Showing papers on "Nitrogen fixation published in 1994"
TL;DR: General features of the nitrogenase system, including conformational coupling of nucleotide hydrolysis, aspects of the cluster structures, and the general spatial organization of redox centers within the protein subunits, are relevant to a wide range of biochemical systems.
Abstract: Biological nitrogen fixation is catalyzed by the nitrogenase enzyme system which consists of
two metalloproteins, the iron (Fe-) protein and the molybdenum-iron (MoFe-) protein. Together, these
proteins mediate the ATP-dependent reduction of dinitrogen to ammonia. Recent crystallographic analyses
of Fe-protein and MoFe-protein have revealed the polypeptide fold and the structure and organization of
the unusual metal centers in nitrogenase. These structure provide a molecular framework for addressing
the mechanism of the nitrogenase-catalyzed reaction. General features of the nitrogenase system, including
conformational coupling of nucleotide hydrolysis, aspects of the cluster structures, and the general spatial
organization of redox centers within the protein subunits, are relevant to a wide range of biochemical
systems.
378 citations
TL;DR: Hypotheses concerning factors causing cyanobacterial dominance in freshwaters are reviewed against evidence from four different types of lakes and it is argued that either cyanobacteria should not be treated as a group or that more information is needed.
Abstract: Hypotheses concerning factors causing cyanobacterial dominance in freshwaters are reviewed against evidence from four different types of lakes. It is argued that either cyanobacteria should not be treated as a group or that more information is needed. It
293 citations
TL;DR: Rates of release of DON showed considerable variation within replicate experiments and were variable depending on time of day and duration of time course experiments, likely a significant source of new nitrogen for the associated bacteria or the non-nitrogen-fixing filaments of the Trichodesmium colonies.
Abstract: Trichodesmium sp. is a filamentous, colonial cyanobacterium which contributes substantially to the input of nitrogen in tropical and subtropical oceanic waters through nitrogen fixation (N2 fixation). We applied a 15N tracer technique to assess the rate of release of dissolved organic nitrogen (DON) from this cyanobacterium and compared those rates with rates of N2 fixation determined for the same assemblages at the same times of day. Rates of release of DON showed considerable variation within replicate experiments and were variable depending on time of day and duration of time course experiments. On average, rates of DON release were ca. 50% the rates of N2 fixation. We also fractionated the DON released by using ultrafiltration and found that 60 to 80% of the total organic release was of the size class <10,000 Da. The release of these organic compounds by Trichodesmium spp. is likely a significant source of new nitrogen for the associated bacteria or the non-nitrogen-fixing filaments of the Trichodesmium colonies.
257 citations
TL;DR: In this paper, the authors present results on a number of structural and physiological characteristics that allow arctic legumes to function in extreme environments, indicating that both the plants and their rhizobia have successfully adapted to arctic conditions.
Abstract: Biological nitrogen fixation is a phenomenon occurring in all known ecosystems. Symbiotic nitrogen fixation is dependent on the host plant genotype, the Rhizobium strain, and the interaction of these symbionts with the pedoclimatic factors and the environmental conditions. Extremes of pH affect nodulation by reducing the colonization of soil and the legume rhizosphere by rhizobia. Highly acidic soils (pH 8.0) tend to be high in sodium chloride, bicarbonate, and borate, and are often associated with high salinity which reduce nitrogen fixation. Nodulation and N-fixation are observed under a wide range of temperatures with optima between 20–30°C. Elevated temperatures may delay nodule initiation and development, and interfere with nodule structure and functioning in temperate legumes, whereas in tropical legumes nitrogen fixation efficiency is mainly affected. Furthermore, temperature changes affect the competitive ability of Rhizobium strains. Low temperatures reduce nodule formation and nitrogen fixation in temperate legumes; however, in the extreme environment of the high arctic, native legumes can nodulate and fix nitrogen at rates comparable to those observed with legumes in temperate climates, indicating that both the plants and their rhizobia have successfully adapted to arctic conditions. In addition to low temperatures, arctic legumes are exposed to a short growing season, a long photoperiod, low precipitation and low soil nitrogen levels. In this review, we present results on a number of structural and physiological characteristics which allow arctic legumes to function in extreme environments.
245 citations
TL;DR: Besides the relatively small contribution from commercial ammonical fertilizer production, replenishing of the nitrogen pool falls mainly to a limited number of physiologically diverse microbes that contain the nitrogenase enzyme system.
