Showing papers on "Nitrogen fixation published in 2005"

Structural basis of biological nitrogen fixation

Abstract: Biological nitrogen fixation is mediated by the nitrogenase enzyme system that catalyses the ATP dependent reduction of atmospheric dinitrogen to ammonia. Nitrogenase consists of two component metalloproteins, the MoFe-protein with the FeMo-cofactor that provides the active site for substrate reduction, and the Fe-protein that couples ATP hydrolysis to electron transfer. An overview of the nitrogenase system is presented that emphasizes the structural organization of the proteins and associated metalloclusters that have the remarkable ability to catalyse nitrogen fixation under ambient conditions. Although the mechanism of ammonia formation by nitrogenase remains enigmatic, mechanistic inferences motivated by recent developments in the areas of nitrogenase biochemistry, spectroscopy, model chemistry and computational studies are discussed within this structural framework.

Nitrogen assimilation and nitrogen control in cyanobacteria

Abstract: Nitrogen sources commonly used by cyanobacteria include ammonium, nitrate, nitrite, urea and atmospheric N2, and some cyanobacteria can also assimilate arginine or glutamine. ABC (ATP-binding cassette)-type permeases are involved in the uptake of nitrate/nitrite, urea and most amino acids, whereas secondary transporters take up ammonium and, in some strains, nitrate/nitrite. In cyanobacteria, nitrate and nitrite reductases are ferredoxin-dependent enzymes, arginine is catabolized by a combination of the urea cycle and arginase pathway, and urea is degraded by a Ni2+-dependent urease. These pathways provide ammonium that is incorporated into carbon skeletons through the glutamine synthetase–glutamate synthase cycle, in which 2-oxoglutarate is the final nitrogen acceptor. The expression of many nitrogen assimilation genes is subjected to regulation being activated by the nitrogen-control transcription factor NtcA, which is autoregulatory and whose activity appears to be influenced by 2-oxoglutarate and the signal transduction protein PII. In some filamentous cyanobacteria, N2 fixation takes place in specialized cells called heterocysts that differentiate from vegetative cells in a process strictly controlled by NtcA. Abbreviations: ABC, ATP-binding cassette; CAP, catabolite gene activator protein

Enterobacteria‐mediated nitrogen fixation in natural populations of the fruit fly Ceratitis capitata

Abstract: Nitrogen, although abundant in the atmosphere, is paradoxically a limited resource for multicellular organisms. In the Animalia, biological nitrogen fixation has solely been demonstrated in termites. We found that all individuals of field-collected Mediterranean fruit flies (Ceratitis capitata) harbour large diazotrophic enterobacterial populations that express dinitrogen reductase in the gut. Moreover, nitrogen fixation was demonstrated in isolated guts and in live flies and may significantly contribute to the fly's nitrogen intake. The presence of similar bacterial consortia in additional insect orders suggests that nitrogen fixation occurs in vast pools of terrestrial insects. On such a large scale, this phenomenon may have a considerable impact on the nitrogen cycle.

The sulfate transporter SST1 is crucial for symbiotic nitrogen fixation in Lotus japonicus root nodules

Abstract: Symbiotic nitrogen fixation (SNF) by intracellular rhizobia within legume root nodules requires the exchange of nutrients between host plant cells and their resident bacteria. Little is known at the molecular level about plant transporters that mediate such exchanges. Several mutants of the model legume Lotus japonicus have been identified that develop nodules with metabolic defects that cannot fix nitrogen efficiently and exhibit retarded growth under symbiotic conditions. Map-based cloning of defective genes in two such mutants, sst1-1 and sst1-2 (for symbiotic sulfate transporter), revealed two alleles of the same gene. The gene is expressed in a nodule-specific manner and encodes a protein homologous with eukaryotic sulfate transporters. Full-length cDNA of the gene complemented a yeast mutant defective in sulfate transport. Hence, the gene was named Sst1. The sst1-1 and sst1-2 mutants exhibited normal growth and development under nonsymbiotic growth conditions, a result consistent with the nodule-specific expression of Sst1. Data from a previous proteomic study indicate that SST1 is located on the symbiosome membrane in Lotus nodules. Together, these results suggest that SST1 transports sulfate from the plant cell cytoplasm to the intracellular rhizobia, where the nutrient is essential for protein and cofactor synthesis, including nitrogenase biosynthesis. This work shows the importance of plant sulfate transport in SNF and the specialization of a eukaryotic transporter gene for this purpose.

