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Coral Reefs - Benthic cyanobacterial mats (BCMs) have increased in abundance on coral reefs worldwide. However, their species diversity and role in nitrogen fixation are poorly understood. We...  相似文献   

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The contribution of nitrogen fixation to the nitrogen budget of Lake Mendota has been calculated. On average, the equivalent of 1.28 × 105 kg of NH3 (as determined by the acetylene reduction technique) was added to this eutrophic lake during June, July and August. Diurnal variation (approximately two-thirds of the day's fixation occurs prior to noon) in algal nitrogen fixation, and variation of fixation with depth (3.6% of the fixation in the column occurs in the top decimeter) were characterized as prerequisites to this calculation.  相似文献   

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Summary The ability to fix nitrogen of 10 strains of the yeasts Rhodotorula, Bullera and Torulopsis and 4 strains of Pullularia, all isolated from soils and some supplied by other investigators was examined using both the heavy nitrogen (15N2) and acetylene reduction techniques. Rigorous standards for aseptic culture, freedom from combined nitrogen and precision of analysis were maintained. No fixation was observed in any of the organisms and the ability of any eucaryote cell to fix nitrogen is doubted. Suggestions for the previous reports of fixation are made.  相似文献   

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《植物生态学报》2018,42(4):407
苔藓-蓝藻共生体(BCS)能固氮, 是养分贫瘠地区森林氮输入的不可忽视的来源。BCS关系与固氮能力研究为科学认识生态系统氮输入与氮循环过程和机理提供了新的视角和有效途径, 具有重要的理论价值。然而, BCS关系、固氮作用与机理的研究迄今未受到足够关注, 报道较少, 认识仍然是零星而片段化的。基于系统查阅的相关文献, 该文综述了BCS的种类组成与共生关系类型、固氮能力及所固定氮的去向及其影响因素和作用机理, 指出了存在的问题及需要深入关注和亟待突破的4个研究方向。  相似文献   

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Liverworts, the most ancient group of land plants, form a range of intimate associations with fungi that may be analogous to the mycorrhizas of vascular plants. Most thalloid liverworts contain arbuscular mycorrhizal glomeromycete fungi similar to most vascular plants. In contrast, a range of leafy liverwort genera and one simple thalloid liverwort family (the Aneuraceae) have switched to basidiomycete fungi. These liverwort switches away from glomeromycete fungi may be expected to parallel switches undergone by vascular plants that target diverse lineages of basidiomycete fungi to form ectomycorrhizas. To test this hypothesis, we used a cultivation-independent approach to examine the basidiomycete fungi associated with liverworts in varied worldwide locations by generating fungal DNA sequence data from over 200 field collections of over 30 species. Here we show that eight leafy liverwort genera predominantly and consistently associate with members of the Sebacina vermifera species complex and that Aneuraceae thalloid liverworts associate nearly exclusively with Tulasnella species. Furthermore, within sites where multiple liverwort species co-occur, they almost never share the same fungi. Our analyses reveal a strikingly conservative ecological and evolutionary pattern of liverwort symbioses with basidiomycete fungi that is unlike that of vascular plant mycorrhizas.  相似文献   

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Background  

Nitrogen, a component of many bio-molecules, is essential for growth and development of all organisms. Most nitrogen exists in the atmosphere, and utilisation of this source is important as a means of avoiding nitrogen starvation. However, the ability to fix atmospheric nitrogen via the nitrogenase enzyme complex is restricted to some bacteria. Eukaryotic organisms are only able to obtain fixed nitrogen through their symbiotic interactions with nitrogen-fixing prokaryotes. These symbioses involve a variety of host organisms, including animals, plants, fungi and protists.  相似文献   