Abstract: In the simplest terms, the biological nitrogen cycle is the reduction of atmospheric dinitrogen (N2) to ammonia with the subsequent reoxidation ammonia to dinitrogen (1). At the reduction level of ammonia, nitrogen incorporated into precursors for biological macromolecules such as proteins and nucleic acids. Reoxidation of ammonia to dinitrogen ("denitrification") by a variety of microbes (by way of nitrite and other oxidation levels of nitrogen) leads to the depletion of the "fixed," biologically usable, nitrogen pool. Besides the relatively small contribution from commercial ammonical fertilizer production, replenishing of the nitrogen pool falls mainly to a limited number of physiologically diverse microbes (e.g. eubacteria and archaebacteria; free-living and symbiotic; aerobic and anaerobic) that contain the nitrogenase enzyme system.
213 citations
TL;DR: C Cyanothece sp.
Abstract: It has been shown that some aerobic, unicellular, diazotrophic cyanobacteria temporally separate photosynthetic O2 evolution and oxygen-sensitive N2 fixation. Cyanothece sp. ATCC strain 51142 is an aerobic, unicellular, diazotrophic cyanobacterium that fixes N2 during discrete periods of its cell cycle. When the bacteria are maintained under diurnal light-dark cycles, N2 fixation occurs in the dark. Similar cycling is observed in continuous light, implicating a circadian rhythm. Under N2-fixing conditions, large inclusion granules form between the thylakoid membranes. Maximum granulation, as observed by electron microscopy, occurs before the onset of N2 fixation, and the granules decrease in number during the period of N2 fixation. The granules can be purified from cell homogenates by differential centrifugation. Biochemical analyses of the granules indicate that these structures are primarily carbohydrate, with some protein. Further analyses of the carbohydrate have shown that it is a glucose polymer with some characteristics of glycogen. It is proposed that N2 fixation is driven by energy and reducing power stored in these inclusion granules. Cyanothece sp. strain ATCC 51142 represents an excellent experimental organism for the study of the protective mechanisms of nitrogenase, metabolic events in cyanobacteria under normal and stress conditions, the partitioning of resources between growth and storage, and biological rhythms.
204 citations
TL;DR: Results suggest that nitrogen-fixing bacteria found in the skeleton of corals benefit from organic carbon excreted by the coral tissue, which may be of major importance for the nitrogen budget of the corals.
Abstract: Nitrogen fixation, as measured by acetylene reduct~on, has been detected to be associated with various hermatypic corals. Experiments were carried out on the massive coral Favia f a v u s both in situ and in the laboratory. Nitrogen fixation activity was found to be light dependent and fully inhibited by 5 X 10-6 M DCMU [3-(3,4-dichloropheny1)-1,l-dimethylurea Addition of glucose restored nitrogen fixation activity both in the dark and in the presence of DCMU. Removal of the coral tissue prevented acetylene reduction, while addition of glucose to the coral skeleton restored this activity. Bacteria isolated from the coral skeleton were found by dot blotting to contain the nif H gene. These results suggest that nitrogen-fixing bacteria found in the skeleton of corals benefit from organic carbon excreted by the coral tissue. The interaction between the nitrogen-fixing organisms and the coral may be of major lrnportance for the nitrogen budget of the corals.
182 citations
TL;DR: In this article, the regulatory impact of iron availability on N(2) fixation and production dynamics was investigated in the widespread and biogeochemically important diazotrophic, planktonic cyanobacteria Trichodesmium spp.
Abstract: In light of recent proposals that iron (Fe) availability may play an important role in controlling oceanic primary production and nutrient flux, its regulatory impact on N(2) fixation and production dynamics was investigated in the widespread and biogeochemically important diazotrophic, planktonic cyanobacteria Trichodesmium spp. Fe additions, as FeCl(3) and EDTA-chelated FeCl(3), enhanced N(2) fixation (nitrogenase activity), photosynthesis (CO(2) fixation), and growth (chlorophyll a production) in both naturally occurring and cultured (on unenriched oligotrophic seawater) Trichodesmium populations. Maximum enhancement of these processes occurred under FeEDTA-amended conditions. On occasions, EDTA alone led to enhancement. No evidence for previously proposed molybdenum or phosphorus limitation was found. Our findings geographically extend support for Fe limitation of N(2) fixation and primary production to tropical and subtropical oligotrophic ocean waters often characterized by Trichodesmium blooms.
150 citations
TL;DR: Comparing the effects of heat on symbiotic nitrogen fixation, cell survival, amino acid uptake, and protein synthesis in a heat-tolerant and aHeat-sensitive bean-nodulating Rhizobium strain indicated the higher tolerance of CIAT899 than of CNPAF512 to thermal stress, and acquired thermotolerance inCIAT899 was shown to depend on protein synthesis.
