotenoid content increased under a combination of high light intensity and low N concentration. WhenD. bardawil was exposed to the same combination of growth conditions, there was an increase in its carotenoid content. The light saturation and the light inhibition constants (K s andK i, respectively) for growth, and the saturation constant (K m) for NH4Cl were determined. TheK s andK i values were higher inT. odorata (66.7 and> 122 mol m–2 s–1, respectively) than inD. bardawil (5.1 and 14.7 µmol m–2 s–1, respectively). TheK m value determined at 122 µmol m–2 s–1, however, was lower inT. odorata (0.048 µM) than inD. bardawil (0.062 µM).Author for correspondence  相似文献   
4.
Small birch plants were grown for up to 80 d in a climate chamber at varied relative addition rates of nitrogen in culture solution, and at ambient (350 μmol mol-1) or elevated (700 μmol mol-1) concentrations of CO2. The relative addition rate of nitrogen controlled relative growth rate accurately and independently of CO2 concentration at sub-optimum levels. During free access to nutrients, relative growth rate was higher at elevated CO2. Higher values of relative growth rate and net assimilation rate were associated with higher values of plant N-concentration. At all N-supply rates, elevated CO2 resulted in higher values of net assimilation rate, whereas leaf weight ratio was independent of CO2. Specific leaf area (and leaf area ratio) was less at higher CO2 and at lower rates of N-supply. Lower values of specific leaf area were partly because of starch accumulation. Nitrogen productivity (growth rate per unit plant nitrogen) was higher at elevated CO2. At sub-optimal N-supply, the higher net assimilation rate at elevated CO2 was offset by a lower leaf area ratio. Carbon dioxide did not affect root/shoot ratio, but a higher fraction of plant dry weight was found in roots at lower N-supply. In the treatment with lowest N-supply, five times as much root length was produced per amount of plant nitrogen in comparison with optimum plants. The specific fine root length at all N-supplies was greater at elevated CO2. These responses of the root system to lower N-supply and elevated CO2 may have a considerable bearing on the acquisition of nutrients in depleted soils at elevated CO2. The advantage of maintaining steady-state nutrition in small plants while investigating the effects of elevated CO2 on growth is emphasized.  相似文献   
5.
This study was undertaken to determine which of the two NO3? fluxes (influx or efflux) across plasma membranes of root cells is the target of those amino acids which have been shown to inhibit net NO3? uptake (Muller & Touraine 1992, Journal of Experimental Botany 43 , 617–623). Parallel experiments were performed to mea-sure either the time course of 15NO3? release from roots of soybean seedlings previously labelled with this isotope into non-labelled solution, or the time course of 15N accumulation from labelled 15NO3? solution in non-labelled seedlings. Focusing on the fate of 15NO3? in the cytoplasmic compartment, a model is developed to describe the time courses of the accumulation and release of tracer across the plasma membranes of root cells. Both time courses can be described by the sum of an exponential plus a linear term. In our material, the linear part of the accumulation time course is obscured by the NO3? fluxes exiting the cytoplasm, and the curve thus appears to be quasilinear over several minutes. However, we show that the use of the net tracer accumulation rate during this time period as an estimate of NO3? influx does not provide accurate estimates of influx and efflux. By contrast, 15NO3? efflux analysis permits calculation of the unidirectional fluxes across plasma membranes of root cells and the kinetic parameters of the cytoplasmic NO3? pool. Under our experimental conditions, efflux accounted for 30 to 50% of influx, and the cytoplasmic NO3? content was found to be in the 70–400nmol g?1 fw range. Using this methodology, the effect of amino acid accumulation on unidirectional fluxes of nitrate was then examined. Pretreatments of the seedlings with an amino acid which has been shown to inhibit net NO3? uptake led to concomitant decreases in net accumulation rates of 15NO3? and of reduced 15N in roots and total 15N in cotyledons. NO3? influx was markedly inhibited by these treatments, while NO3? efflux remained essentially unaffected, or even decreased. It is concluded that the target of the regulation of NO3? uptake by phloemtranslocated amino acids is the influx system.  相似文献   
6.
7.
