A possible nitrogen crisis for Archaean life due to reduced nitrogen fixation by lightning.

Nitrogen is an essential element for life and is often the limiting nutrient for terrestrial ecosystems. As most nitrogen is locked in the kinetically stable form, N2, in the Earth's atmosphere, processes that can fix N2 into biologically available forms-such as nitrate and ammonia-control the supply of nitrogen for organisms. On the early Earth, nitrogen is thought to have been fixed abiotically, as nitric oxide formed during lightning discharge. The advent of biological nitrogen fixation suggests that at some point the demand for fixed nitrogen exceeded the supply from abiotic sources, but the timing and causes of the onset of biological nitrogen fixation remain unclear. Here we report an experimental simulation of nitrogen fixation by lightning over a range of Hadean (4.5-3.8 Gyr ago) and Archaean (3.8-2.5 Gyr ago) atmospheric compositions, from predominantly carbon dioxide to predominantly dinitrogen (but always without oxygen). We infer that, as atmospheric CO2 decreased over the Archaean period, the production of nitric oxide from lightning discharge decreased by two orders of magnitude until about 2.2 Gyr. After this time, the rise in oxygen (or methane) concentrations probably initiated other abiotic sources of nitrogen. Although the temporary reduction in nitric oxide production may have lasted for only 100 Myr or less, this was potentially long enough to cause an ecological crisis that triggered the development of biological nitrogen fixation.     

Energy requirement for symbiotic nitrogen fixation

Comparing the energy required by legumes for symbiotic nitrogen fixation with that of assimilation of nitrate, Gibson concluded the costs to be about the same. About 15% of net photosynthetic production by the plant may be used in meeting its nitrogen requirements. If energy for the fixation of nitrogen symbiotically and that for the assimilation of NH4+ or NO3- from the soil solution are both provided by the chemical products of photosynthesis, then the CO2 respired in supplying that energy must contribute to the total efflux of CO2 from the plant in the dark. McCree and Thornley have discussed methods of partitioning the dark CO2 efflux into a growth (synthesis) and a maintenance component. Both nitrogen fixation and nitrogen assimilation can be expected to contribute to the CO2 flux associated with synthesis. If Gibson is correct, nodulated plants using only symbiotically fixed nitrogen should have the same growth coefficient as non-nodulated plants supplied with exogenous mineral nitrogen when grown in the same conditions. I have examined this hypothesis using a modification of the method used by McCree with Trifolium subterraneum L. cultivar Woogenellup as test material.     

Buckminsterfullerenes: a non-metal system for nitrogen fixation

In all nitrogen-fixation processes known so far--including the industrial Haber-Bosch process, biological fixation by nitrogenase enzymes and previously described homogeneous synthetic systems--the direct transformation of the stable, inert dinitrogen molecule (N2) into ammonia (NH3) relies on the powerful redox properties of metals. Here we show that nitrogen fixation can also be achieved by using a non-metallic buckminsterfullerene (C60) molecule, in the form of a water-soluble C60:gamma-cyclodextrin (1:2) complex, and light under nitrogen at atmospheric pressure. This metal-free system efficiently fixes nitrogen under mild conditions by making use of the redox properties of the fullerene derivative.     

Phosphorus limitation of nitrogen fixation by Trichodesmium in the central Atlantic Ocean

Marine fixation of atmospheric nitrogen is believed to be an important source of biologically useful nitrogen to ocean surface waters, stimulating productivity of phytoplankton and so influencing the global carbon cycle. The majority of nitrogen fixation in tropical waters is carried out by the marine cyanobacterium Trichodesmium, which supplies more than half of the new nitrogen used for primary production. Although the factors controlling marine nitrogen fixation remain poorly understood, it has been thought that nitrogen fixation is limited by iron availability in the ocean. This was inferred from the high iron requirement estimated for growth of nitrogen fixing organisms and the higher apparent densities of Trichodesmium where aeolian iron inputs are plentiful. Here we report that nitrogen fixation rates in the central Atlantic appear to be independent of both dissolved iron levels in sea water and iron content in Trichodesmium colonies. Nitrogen fixation was, instead, highly correlated to the phosphorus content of Trichodesmium and was enhanced at higher irradiance. Furthermore, our calculations suggest that the structural iron requirement for the growth of nitrogen-fixing organisms is much lower than previously calculated. Although iron deficiency could still potentially limit growth of nitrogen-fixing organisms in regions of low iron availability-for example, in the subtropical North Pacific Ocean-our observations suggest that marine nitrogen fixation is not solely regulated by iron supply.     