Abstract: High soil temperatures in tropical areas limit nodulation and dinitrogen fixation by strains of Rhizobium. Several heat-tolerant bean-nodulating Rhizobium strains have been isolated previously. However, the basis of their resistance to heat remains unknown. In this study, we compared the effects of heat on symbiotic nitrogen fixation, cell survival, amino acid uptake, and protein synthesis in a heat-tolerant (CIAT899) and a heat-sensitive (CNPAF512) bean-nodulating Rhizobium strain. Acetylene reduction activity of nodulated roots excised from unstressed plants was strongly diminished at 35 or 40 degrees C when plants were nodulated either by CIAT899 or by CNPAF512. When these strains were tested under free-living conditions, survival at 40 degrees C as well as the kinetics of l-[S]methionine uptake and protein synthesis at 35 and 40 degrees C indicated the higher tolerance of CIAT899 than of CNPAF512 to thermal stress. The synthesis of heat shock proteins was detected in both strains, although at different temperatures. Increased synthesis of 14 heat shock proteins in CNPAF512 and of 6 heat shock proteins in CIAT899 was observed at 40 and 45 degrees C, respectively. A heat shock protein of approximately 21 kDa, of which the synthesis was strongest in both Rhizobium strains upon a temperature shift up, was also conserved in several other bean-nodulating rhizobia. Acquired thermotolerance in CIAT899 was shown to depend on protein synthesis.
134 citations
TL;DR: Examination of boron-deficient nodules showed dramatic changes in cell walls and in both peribacteroid and infection thread membranes, suggesting a role for bor on in the stability of these structures.
Abstract: The effect of boron deficiency on symbiotic nitrogen fixation in pea (Pisum sativum) was examined. The absence of boron in the culture medium resulted in a decrease of the number of nodules and an alteration of nodule development leading to an inhibition of nitrogenase activity. Examination of boron-deficient nodules showed dramatic changes in cell walls and in both peribacteroid and infection thread membranes, suggesting a role for boron in the stability of these structures. These results indicate that boron is a requirement for normal nodule development and functionality.
127 citations
TL;DR: Estimates of transfer of atmospheric derived nitrogen from clover to grass obtained by the natural 15N abundance method were consistently higher than those obtained byThe enriched 15N dilution method, which showed that application of 400 kg N ha-1 significantly reduced dinitrogen fixation, while cutting frequency had no effect.
Abstract: A plant mixture of white clover (Trifolium repens L.), red clover (Trifolium pratense L.), and ryegrass (Lolium perenne L.) was established in the spring of 1991 under a cover-crop of barley. Treatments were two levels of nitrogen (400 and 20 kg N ha-1) and two cutting intensities (3 and 6 cuts per season). Fixation of atmospheric derived nitrogen was estimated by two 15N dilution methods, one based on application of 15N to the soil, the other utilising small differences in natural abundance of 15N.
TL;DR: It is concluded that RMBPC -2 can increase alfalfa yields under field conditions of nitrogen limitation, low endogenous rhizobial competitors, and sufficient moisture.
Abstract: The construction of rhizobial strains which increase plant biomass under controlled conditions has been previously reported. However, there is no evidence that these newly constructed strains increase legume yield under agricultural conditions. This work tested the hypothesis that carefully manipulating expression of additional copies of nifA and dctABD in strains of Rhizobium meliloti would increase alfalfa yield in the field. The rationale for this hypothesis is based on the positive regulatory role that nifA plays in the expression of the nif regulon and the fact that a supply of dicarboxylic acids from the plant is required as a carbon and energy source for nitrogen fixation by the Rhizobium bacteroids in the nodule. These recombinant strains, as well as the wild-type strains from which they were derived, are ideal tools to examine the effects of modifying or increasing the expression of these genes on alfalfa biomass. The experimental design comprised seven recombinant strains, two wild-type strains, and an uninoculated control. Each treatment was replicated eight times and was conducted at four field sites in Wisconsin. Recombinant strain RMBPC-2, which has an additional copy of both nifA and dctABD, increased alfalfa biomass by 12.9% compared with the yield with the wild-type strain RMBPC and 17.9% over that in the uninoculated control plot at the site where soil nitrogen and organic matter content was lowest. These increases were statistically significant at the 5% confidence interval for each of the three harvests made during the growing season. Strain RMBPC-2 did increase alfalfa biomass at the Hancock site; however, no other significant increases or decreases in alfalfa biomass were observed with the seven other recombinant strains at that site. At three sites where this experiment was conducted, either native rhizobial populations or soil nitrogen concentrations were high. At these sites, none of the recombinant strains affected yield. We conclude that RMBPC -2 can increase alfalfa yields under field conditions of nitrogen limitation, low endogenous rhizobial competitors, and sufficient moisture.
TL;DR: Using this approach, it was shown that microbial denitrification can represent a significant loss of combined nitrogen from mats on daily as well as monthly time scales and underscore the importance of determining integrated diel rates of both nitrogen fixation andDenitrification when constructing N budgets.