Effects of model root exudates on structure and activity of a soil diazotroph community

Abstract: Nitrogen fixation is often enhanced in the rhizosphere as compared with bulk soil, due to asymbiotic microorganisms utilizing root exudates as an energy source. We have studied the activity and composition of asymbiotic soil diazotrophs following pulse additions of artificial root exudates and single carbon sources, simulating the situation of bulk soil coming into contact with exudates from growing roots. Artificial root exudates and single sugars rapidly induced nitrogen fixation. The population of potential diazotrophs was studied using universal and group-specific nifH polymerase chain reaction (PCR) and denaturing gradient gel electrophoresis (DGGE) analysis. Reverse transcription PCR of nifH mRNA confirmed that phylotypes with an apparently increasing population size also expressed the nitrogenase system. According to our results, the actively nitrogen-fixing population represents only a fraction of the total diazotroph diversity, and the results of group-specific nifH PCR and phylogenetic analysis of cloned nifH and 16S rRNA gene fragments identified active species that belonged to the genus Azotobacter. Rapid changes of transcriptional activity over time were observed, indicating different growth and activation strategies in different Azotobacter strains. Only sugar-containing substrates were able to induce nitrogen fixation, but substrate concentration and the presence of organic acids may have additional selective effects on the active diazotroph population.

Maturation of Nitrogenase: a Biochemical Puzzle

Abstract: The nitrogenase enzyme catalyzes the reductive breakage of the very strong triple bond of N2 to generate NH3 in a process known as biological nitrogen fixation. Biological nitrogen fixation is an essential step in the nitrogen cycle in the biosphere, and it is a major contributor to the nitrogen

Nitrogen fixation in agriculture, forestry, ecology, and the environment

Abstract: Preface to the Series. Preface. List of Contributors. 1: Production and Biological Nitrogen Fixation of Tropical Legumes D. Werner 1. Introduction 2. Phaseolus sp. and Vigna sp. (Beans) 3. Arachis hypogaea (Groundnut, Peanut) 4. Cicer arietinum (Chickpea) 5. Cajanus cajan (Pigeon pea) 6. Mucuna pruriens (Velvet bean) and Other Legumes Acknowledgements References 2: Nitrogen Fixation by Soybean in North America S. G. Pueppke 1. Soybean: Pathways to North America and Establishment as a Crop 2. Soybean Production in North America 3. Major Soybean Cropping Systems 4. Biological Nitrogen Fixation by Soybean in North America 5. Perspectives Acknowledgements References 3: The Importance of Nitrogen Fixation to Soybean Cropping in South America M. Hungria, J. C. Franchini, R. J. Campo and P. H. Graham 1. Introduction 2. Taxonomy, Origins, and Importance of Soybean 3. Biological Nitrogen Fixation 4. Economic Importance of Biological Nitrogen Fixation (BNF) in South America 5. Crop Management in South America 6. Final Considerations Acknowledgement References 4: Production, Regional Distribution of Cultivars, and Agricultural Aspects of Soybean in India S. K. Mahna 1. Introduction and Historical Background 2. All-India Area Coverage, Productivity, and Production of Soybean between 1970- 2003 3. All-India State-wise Area Coverage, Productivity, and Production of Soybean 4. Regional Distribution of Soybean Cultivars 5. Regional Agricultural Aspects of Soybean Cultivation Acknowledgements References 5: Soybean Cultivation and BNF in China J. E. Ruiz Sainz, J. C. Zhou, D.-N. Rodriguez-Navarro, J. M. Vinardell and J. E. Thomas-Oates 1. Summary 2. Soybean Cultivation in China: Historical Aspects and Current Situation 3. Nitrogen-Fixing Bacteria that Nodulate Soybean 4. The Soybean Germplasm Collection in China 5. Soybean in Crop Rotation and in Continuous Culture 6. Conclusions Acknowledgement References 6: Soil Stress Factors Influencing Symbiotic Nitrogen Fixation M. J. Sadowsky 1. Introduction 2. Importance of Symbiotic Nitrogen Fixation 3. Symbiotic Interaction of Legumes with Rhizobia 4. Nodulation and Nitrogen-Fixation Genetics in the Rhizobia and Bradyrhizobia 5. Rhizobia in the Soil Environment 6. Stress Factors in the Soil Environment that Influence N2 Fixation 7. Concluding Remarks References 7: Nodulated Legume Trees J. I. Sprent 1. Introduction 2. Leguminosae 3. Rhizobia that Nodulate Legume Trees 4. Types of Nodules formed on Trees 5. Mycorrhizas and Other Nutrient-Acquisition Systems 6. Measurement of Nitrogen Fixation by Trees 7. Role of Legume Trees in Natural and Managed Systems References 8. Nitrogen Fixing Trees with Actinorhiza in Forestry and Agroforestry R. O. Russo 1. Introduction 2. General Characteristics of the Actinorhizal Symbiosis 3. Host Botanical Families 4. Nitrogen Fixation in Actinorhizal Trees 5. Mycorrhizal Associations with Actinorhizal Trees 6. Actinorhizal Trees in Agroforestry 7. The Genus Casaurina 8. The Experience of the Central America Fuelwood Project 9. The Case of Alnus acuminata in Tropical Highlands 10. Other Uses of Actinorhizal Trees 11. Concluding Considerations References 9: Molecular Ecology of N2-fixing Microbes Associated with Gramineous Plants: Hidden Activities of Unknown Bacteria T. Hurek and B. Reinhold-Hurek

Functional Genomic Analysis of Three Nitrogenase Isozymes in the Photosynthetic Bacterium Rhodopseudomonas palustris

