Nitrogen fixation by gloeocapsa

The continuous growth in a medium free of combined nitrogen and the experimental production of ethylene via acetylene reduction indicate that nitrogen fixation by blue-green algae is not solely confined to filamentous genera with heterocysts. Axenic cultures of Gloeocapsa sp., adapted to nitrate-free medium, form ethylene at rates comparable to those of species known to fix nitrogen.     

Nitrogen fixation in termites

Nitrogen fixation, measured by the reduction of acetylene to ethylene, was found in workers of the dry-wood termite Kalotermes minor. The soldiers and reproductive castes fixed little or no nitrogen. The fixation rates ranged between 24 and 566 micrograms of nitrogen fixed per month per gram (wet weight) of termite. Nitrogen fixation can be a significant source of nitrogen for these termites.     

Nitrogen fixation in marine shipworms

Nitrogen fixation is associated with four shipworl species. A bacterium capable of fixing nitrogen under anaerobic conditions and of liquefying cellulose in culture has been isolated from the gut of one species. High fixation rates (up to 1.5 micrograms of nitrogen per milligram dry weight per hour), which resulted in a doubling of cellular nitrogen in as little as 1.4 days, was associated with Teredora malleolus from the Sargasso Sea. Three species from coastal waters were assayed, and of these juveniles showed the highest fixation rates. Nitrogen fixation activity appeared to be inversely related to the ability of shipworms to obtain combined-nitrogen compounds in their diet. It could be a significant source of nitrogen for shipwornms and perhaps other oceanic organisms that ingest terrestrial plant material.     

Nitrogen fixation in the marine environment

Cyanobacteria of the genus Oscillatoria (Trichodesmium) account for annual inputs of nitrogen to the world's oceans of about 4.8 x 10(12) grams while benthic environments contribute 15 x 10(12) grams. The sum of these inputs is one-fifth of current estimates of nitrogen fixation in terrestrial environments and one-half of the present rate of industrial synthesis of ammonia. When the total of all nitrogen inputs to the sea is compared with estimated losses through denitrification, the marine nitrogen cycle approximates a steady state. Oceanic nitrogen fixation can supply less than 0.3 percent of the calculated demand of marine phytoplankton. The minor contribution by nitrogen fixation to the overall nitrogen economy of the sea is not consistent with the supposition that nitrogen is the primary limiting nutrient and suggests that factors other than nitrogen availability limit phytoplankton growth rates.     

Nitrogen fixation by anaerobic cellulolytic bacteria

Four strains of anaerobic nitrogen-fixing, cellulose-fermenting bacteria were isolated in pure culture from freshwater mud and soil. Nitrogenase activity was demonstrated in these strains and also in several previously described anaerobic cellulolytic bacteria isolated from various natural environments. These are the first anaerobic bacteria known to use cellulose as an energy source for nitrogen fixation. Because cellulose is a plant polysaccharide that abounds in nature, these results raise the possibility that nitrogen-fixing, cellulose-fermenting bacteria may be widespread and thus play a major role in carbon and nitrogen cycling.     

Nitrogen fixation in some anoxic lacustrine environments

Low rates of acetylene reduction to ethylene in water samples from two dystrophic lakes indicate the presence of nitrogenase and in situ nitrogen fixation. Highest rates were found in anoxic water from the aphotic zone. Environmental conditions in these lakes suggest the agents of fixation were bacteria.     

Nitrogen fixation in a coral reef community

Algal reef flats at Enewetak Atoll, Marshall Islands, fix atmospheric nitrogen at rates comparable to those in managed agriculture. The dominant nitrogen fixer appears to be the blue-green alga Calothrix crustacea. Since this nutrient enrichment contributes to the high productivity of adjacent coral reefs and undoubtedly to atoll lagoons, it is recommended that the algal reef flats receive increased conservation priority.     

Nitrogen fixation and delayed leaf senescence in soybeans

Delayed leaf senescence has been found in a soybean population which maintains its chlorophyll and ribulosebisphosphate carboxylase activity in leaves and nitrogen fixation (acetylene reduction) activity in root nodules throughout seed maturation. Incorporation of delayed leaf senescence into an agronomically desirable genetic background may help to increase seed yield and symbiotic nitrogen fixation during seed development.     