Abstract: Spatial and temporal variations in nitrogen fixation and denitrification rates were examined between July 1991 and September 1992 in the intertidal regions of Tomales Bay (California, USA). Microbial mat communities inhabited exposed mudflat and vegetated marsh surface sediments. Mudflat and marsh sediments exhibited comparable rates of nitrogen fixation. Denitrification rates were higher in marsh sediments. Nitrogen fixation rates were lowest during January at both sites, whereas highest rates occurred during summer and fall. Denitrification rates were highest during fall and winter months in marsh sediments, while rates in mudflat sediments were highest during summer and fall. In mudflat sediments, nitrogen fixation and denitrification rates, integrated over 24 h, ranged from 6 to 79 mg N m-1 d-1 and 1 to 10 mg N m-2 d-1, respectively. Rates of denitrification represented between 6 and 20% of nitrogen fixation rates during the day, but exceeded or were equivalent to nitrogen fixation rates at night. The highest integrated rates of both nitrogen fixation and denitrification occurred during July, whereas, the highest percent loss occurred during spring when denitrification rates amounted to 20% of nitrogen fixation rates during the day. Over an annual cycle, inputs of fixed N to mudflat communities occurred exclusively during daylight. These results underscore the importance of determining integrated diel rates of both nitrogen fixation and denitrification when constructing N budgets. Using this approach, it was shown that microbial denitrification can represent a significant loss of combined nitrogen from mats on daily as well as monthly time scales.
TL;DR: Nitrogen inputs, outputs and compartamentalization were quantified in a freshwater fish pond stocked with hybrid Oreochromis throughout a production cycle, finding that nitrogen transformation constituted a major pathway for nitrogen transformation, but only 1% of the nitrogen input was lost through denitrification.
Abstract: Nitrogen inputs, outputs and compartamentalization were quantified in a freshwater fish pond stocked with hybrid Oreochromis throughout a production cycle. The budget accounts for 91% of the nitrogen added to the system. Feed addition accounted for 87% of the nitrogen input and an additional 11% was attributable to nitrogen fixation, mainly in the water column. The balance of the nitrogen input was contained in the source water for the pond. Commercial-size fish accumulated 17.5% of the nitrogen added to the system. Most of the nitrogen was eventually deposited in the sediments. Nitrification constituted a major pathway for nitrogen transformation, but only 1% of the nitrogen input was lost through denitrification.
TL;DR: Effects of boron (B) on soybean growth, nodule development, and nitrogen fixation were studied and Boron concentration in the whole soybean plant increased when the amount of B in the culture media increased.
Abstract: Effects of boron (B) on soybean growth, nodule development, and nitrogen fixation were studied. When soybean plants were grown in B-free medium for 35,40, or 72 d, B deficiency symptoms appeared. Nodules of B-deficient soybeans were damaged and showed low acetylene reduction activities. Soybean seed production was seriously depressed. Production of whole plant, nodules and pods was maximum at concentrations of 15–180 μg B L-1 (1.4–16 μm). At concentrations of 29–88 μg B L-1 nitrogen fixation on a plant and nodule weight basis was markedly enhanced. Element absorption into soybean plants grown without boron and at 22, 44, 440, and 1,300 p.g B L-1 was examined. At 22–44 μg B L-1, the total amount of K, Ca, and Mg absorbed into soybean plants was relatively high, although the concentrations of these elements on a dry weight basis were almost the same. Boron concentration in the whole soybean plant increased when the amount of B in the culture media increased. The B exogenously supplied accumulated i...
TL;DR: It is shown that, although homologous regulatory molecules allow response to oxygen in different species, they are assembled in various combinations resulting in a variable regulatory coupling between genes for microaerobic respiration and nitrogen fixation genes.
Abstract: Rhizobia are gram-negative bacteria with two distinct habitats: the soil rhizosphere in which they have a saprophytic and, usually, aerobic life and a plant ecological niche, the legume nodule, which constitutes a microoxic environment compatible with the operation of the nitrogen reducing enzyme nitrogenase. The purpose of this review is to summarize the present knowledge of the changes induced in these bacteria when shifting to a microoxic environment. Oxygen concentration regulates the expression of two major metabolic pathways: energy conservation by respiratory chains and nitrogen fixation. After reviewing the genetic data on these metabolic pathways and their response to oxygen we will put special emphasis on the regulatory molecules which are involved in the control of gene expression. We will show that, although homologous regulatory molecules allow response to oxygen in different species, they are assembled in various combinations resulting in a variable regulatory coupling between genes for microaerobic respiration and nitrogen fixation genes. The significance of coordinated regulation of genes not essential for nitrogen fixation with nitrogen fixation genes will also be discussed.