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Summary Experiments carried out in a sand culture have demonstrated that during the growth ofVicia faba andLupinus luteus inoculated with effective strains of Rhizobium, and when the behaviour of bacteroids isolated from nodules ofLupinus luteus, Pisum sativum andVicia faba which had been inoculated with effective and ineffective strains and when comparisons were made between bacteriods isolated from effective nodules ofVicia faba andLupinus luteus either at midday or at midnight there is a reverse correlation between the intensity of nitrogen fixation and respiration, on the one hand, and the content of poly--hydroxybutyric acid (PHB), on the other. This evidence suggests an important role played by PHB in the supply of symbiotic fixation with energy and carbon substrates.Glucose and -hydroxybutyrate were the best substrates for PHB synthesis in the suspension of bacteroids of an effective strain ofR. lupini at all stages of plant growth. At the stage of active nitrogen fixation (flowering) PHB was actively synthesized in the presence of succinate. In the absence of exogenous substrates the polymer degraded, the process being enhanced in the presence of ammonium ions. When ammonium was added together with glucose, PHB synthesis did not occur and at the flowering stage the polymer broke down particularly rapidly. re]19760505  相似文献   

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We measured the δ98Mo of cells and media from molybdenum (Mo) assimilation experiments with the freshwater cyanobacterium Anabaena variabilis, grown with nitrate as a nitrogen (N) source or fixing atmospheric N2. This organism uses a Mo‐based nitrate reductase during nitrate utilization and a Mo‐based dinitrogenase during N2 fixation under culture conditions here. We also demonstrate that it has a high‐affinity Mo uptake system (ModABC) similar to other cyanobacteria, including marine N2‐fixing strains. Anabaena variabilis preferentially assimilated light isotopes of Mo in all experiments, resulting in fractionations of ?0.2‰ to ?1.0‰ ± 0.2‰ between cells and media (εcells–media), extending the range of biological Mo fractionations previously reported. The fractionations were internally consistent within experiments, but varied with the N source utilized and for different growth phases sampled. During growth on nitrate, A. variabilis consistently produced fractionations of ?0.3 ± 0.1‰ (mean ± standard deviation between experiments). When fixing N2, A. variabilis produced fractionations of ?0.9 ± 0.1‰ during exponential growth, and ?0.5 ± 0.1‰ during stationary phase. This pattern is inconsistent with a simple kinetic isotope effect associated with Mo transport, because Mo is likely transported through the ModABC uptake system under all conditions studied. We present a reaction network model for Mo isotope fractionation that demonstrates how Mo transport and storage, coordination changes during enzymatic incorporation, and the distribution of Mo inside the cell could all contribute to the total biological fractionations. Additionally, we discuss the potential importance of biologically incorporated Mo to organic matter‐bound Mo in marine sediments.  相似文献   

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Nitrogen fixation is an important biological process in terrestrial ecosystems and for global crop production. Legume nodulation and N2 fixation have been improved using nodule-enhancing rhizobacteria (NER) under both regular and stressed conditions. The positive effect of NER on legume–rhizobia symbiosis can be facilitated by plant growth-promoting (PGP) mechanisms, some of which remain to be identified. NER that produce aminocyclopropane-1-carboxylic acid deaminase and indole acetic acid enhance the legume–rhizobia symbiosis through (i) enhancing the nodule induction, (ii) improving the competitiveness of rhizobia for nodulation, (iii) prolonging functional nodules by suppressing nodule senescence and (iv) upregulating genes associated with legume–rhizobia symbiosis. The means by which these processes enhance the legume–rhizobia symbiosis is the focus of this review. A better understanding of the mechanisms by which PGP rhizobacteria operate, and how they can be altered, will provide opportunities to enhance legume–rhizobial interactions, to provide new advances in plant growth promotion and N2 fixation.  相似文献   