固氮类微生物产氢机理研究进展

氢是一种无污染的新型、高效能源,受到广泛的重视。微生物制氢是氢能开发研究的一项重要内容。至今已知的具有产氢活性的微生物有“光合细菌”（photosynthetic bacteria,PSB）、藻类（algae）和非光合细菌（non-photo-synthetic bacteria）。对固氮类微生物的产氢机理及影响产氢的因素、光合产氢的能量利用等进行综述。     

生物固氮的研究现状初探

初步探索了生物固氮目前的研究进展,并着重研究了豆科植物根毛与根瘤菌接触到最后形成成熟的可以固氮的根瘤组织的途径,为将来把非豆科植物,特别是禾谷类农作物转变为固氮植物的可行性提供参考。     

Amino-acid cycling drives nitrogen fixation in the legume-Rhizobium symbiosis

The biological reduction of atmospheric N2 to ammonium (nitrogen fixation) provides about 65% of the biosphere's available nitrogen. Most of this ammonium is contributed by legume-rhizobia symbioses, which are initiated by the infection of legume hosts by bacteria (rhizobia), resulting in formation of root nodules. Within the nodules, rhizobia are found as bacteroids, which perform the nitrogen fixation: to do this, they obtain sources of carbon and energy from the plant, in the form of dicarboxylic acids. It has been thought that, in return, bacteroids simply provide the plant with ammonium. But here we show that a more complex amino-acid cycle is essential for symbiotic nitrogen fixation by Rhizobium in pea nodules. The plant provides amino acids to the bacteroids, enabling them to shut down their ammonium assimilation. In return, bacteroids act like plant organelles to cycle amino acids back to the plant for asparagine synthesis. The mutual dependence of this exchange prevents the symbiosis being dominated by the plant, and provides a selective pressure for the evolution of mutualism.     

农业生态系统非共生固氮研究进展

非共生固氮作用被认为是生物圈中最重要的反应之一.笔者从农业生态系统非共生固氮切入,通过梳理土壤环境因子对非共生固氮的影响、农业管理方式对非共生固氮的影响及气候变化对非共生固氮的影响的相关文献,重点探讨了非共生固氮的特征及其主要的影响因素.同时,还根据农业生态系统的一些研究进展,提出从新技术的应用(例如,结合15 N2-...     

Ball lightning caused by oxidation of nanoparticle networks from normal lightning strikes on soil

Observations of ball lightning have been reported for centuries, but the origin of this phenomenon remains an enigma. The 'average' ball lightning appears as a sphere with a diameter of 300 mm, a lifetime of about 10 s, and a luminosity similar to a 100-W lamp. It floats freely in the air, and ends either in an explosion, or by simply fading from view. It almost invariably occurs during stormy weather. Several energy sources have been proposed to explain the light, but none of these models has succeeded in explaining all of the observed characteristics. Here we report a model that potentially accounts for all of those properties, and which has some experimental support. When normal lightning strikes soil, chemical energy is stored in nanoparticles of Si, SiO or SiC, which are ejected into the air as a filamentary network. As the particles are slowly oxidized in air, the stored energy is released as heat and light. We investigated this basic process by exposing soil samples to a lightning-like discharge, which produced chain aggregates of nanoparticles: these particles oxidize at a rate appropriate for explaining the lifetime of ball lightning.     

一种防直击雷方法——电感型限流接闪体

富兰克林 (Franklin)避雷针的二次雷击效应对信息技术设备的危害极大 ,对直击雷限流是当今防雷工程研究极有实际价值的课题。运用麦克斯韦 (Maxwell)电磁场理论 ,着重于场的观念及高频电磁波的弥散与光波的色散的一致性 ,对闪电过程和防直击雷机理作了分析 ,并与实验结果相吻合 ,从而阐明 :电感型限流接闪体取代 Franklin接闪针是可行的很有价值的一种防直击雷方法 ,可大大削弱二次雷击作用 ,适应当前信息防雷之需     

Rearrangement of nitrogen fixation genes during heterocyst differentiation in the cyanobacterium Anabaena

Nitrogen fixation by the cyanobacterium Anabaena is carried out in heterocysts, specialized, non-dividing cells which differentiate under conditions of ammonia or nitrate deprivation. In Anabaena, heterocyst differentiation is accompanied by rearrangement of some nitrogen fixation genes. A site-specific recombination between an 11 base-pair direct repeat sequence flanking the nifK and nifD genes removes 11 kilobases of intervening DNA, resulting in juxtaposition of the two genes and an alteration of the nifD protein-coding sequence.     