Patent•
28 Dec 1994
TL;DR: In this paper, a method and compositions for increasing the availability of soluble phosphate and fixed nitrogen for legume: Rhizobium symbioses involving co-inoculating legume seeds with a phosphate-solubilizing soil fungus, Penicillium bilaii, and Rhizobalium spp. prior to planting.
Abstract: The invention provides methods and compositions for increasing the availability of soluble phosphate and fixed nitrogen for legume: Rhizobium symbioses involving co-inoculating legume seeds with a phosphate-solubilizing soil fungus, Penicillium bilaii, and Rhizobium spp. prior to planting. The two types of microorganisms do not appear to compete for nutrients in the rhizosphere of legumes and the P. bilaii provides a source of available phosphorus for use by the plant without adversely affecting the nitrogen fixation ability of the Rhizobium spp.; and indeed such ability is enhanced. The invention is used to increase the efficiencies of nodulation, nitrogen fixation and legume crop production.
TL;DR: Rates of acetylene reduction and estimated rates of nitrogen fixation in three sites that varied in soil age and nitrogen accumulation on Kilauea Volcano, Hawai'i, showed that the sum of nitrogen fixed by these sources plus nitrogen inputs in precipitation was insufficient to explain the quantity of nitrogen accumulated in the 200 yr old site.
Abstract: I determined rates of acetylene reduction and estimated rates of nitrogen fixation in three sites that varied in soil age and nitrogen accumulation on Kilauea Volcano, Hawai'i. The major substrates in which acetylene reduction occurred were lichens of the genus Stereocaulon, liverworts, decaying leaf litter, and decaying wood. Estimated rates of nitrogen fixation were 0.3, 1.2, and 2.8 kg ha-' yr-' in an open-canopied 27 yr old site, a 200 yr old primary successional montane rain forest, and an -2000 yr old montane rain forest, respectively. The sum of nitrogen fixed by these sources plus nitrogen inputs in precipitation was insufficient to explain the quantity of nitrogen accumulated in the 200 yr old site.
TL;DR: Intercropping pea and mustard has demonstrated the capacity to increase economic returns by achieving land equivalent ratios (LER) > 1.
Abstract: Intercropping pea and mustard has demonstrated the capacity to increase economic returns by achieving land equivalent ratios (LER) > 1. Mineral N is essential to produce adequate mustard stands, however, significant N additions are inhibitory to N 2 fixation. Yield and N 2 fixation studies characterized the response of pea and mustard in sole and intercrop conditions at four fertilizer N concentrations (10, 30, 60 and 90 kg N ha −1 ). The yield of peas and LERs of the intercrop did not increase with increasing N rates. The mustard yields increased with N rate in sole-crop but not in intercrop plots. Seasonal patterns of nitrogenase activity and total N 2 fixation estimates in pea were made using acetylene reduction assays and the 15 N isotope dilution technique, respectively. Nitrogenase activity early in the growing season was negatively correlated with N rate in both years. In 1990, nitrogenase activity increased later in the season in the treatments receiving 30 or 60kg N ha −1 In 1991, the inhibition of nitrogenase activity with increasing N rate was sustained throughout the season. 15 N studies aimed at quantifying total N 2 fixation indicated no significant response to N rate in 1990 and a negative correlation to N rate in 1991. N transfer between the pea and the mustard was not identified.
TL;DR: The present data are consistent with the hypothesis that after defoliation nitrogenase activity is adjusted in response to the reduced demand for symbiotically fixed nitrogen (nitrogen-sink strength), and it is proposed that such an adjustment could be made by a nitrogen feedback mechanism that regulates a variable oxygen-diffusion barrier in the nodules.
Abstract: In an attempt to manipulate plant nitrogen-sink strength, various defoliation treatments were applied to white clover (Trifolium repent L. cv. Ladino) grown in a controlled environment. Nitrogenase activity and its oxygen limitation were measured as H; evolution in Ar:O2 using a flow-through gas exchange system. An experiment to monitor the response of nitrogenase activity to various degrees of defoliation showed that the removal of up to 50% of the leaf are a had no effect on nitrogenase activity within 6 h. If more than 50% of the leaf area was removed. the nitrogenase activity decreased in relation to the loss of leaf area. This was accompanied by a corresponding increase in the O2 limitation of nitrogenase activity. In the experiment to determine the N-sink strength after defoliation, the dry weight increase w as initially unaffected by the removal of 4% of the leaf area, whereas removal of 85% or 100% of the leaf area resulted in a dry weight loss for several days. The time course of nitrogen assimilation was similar to that of dry weight increase. This study provides substantial evidence that after a severe defoliation the nitrogen demand is temporarily restricted due to the lack of dry weight increase. Since the plant's ability to store organic nitrogen is very limited. it seems plausible that nitrogen assimilation in the still fully intact symbiotic system had to be down-regulated. Consequently, to avoid ammonia toxicity. nitrogenase activity had to be reduced. Such an interpretation is supported by the fact that complete defoliation of nitrogen-Starved plants caused a much milder decrease in nitrogenase activity compared to the decrease in plants well supplied with nitrogen. The present data are consistent with the hypothesis that after defoliation nitrogenase activity is adjusted in response to the reduced demand for symbiotically fixed nitrogen (nitrogen-sink strength), It is proposed that such an adjustment could he made by a nitrogen feedback mechanism that regulates a variable oxygen-diffusion barrier in the nodules.