Solution culture studies have shown that plant uptake of NH4 + and NO3 - can be improved by increasing the concentration of Ca2+ in the root environment: the same may be true for grass grown in soil culture. An experiment was set up to see whether gypsum (CaSO4 2H2O) increased the rate at which perennial ryegrass absorbed 15NH4 + and 15NO3 - from soil.The results demonstrated that gypsum increases the rates of uptake of both NH4 + and NO3 - by perennial ryegrass. However because there was little potential for mineral-N loss from the experimental system, either by gaseous emission or by N immobilization, long term improvements in fertilizer efficiency were not observed. Nitrogen cycling from shoots to roots commenced once net uptake of N into plants had ceased. Labelled N transferred thus to roots underwent isotopic exchange with unlabelled soil N. It was suggested that this exchange of N might constitute an energy drain from the plant, if plant organic N was exchanged for soil inorganic N. The fact that the exchange occurred at all cast doubt on the suitability of the 15N-isotope dilution technique for assessing fertilizer efficiency in medium to long term experiments. There was evidence that the extra NO3 --N taken up by plants on the all-nitrate treatments as a result of gypsum application, was reduced in root tissue rather than in shoots, but to the detriment of subsequent root growth and N uptake.  相似文献   
8.
模拟增温对西藏高原高寒草甸土壤供氮潜力的影响   总被引:3,自引:0,他引:3  
宗宁  石培礼 《生态学报》2019,39(12):4356-4365
过去几十年青藏高原呈现显著的增温趋势,冬季增温幅度显著高于生长季的季节非对称特征。气候变暖会对生态系统氮素循环产生重要影响,但关于全年增温与冬季增温对高寒生态系统氮循环的不同影响仍缺乏研究。在青藏高原高寒草甸区开展模拟增温试验,研究季节非对称增温对高寒草甸生态系统氮循环的影响。该试验布设于2010年7月,设置3种处理(不增温、冬季增温与全年增温)。研究结果发现,开顶箱增温装置造成了小环境的暖干化:显著提高了地表空气温度和表层土壤温度,降低了表层土壤含水量。冬季增温会加剧土壤中氮素的流失,所以在经历了冬季增温后土壤氮含量显著降低;在生长季节,土壤氮素周转速率受土壤水分的调控,在降雨较少的季节,增温引起的土壤含水量降低会抑制土壤氮周转速率。对于土壤微生物量而言,高寒草甸土壤微生物量碳表现出明显的季节动态,在生长季旺盛期较低,在生长季末期和初冬季节反而较高,这说明为了降低对土壤养分的竞争,高寒草甸植物氮吸收与土壤微生物氮固持在时间上存在分离。研究结果表明,冬季增温导致的土壤养分含量变化会影响随后生长季植物群落的生产力、结构组成与碳氮循环等过程,对生态系统过程产生深远的影响。  相似文献   
9.
Mycobacterium tuberculosis (M.tb.), the causative agent of tuberculosis (TB), cannot synthesize GSH, but synthesizes two major low molecular weight thiols namely mycothiol (MSH) and ergothioneine (ERG). Gamma-glutamylcysteine (GGC), an intermediate in GSH synthesis, has been implicated in the protection of lactic acid bacteria from oxidative stress in the absence of GSH. In mycobacteria, GGC is an intermediate in ERG biosynthesis, and its formation is catalysed by EgtA (GshA). GGC is subsequently used by EgtB in the formation of hercynine-sulphoxide-GGC. In this study, M.tb. mutants harbouring unmarked, in-frame deletions in each of the fives genes involved in ERG biosynthesis (egtA, egtB, egtC, egtD and egtE) or a marked deletion of the mshA gene (required for MSH biosynthesis) were generated. Liquid chromatography tandem mass spectrometry analyses (LC-MS) revealed that the production of GGC was elevated in the MSH-deficient and the ERG-deficient mutants. The ERG-deficient ΔegtB mutant which accumulated GGC was more resistant to oxidative and nitrosative stress than the ERG-deficient, GGC-deficient ΔegtA mutant. This implicates GGC in the detoxification of reactive oxygen and nitrogen species in M.tb.  相似文献   
10.
Food–web complexity often hinders disentangling functionally relevant aspects of food–web structure and its relationships to biodiversity. Here, we present a theoretical framework to evaluate food–web complexity in terms of biodiversity. Food network unfolding is a theoretical method to transform a complex food web into a linear food chain based on ecosystem processes. Based on this method, we can define three biodiversity indices, horizontal diversity (DH), vertical diversity (DV) and range diversity (DR), which are associated with the species diversity within each trophic level, diversity of trophic levels, and diversity in resource use, respectively. These indices are related to Shannon's diversity index (H′), where H′ = DH + DV ? DR. Application of the framework to three riverine macroinvertebrate communities revealed that D indices, calculated from biomass and stable isotope features, captured well the anthropogenic, seasonal, or other within‐site changes in food–web structures that could not be captured with H′ alone.  相似文献   
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