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Agriculture in the United States (US) cycles large quantities of nitrogen (N) to produce food, fuel, and fiber and is a major source of excess reactive nitrogen (Nr) in the environment. Nitrogen lost from cropping systems and animal operations moves to waterways, groundwater, and the atmosphere. Changes in climate and climate variability may further affect the ability of agricultural systems to conserve N. The N that escapes affects climate directly through the emissions of nitrous oxide (N2O), and indirectly through the loss of nitrate (NO3 ?), nitrogen oxides (NO x ) and ammonia to downstream and downwind ecosystems that then emit some of the N received as N2O and NO x . Emissions of NO x lead to the formation of tropospheric ozone, a greenhouse gas that can also harm crops directly. There are many opportunities to mitigate the impact of agricultural N on climate and the impact of climate on agricultural N. Some are available today; many need further research; and all await effective incentives to become adopted. Research needs can be grouped into four major categories: (1) an improved understanding of agricultural N cycle responses to changing climate; (2) a systems-level understanding of important crop and animal systems sufficient to identify key interactions and feedbacks; (3) the further development and testing of quantitative models capable of predicting N-climate interactions with confidence across a wide variety of crop-soil-climate combinations; and (4) socioecological research to better understand the incentives necessary to achieve meaningful deployment of realistic solutions.  相似文献   

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Induction of assimilatory NO 3 reduction through the application of an easily decomposable substrate in alkaline–saline soils of the former lake Texcoco (Mexico) resulted in a fast immobilization of NO 3 in excess of N required for metabolic activity and the release of large concentrations of NO 2 and smaller amounts of NH 4 + . We postulated that this was regulated by the amounts of NO 3 and glucose added, and affected by the specific characteristics of soil from the former lake Texcoco. This was investigated by spiking soils of different electrolytic conductivity (EC) 56.0 dS m−1 (soil A of Texcoco) and 11.6 dS m−1 (soil B of Texcoco) with different concentrations of NO 3 and glucose while dynamics of CO2, NH 4 + , NO 2 and NO 3 were monitored in an aerobic incubation for 7 days. For comparison reasons (control) an agricultural soil with low EC (0.3 dS m−1) was included as well. In the agricultural soil, 67% of the added glucose mineralized within 7 days, but only 15% in soil A of Texcoco and 20% in soil B of Texcoco. The application of NO 3 to the agricultural soil added with glucose increased cumulative production of CO2 1.2 times, 1.5 times in soil A of Texcoco and 1.8 times in soil B of Texcoco. Concentration of NO 2 increased to > 100 mg NO 2 -N kg−1 when 1000 mg glucose-C kg−1 and 500 mg NO 3 -N kg−1 were added to soil A and B of Texcoco, but remained < 3 mg NO 2 -N kg−1 in the agricultural soil. The ratio between the cumulative production of CO2 and the decrease in concentration of NO 3 was approximately one in soil A and B of Texcoco, but 10 in the agricultural soil after 3 days. It was found that micro-organisms in the alkaline–saline soil of the former lake Texcoco were capable of immobilizing large quantities of NO 3 when an easy decomposable substrate was available in excess of what might be required for metabolic activity while producing large concentrations of NO 2 , but these phenomena were absent in an agricultural soil. In soil of Texcoco, concentrations of NO 2 and NH 4 + increased with increased salinity and availability of NO 3 . This ability to remove large quantities of NO 3 under these conditions and then utilize it at a later time might benefit micro-organisms of the N limited alkaline–saline soils of Texcoco.  相似文献   

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Nitrogen fixation (acetylene reduction) was found in intact tomato (Lycopersicon esculentum Mill ‘Pusa Ruby’) plants in the field, in pots and also in aseptic cultures. The unsterilized as well as sterilized rhizoplane and phylloplane of the plant when assayed separately also responded to the test. From root bits of tomato sterilized upto 20 minutes with 0.1% mercuric chloride, growth of the bacterium from the interior of the root into the medium was observed thereby indicating their presence within the endorhizosphere. Phase contrast and electron microscopic studies of the root system of tomato revealed the presence of bacterial colonies in the epidermis, cortex and vascular bundles. Bacterial numbers in the endosphere, of root and leaf were 30×104 and 12×104, respectively, per gram fresh weight of tissue. The bacteria were predominantly rod-shaped 1.4–4.8×0.9–1.95 μm in 24-h-old cultures, pleomorphic, polar or bipolary flagellated having β-hydroxy butyrate granules. The bacterium has been identified as a new species of Azospirillum.  相似文献   