TL;DR: The potential contribution by N2-fixing pseudomonads to the sinks and sources of soil nitrogen is considered small in the short term but essentially remains unclear in the absence of experimental data.
Abstract: Pseudomonas-like organisms form a highly heterogeneous and ubiquitous group of bacteria. Recent identification of an authentic Pseudomonas genus should help to decrease difficulties which arose from taxonomic uncertainties. Further difficulties in recognising N2-fixing Pseudomonas species lie in the confirmation of diazotrophy. Optimised conditions are often needed for the detection of nitrogenase activity since it is controlled by specific environmental factors and physiological requirements. However, genetically constructed N2-fixing strains from authentic Pseudomonas species have demonstrated that at least some members of the genus possess mechanisms to accommodate and express nif (N2 fixation) genes from a well-studied diazotroph, Klebsiella pneumoniae. Renowned for their catabolic versatility, pseudomonads can use a wide range of carbon and energy substrates. Hence, potential N2-fixing pseudomonads are conceivably less limited by carbon and energy sources available in the environment compared to other N2-fixing organisms. Pseudomonas species dominate in the rhizosphere of some plants from which isolates have been shown to be diazotrophic. Several strains are also chemolithotrophs, capable of using H2 as energy and electron source and CO2 as carbon source. Besides assays for N2-fixing activity, DNA hybridisation to the well conserved molybdo-nitrogenase structural gene probe is an indicator of diazotrophy. However, absence of hybridisation to this probe, despite N2 fixation activity, has been reported. Although the genetics of N2 fixation in pseudomonads have hardly been studied, some nif genes have been shown to be plasmid-borne. Pseudomonas species are also predominant soil denitrifiers, reducing nitrate and nitrite to gaseous forms of nitrogen during anaerobic respiration. Hence, they play an important role in the global biological nitrogen cycle. Several diazotrophic species including a few pseudomonads can also denitrify. The potential contribution by N2-fixing pseudomonads to the sinks and sources of soil nitrogen is considered small in the short term but essentially remains unclear in the absence of experimental data. Reliable rapid methods for their specific enumeration are indispensable for assessing their population dynamics and ascertaining their ecological significance.
TL;DR: The 15N-enrichment technique for estimating biological nitrogen fixation in Casuarinaequisetifolia was evaluated under field conditions in single-species and mixed-species planting conditions.
Abstract: The 15N-enrichment technique for estimating biological nitrogen fixation in Casuarinaequisetifolia J.R. & G. Forst. was evaluated under field conditions in single-species and mixed-species planting...
TL;DR: Strain BH72 used an increasing amount of energy generated by respiration for N2 fixation, enabling them to develop ecological niches at very low pO2, in which they may respire actively and fix nitrogen efficiently at comparatively high rates.
Abstract: Azoarcus sp. strain BH72 is an aerobic diazotrophic bacterium that was originally found as an endophyte in Kallar grass. Anticipating that these bacteria are exposed to dissolved O2 concentrations (DOCs) in the nanomolar range during their life cycle, we studied the impact of increasing O2 deprivation on N2 fixation and respiration. Bacteria were grown in batch cultures, where they shifted into conditions of low pO2 upon depletion of O2 by respiration. During incubation, specific rates of respiration (qO2) and efficiencies of carbon source utilization for N2 reduction increased greatly, while the growth rate did not change significantly, a phenomenon that we called "hyperinduction." To evaluate this transition from high- to low-cost N2 fixation in terms of respiratory kinetics and nitrogenase activities at nanomolar DOC, bacteria which had shifted to different gas-phase pO2s in batch cultures were subjected to assays using leghemoglobin as the O2 carrier. As O2 deprivation in batch cultures proceeded, respiratory Km (O2) decreased and Vmax increased. Nitrogenase activity at nanomolar DOC increased to a specific rate of 180 nmol of C2H4 min-1 mg of protein-1 at 32 nM O2. Nitrogenase activity was proportional to respiration but not to DOC in the range of 12 to 86 nM O2. Respiration supported N2 fixation more efficiently at high than at low respiratory rates, the respiratory efficiency increasing from 0.14 to 0.47 mol of C2H4 mol of O2 consumed-1. We conclude that (i) during hyperinduction, strain BH72 used an increasing amount of energy generated by respiration for N2 fixation, and (ii) these bacteria have a high respiratory capacity, enabling them to develop ecological niches at very low pO2, in which they may respire actively and fix nitrogen efficiently at comparatively high rates.