Nitrogen fixation by a blue-green epiphyte on pelagic sargassum

Nitrogen fixation by Dichothrix fucicola, an epiphyte on pelagic Sargassum, was measured in May and June 1972 in the western Sargasso Sea and the Gulf Stream. This is the first report of nitrogen fixation by a heterocyst-bearing blue-green alga in the open ocean, and also the first observation of nitrogen fixation in the genus Dichothrix. Cellular carbon/nitrogen ratios suggested that the Dichothrix was nitrogen-starved. In dense aggregations of Sargassum, such as rafts or windrows, the enrichment of surface seawater with combined nitrogen from nitrogen fixation may be pronounced.     

白蚁及其肠道细菌的固氮作用研究

台湾家白蚁(Coptotermes formosanus Shiraki)工蚁具有较高的固氮活性,刚收集的工蚁其固氮活性达0.094 4 μg/g*h-1.固氮活性的高低受白蚁食物含氮量、白蚁生理状态的影响,某些杀菌剂能使白蚁固氮活性丧失.从台湾家白蚁肠道中分离到2株兼性厌氧固氮菌HN1和YN菌株,经鉴定皆为柠檬酸菌属的菌株(Citrobacter sp.),均在厌氧条件下表现固氮活性.最大或然数法计数表明,每只白蚁肠道固氮菌数量约为7 500个.     

Nitrogen fixation at 92 degrees C by a hydrothermal vent archaeon

A methanogenic archaeon isolated from deep-sea hydrothermal vent fluid was found to reduce N(2) to NH(3) at up to 92 degrees C, which is 28 degrees C higher than the current upper temperature limit of biological nitrogen fixation. The 16S ribosomal RNA gene of the hyperthermophilic nitrogen fixer, designated FS406-22, was 99% similar to that of non-nitrogen fixing Methanocaldococcus jannaschii DSM 2661. At its optimal growth temperature of 90 degrees C, FS406-22 incorporated (15)N(2) and expressed nifH messenger RNA. This increase in the temperature limit of nitrogen fixation could reveal a broader range of conditions for life in the subseafloor biosphere and other nitrogen-limited ecosystems than previously estimated.     

生物固氮基因簇结构与进化研究进展

在固氮菌基因组中,固氮相关基因都是作为一个或几个基因簇存在,是细菌生物固氮的遗传基础。这其中包括固氮酶结构基因nifHDK,以及参与电子转运和铁钼辅因子合成及组装的其他相关基因。对不同菌中固氮基因簇的结构和进化进行了简要的比较分析。比较所有已测序固氮菌中的固氮基因簇,发现均至少含有六个保守基因:nifH、nifD、nifK、nifE、nifN和nifB,并且这6个基因在所有已鉴定的系统中都是固氮所必需的。尽管在不同种类的固氮微生物中,固氮基因簇的构成及组织模式有所不同,但是它们在结构、催化机制和系统进化上紧密相关。同时也对固氮基因的研究方向提出了展望。旨在为今后进一步研究生物固氮机制,了解固氮微生物的进化,扩展生物固氮多样性提供理论基础。     

羊奶果的共生固氮及氮素的运转形式

对羊奶果根瘤的固氮、吸氧、防氧保护及固氮产物的运转进行研究。羊奶果是热带植物，离体根瘤固氮活性在25－30℃时最高，且在离体3h以内无显著变化；35－40℃的高温对根瘤吸氢活性有促进作用，离体根瘤的吸氢活性可保持6h 以上，低温对固氮活性有明显的抑制作用。羊奶果为酰脲型植物，根、根瘤和木质部中都含有大量的尿囊酸，酰脲含量均占氮化合物总量的50％以上，说明它的根瘤和根都能将氮素转化成酰脲并经过木质部 向上转运，还有部分氮素通过丝氨酸、蛋氨酸为主的氨基酸形式运输。根瘤中未检测到血红蛋白，但有较高含量的SOD、ASA和GSH.     

Nitrogen fixation by floating diatom mats: a source of new nitrogen to oligotrophic ocean waters

Nitrogen fixation, apparently by bacterial endosymbionts, is associated with intertwining chains of two species of the diatom Rhizosolenia. In situ fixation rates were enhanced by incubation in the dark, whereas concurrent shipboard experiments either underestimated or did not detect nitrogen fixation. This is the first example of nitrogen fixation associated with a bacteria-diatom symbiosis in the pelagic zone, and it indicates that these systems may contribute a significant amount of "new" nitrogen to oligotrophic waters.