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《Zoology (Jena, Germany)》2015,118(2):125-131
Cnidarian–dinoflagellate photosynthetic symbioses are fundamental to biologically diverse and productive coral reef ecosystems. The hallmark of this symbiotic relationship is the ability of dinoflagellate symbionts to supply their cnidarian host with a wide range of nutrients. Many aspects of this association nevertheless remain poorly characterized, including the exact identity of the transferred metabolic compounds, the mechanisms that control their exchange across the host–symbiont interface, and the precise subcellular fate of the translocated materials in cnidarian tissues. This lack of knowledge is mainly attributed to difficulties in investigating such metabolic interactions both in situ, i.e. on intact symbiotic associations, and at high spatial resolution. To address these issues, we illustrate the application of two in situ and high spatial resolution molecular and ion imaging techniques–matrix-assisted laser desorption ionization mass spectrometry imaging (MALDI-MSI) and the nano-scale secondary-ion mass spectrometry (NanoSIMS) ion microprobe. These imaging techniques provide important new opportunities for the detailed investigation of many aspects of cnidarian–dinoflagellate associations, including the dynamics of cellular interactions.  相似文献   

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As the newest plant hormone, strigolactone research is undergoing an exciting expansion. In less than five years, roles for strigolactones have been defined in shoot branching, secondary growth, root growth and nodulation, to add to the growing understanding of their role in arbuscular mycorrhizae and parasitic weed interactions.1 Strigolactones are particularly fascinating as signaling molecules as they can act both inside the plant as an endogenous hormone and in the soil as a rhizosphere signal.2-4 Our recent research has highlighted such a dual role for strigolactones, potentially acting as both an endogenous and exogenous signal for arbuscular mycorrhizal development.5 There is also significant interest in examining strigolactones as putative regulators of responses to environmental stimuli, especially the response to nutrient availability, given the strong regulation of strigolactone production by nitrate and phosphate observed in many species.5,6 In particular, the potential for strigolactones to mediate the ecologically important response of mycorrhizal colonization to phosphate has been widely discussed. However, using a mutant approach we found that strigolactones are not essential for phosphate regulation of mycorrhizal colonization or nodulation.5 This is consistent with the relatively mild impairment of phosphate control of seedling root growth observed in Arabidopsis strigolactone mutants.7 This contrasts with the major role for strigolactones in phosphate control of shoot branching of rice and Arabidopsis8,9 and indicates that the integration of strigolactones into our understanding of nutrient response will be complex. New data presented here, along with the recent discovery of phosphate specific CLE peptides,10 indicates a potential role for PsNARK, a component of the autoregulation of nodulation pathway, in phosphate control of nodulation.  相似文献   

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《BBA》2013,1827(10):1226-1234
Although the major light harvesting complexes of diatoms, called FCPs (fucoxanthin chlorophyll a/c binding proteins), are related to the cab proteins of higher plants, the structures of these light harvesting protein complexes are much less characterized. Here, a structural/functional model for the “core” of FCP, based on the sequence homology with LHCII, in which two fucoxanthins replace the central luteins and act as quenchers of the Chl a triplet states, is proposed. Combining the information obtained by time-resolved EPR spectroscopy on the triplet states populated under illumination, with quantum mechanical calculations, we discuss the chlorophyll triplet quenching in terms of the geometry of the chlorophyll–carotenoid pairs participating to the process. The results show that local structural rearrangements occur in FCP, with respect to LHCII, in the photoprotective site.  相似文献   

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