TL;DR: A closed-system flow-through enclosure apparatus was used to measure symbiotic nitrogen fixation directly and indirect estimates of N 2 fixation were carried out concurrently using N difference and 15 N isotope dilution techniques.
Abstract: A closed-system flow-through enclosure apparatus was used to measure symbiotic nitrogen fixation directly. A legume-based system comprising 6-week-old Trifolium repens L. (white clover cv. Blanca) growing with Lolium perenne L. (perennial ryegrass cv. Trani) in an agricultural soil was incubated for 19 d in a 15 N-enriched atmosphere (mean value 3.663 atom%). An actinorhizal-based system comprising 1 -year-old Alnus glutinosa L. (alder) saplings growing with Festuca rubra L. (red fescue) in open-cast coal spoil was incubated for 21 d in a 15 N-enriched atmosphere (mean value 3.265 atom%). Indirect estimates of N 2 fixation were carried out concurrently using N difference and 15 N isotope dilution techniques
TL;DR: Nitrogen (N2) fixation in the phyllosphere and in the rhizomes/upper-sediment niche was light dependent, suggesting its dependence on photosynthesis, and diazotrophs involved here probably possess two photosystems (I and II) and may be Cyanobacteria.
Abstract: Halophila stipulacea beds in the northern Gulf of Elat (Red Sea) may be subdivided into three different sub-habitats, each harboring a distinct population of diazotrophs. Nitrogen (N2) fixation in the phyllosphere and in the rhizomes/upper-sediment niche was light dependent, suggesting its dependence on photosynthesis. N2 fixation in the phyllosphere was not affected by the addition of either glucose or 3-3,4-dichloro-phenyl-1,1-dimethyl-urea (DCMU), indicating that the diazotrophs involved carried out non-oxygenic photosynthesis. They may, thus, have been photosynthetic bacteria. N2 fixation in the rhizomes/upper-sediment niche, however, was greatly enhanced by the addition of glucose, but was suppressed in the presence of DCMU. This indicates that the diazotrophs involved here probably possess two photosystems (I and II) and may be Cyanobacteria. The anaerobic rhizosphere, in which fixation rates in light were very slow but were greatly enhanced by the addition of glucose, is probably populated by heterotrophic diazotrophs. Plant and sediment samples used in the present study were collected from the Gulf of Elat between 1990 and 1992.
TL;DR: Ability of the isolates to utilize a range of mono-, di-, and polysaccharides largely reflected the predicted availability of organic carbon and energy in the lagoon, except that chitin was not utilized.
Abstract: Nitrogen-fixing bacteria were isolated from sediments and water of a saltmarsh lagoon on the west coast of South Africa, and characterized according to factors that regulate nitrogen fixation in the marine environment. The majority of isolates were assigned to the Photobacterium or Vibrio genera on the basis of physiological and biochemical characteristics. One isolate was further assigned to the species Vibrio diazotrophicus. Carbohydrate utilization by each diazotrophic isolate was examined. Abilities of the isolates to utilize a range of mono-, di-, and polysaccharides largely reflected the predicted availability of organic carbon and energy in the lagoon, except that chitin was not utilized. Biochemical tests on the utilization of combined nitrogen showed that one isolate could utilize nitrate, and that this strain was susceptible to full repression of nitrogenase activity by 10mm nitrate. Urease activity was not detected in any of the isolates. In the absence of molybdenum two of the isolates, a Photobacterium spp. and V. diazotrophicus, reduced acetylene to ethylene and ethane, a property frequently associated with the activity of alternative nitrogenases. Addition of 25µM molybdenum inhibited ethane production by V. diazotrophicus, but stimulated ethylene and ethane production by the Photobacterium isolate. Addition of 28µM vanadium did not appear to regulate ethane production by either strain. Assays of nitrogenase activity in sediments from which some isolates were obtained indicated that molybdenum was not limiting nitrogenase activity at naturally-occurring concentrations. Southern hybridizations of the chromosomes of these strains with the anfH and vnfH genes of Azotobacter vinelandii and the nifH gene of Klebsiella pneumoniae indicated the presence of only one nitrogenase in these isolates.
TL;DR: In vitro self-transmission of symbiotic plasmids was demonstrated from three Rhizobium leguminosarum bv.