Reduced nitrogen fixation in the glacial ocean inferred from changes in marine nitrogen and phosphorus inventories.

To explain the lower atmospheric CO2 concentrations during glacial periods, it has been suggested that the productivity of marine phytoplankton was stimulated by an increased flux of iron-bearing dust to the oceans. One component of this theory is that iron-an essential element/nutrient for nitrogen-fixing organisms-will increase the rate of marine nitrogen fixation, fuelling the growth of other marine phytoplankton and increasing CO2 uptake. Here we present data that questions this hypothesis. From a sediment core off the northwestern continental margin of Mexico, we show that denitrification and phosphorite formation-processes that occur in oxygen-deficient upwelling regions, removing respectively nitrogen and phosphorus from the ocean-declined in glacial periods, thus increasing marine inventories of nitrogen and phosphorus. But increases in phosphorus were smaller and less rapid, leading to increased N/P ratios in the oceans. Acknowledging that phytoplankton require nitrogen and phosphorus in constant proportions, the Redfield ratio, and that N/P ratios greater than the Redfield ratio are likely to suppress nitrogen fixation, we suggest therefore that marine productivity did not increase in glacial periods in response to either increased nutrient inventories or greater iron supply.     

Iron and phosphorus co-limit nitrogen fixation in the eastern tropical North Atlantic

The role of iron in enhancing phytoplankton productivity in high nutrient, low chlorophyll oceanic regions was demonstrated first through iron-addition bioassay experiments and subsequently confirmed by large-scale iron fertilization experiments. Iron supply has been hypothesized to limit nitrogen fixation and hence oceanic primary productivity on geological timescales, providing an alternative to phosphorus as the ultimate limiting nutrient. Oceanographic observations have been interpreted both to confirm and refute this hypothesis, but direct experimental evidence is lacking. We conducted experiments to test this hypothesis during the Meteor 55 cruise to the tropical North Atlantic. This region is rich in diazotrophs and strongly impacted by Saharan dust input. Here we show that community primary productivity was nitrogen-limited, and that nitrogen fixation was co-limited by iron and phosphorus. Saharan dust addition stimulated nitrogen fixation, presumably by supplying both iron and phosphorus. Our results support the hypothesis that aeolian mineral dust deposition promotes nitrogen fixation in the eastern tropical North Atlantic.     

曲麻莱土壤水分自动站雷击分析与防雷方案设计

通过对玉树州曲麻莱气象局的土壤水分自动站雷击事故的调查,分析了该系统遭受雷击的原因,以曲麻莱气象局土壤水分自动站雷击故障为借鉴,提出农气土壤水分自动站的土壤水分数据采集系统雷电防护的方法和措施,以达到该系统防雷的目的.     

种植转Bt水稻对固氦细菌多样性和固氮酶铁蛋白基因nifH的水平转移的影响

通过对具有不同种植年限和不同种植强度的转Bt基因水稻与非转Bt基因水稻土壤中的细菌以及固氮细菌群落结构进行研究,发现转Bt水稻的种植可能会影响土壤中微生物群落的多样性,但是这种影响的可能只是暂时的,通过对测量种植水稻的芽长实验也得出相似的结论．另外,根据16SrDNA基因构建的系统发育进化树揭示了本实验分离的固氮细菌的遗传多样性,发现实验土壤中的固氮细菌主要分为放线菌门（Actinobacteria）（92％）和α-变形菌门（α-Pro—teobacteria）两大类．分离出的8株典型的固氮菌株,其16SrDNA基因和固氮基因nifH的序列两者的分布不一致,nifH的分布更为紧凑,为固氮基因可能发生了原位水平基因转移提供了证据．     

建筑群的雷击选择性分析及直击雷防护

以电气几何模型的滚球法为理论依据,分析建筑物雷击点的选择性,定性推导建筑群之间的相互屏蔽性,总结建筑群直击雷防护的一些措施,为直击雷的防护提供更加合理和有效的方法.     

利用Saha方程和H线的Stark加宽研究闪电放电通道的电子密度

依据在西藏那曲用无狭缝光栅摄谱仪获得的1次云对地(CG)闪电回击过程的光谱,结合Gigosos得到的等离子体中电子密度的诊断方法,分别利用Hα和Hβ线的Stark加宽研究了闪电放电等离子体的电子密度.同时,运用Saha方程以及以往使用的谱线加宽公式计算了此次闪电放电等离子体的电子密度,并将3种方法所得结果进行了比较、分析,为完善闪电放电等离子体电子密度的诊断方法,进一步研究闪电等离子体特性参数提供了理论依据.