Abstract: In vitro self-transmission of symbiotic plasmids was demonstrated from three Rhizobium leguminosarum bv. trifolii strains used as clover seed inoculants and from two other strains to a cured derivative of R. leguminosarum bv. trifolii . Tn5-marked symbiotic plasmids from strain ICMP2163 and ICMP2668, transferred to three strains of native soil bacteria at frequencies of 10 −4 . Plasmid transfer to one of the native soil bacterial isolates was also demonstrated in a soil microcosm containing white clover seedlings. Insertion of Tn5 into the symbiotic plasmid of the strain ICMP2163 impaired nitrogen fixation in some Tn5-labelled derivatives and effective, ineffective or partly effective Rhizobium :: TnS derivatives were identified. When representative derivatives were crossed with the non-nodulating native soil bacterium NR40 and crosses were inoculated on white clover seedlings, all transconjugants formed nodules. Electron micrographs showed that ineffective or partly effective associations invaded fewer plant cells. Some of the bacteroids formed appeared damaged in comparison to the bacteroids of effective transconjugants which formed nodules and invaded cortical cells in a similar manner to ICMP2163. Competition experiments between the inoculant Rhizobium strain ICMP2163 and strains of transconjugant soil bacteria showed that the transconjugant soil bacteria outcompeted the inoculant strain to form nodules on clover roots. The implications of lateral transfer of symbiotic plasmids from inoculants to other bacteria in virgin native soils and the biogenesis of indigenous rhizobial populations after the introduction of inoculants in such agro-ecosystems is discussed.
TL;DR: In this article, periphyton and plankton were measured on the Amazon flood-plain using the acetylene reduction method calibrated with15N-N2, and the average ratio of moles C2H4 reduced per mole N2-N fixed was 3.4 ± 0.7, similar to other studies.
Abstract: Nitrogen fixation by periphyton and plankton was measured on the Amazon flood-plain using the acetylene reduction method calibrated with15N-N2. The average ratio (± SD) of moles C2H4 reduced per mole N2-N fixed was 3.4 ± 0.7, similar to other studies. Periphyton and plankton had high rates of light-dependent nitrogen fixation, with dark nitrogen fixation averaging 26% of the average rates in the light. The average daily (24 h) rates for periphyton nitrogen fixation in 1989 and 1990 were 1.79 and 0.51 mmol N2-N·m−2·d−1 respectively, which are comparable to summer rates in many temperate cyanobacterial assemblages. Nitrogen fixation was depressed at N03
− concentrations as low as 0.5 μM, and was below detection limits at concentrations of 4 μM, which occurred during periods of river flooding. Planktonic nitrogen fixation rates were high (0.5–0.8 mmol N2-N·m−2·d−1) during the high-water and drainage phases of the annual hydrograph when the floodplain waters were draining towards the river (low NO3
−), but rates were undetectable (< 0.05 mmol N2-N·m−2·d−1) when there was river flooding (high NO3
−). Nitrogen fixation by periphyton and plankton in 1989–1990 accounted for approximately 8% of previously reported total annual nitrogen inputs to the floodplain at Lake Calado.
TL;DR: It is proposed that symbiotic nitrogen fixation in white clover is regulated by the demand for nitrogen rather than by the availability of carbohydrate reserves in the stolons, and should be looked upon as an integrated plant growth factor and not as an isolated phenomenon.
Abstract: The aim of this study was to investigate whether symbiotic nitrogen fixation in white clover nodules limits nitrogen supply and hence clover growth by repeated defoliation at two cutting heights. Other possible factors governing symbiotic nitrogen fixation in the field were also elucidated. Using 15N, a 2-year field experiment including white clover (Trifolium repens L. cv. Ladino) and perennial ryegrass (Loliumperenne L. cv. Bastion) in monocultures and in mixtures was conducted in Eschikon, Switzerland. The effect of two cutting heights (4 cm and 10 cm above ground level) on the performance of symbiotic nitrogen fixation of white clover in the different sward-types was investigated. After each harvest, the plots were fertilized with 3 g N m-2(equivalent to 30 kg N ha-1 cut-1 or 210 kg N ha-1 year-1). In both years, white clover grown in a mixture with grass received a significantly higher percentage of nitrogen from symbiotic fixation compared with clover grown in monoculture. This phenomenon is attributed to the strong competitiveness of ryegrass in soil nitrogen uptake. Consequently, white clover in the clover-ryegrass mixture was more dependent on symbiotic nitrogen fixation than that grown in monoculture. The cutting height did not preferentially influence symbiotic nitrogen fixation, as opposed to the uptake of mineral nitrogen from the soil. From this finding it is suggested that symbiotic nitrogen fixation did not limit the supply of nitrogen to clover and hence its growth. It is proposed that symbiotic nitrogen fixation in white clover is regulated by the demand for nitrogen rather than by the availability of carbohydrate reserves in the stolons. Symbiotic nitrogen fixation should thus be looked upon as an integrated plant growth factor and not as an isolated phenomenon.