Abstract: The photosynthetic bacterium Rhodopseudomonas palustris is one of just a few prokaryotes described so far that has vnf and anf genes for alternative vanadium cofactor (V) and iron cofactor (Fe) nitrogenases in addition to nif genes for a molybdenum cofactor (Mo) nitrogenase. Transcriptome data indicated that the 32 genes in the nif gene cluster, but not the anf or vnf genes, were induced in wild-type and Mo nitrogenase-expressing strains grown under nitrogen-fixing conditions in Mo-containing medium. Strains that were unable to express a functional Mo nitrogenase due to mutations in Mo nitrogenase structural genes synthesized functional V and Fe nitrogenases and expressed vnf and anf genes in nitrogen-fixing growth media that contained Mo and V at concentrations far in excess of those that repress alternative nitrogenase gene expression in other bacteria. Thus, not only does R. palustris have multiple enzymatic options for nitrogen fixation, but in contrast to reports on other nitrogen-fixing bacteria, the expression of its alternative nitrogenases is not repressed by transition metals. Between 95 and 295 genes that are not directly associated with nitrogenase synthesis and assembly were induced under nitrogen-fixing conditions, depending on which nitrogenase was being used by R. palustris. Genes for nitrogen acquisition were expressed at particularly high levels during alternative nitrogenase-dependent growth. This suggests that alternative nitrogenase-expressing cells are relatively starved for nitrogen and raises the possibility that fixed nitrogen availability may be the primary signal that controls the synthesis of the V and Fe nitrogenases.

Interactions between nitrate uptake and nitrogen fixation in continuous cultures of the marine diazotroph trichodesmium (cyanobacteria)1

Abstract: Diazotrophic cyanobacteria can take up combined nitrogen (nitrate, ammonium, amino acids, dissolved organic nitrogen) from solution, but the interaction between N 2 fixation and uptake of combined nitrogen is not well understood. We studied the effects of combined nitrogen (NO - 3 ) additions on N 2 fixation rates in the cyanobacterium Trichodesmium erythraeum (IMS-101) maintained in continuous culture in an N-free medium (YBCII) and a 12:12-h light:dark cycle. We measured acetylene reduction rates, nutrient concentrations, and biomass throughout the 12h of illumination after the addition of nitrate (0.5-20 μM) at the start of the light period. Compared with unamended controls, Trichodesmium showed strong inhibition of acetylene reduction (up to 70%) in the presence of NO - 3 , with apparent saturation of the inhibition effect at an initial NO - 3 concentration of approximately 10 μM. The inhibition of acetylene reduction persisted through much of the light period as NO - 3 concentration in the culture vessel decreased. Recovery of N 2 fixation was observed late in the light period in cultures amended with low concentrations of NO - 3 (<5 μM) when ambient NO - 3 concentrations had decreased to 0.3-0.4 μM in the culture vessel. Nitrate uptake accounted for as much as 86% of total N uptake and, at the higher treatment concentrations, more than made up for the observed decrease in N 2 fixation rates. We conclude that Trichodesmium can obtain significant quantities of N through uptake of nitrate and does so in preference to N 2 fixation when sufficient NO - 3 is available.

Legume species identity and soil nitrogen supply determine symbiotic nitrogen-fixation responses to elevated atmospheric [CO2].

Abstract: In nitrogen (N)-limited systems, the response of symbiotic N fixation to elevated atmospheric [CO2] may be an important determinant of ecosystem responses to this global change. Experimental tests of the effects of elevated [CO2] have not been consistent. Although rarely tested, differences among legume species and N supply may be important. In a field free-air CO2 enrichment (FACE) experiment, we determined, for four legume species, whether the effects of elevated atmospheric [CO2] on symbiotic N fixation depended on soil N availability or species identity. Natural abundance and pool-dilution 15N methods were used to estimate N fixation. Although N addition did, in general, decrease N fixation, contrary to theoretical predictions, elevated [CO2] did not universally increase N fixation. Rather, the effect of elevated [CO2] on N fixation was positive, neutral or negative, depending on the species and N addition. Our results suggest that legume species identity and N supply are critical factors in determining symbiotic N-fixation responses to increased atmospheric [CO2].

Inorganic soil nitrogen under grassland plant communities of different species composition and diversity.

Abstract: We measured aboveground plant biomass and soil inorganic nitrogen pools in a biodiversity experiment in northern Sweden, with plant species richness ranging from 1 to 12 species. In general, biomass increased and nitrate pools decreased with increasing species richness. Transgressive overyielding of mixed plant communities compared to the most productive of the corresponding monocultures occurred in communities with and without legumes. N2-fixing legumes had a fertilizing function, while non-legumes had a N retaining function. Plant communities with only legumes had a positive correlation between biomass and soil nitrate content, whereas in plant communities without legumes they were negatively correlated. Both nitrate and ammonium soil pools in mixed non-legume communities were approximately equal to the lowest observed in the corresponding monocultures. In mixed legume/non-legume communities, no correlation was found for soil nitrate with either biomass or legume biomass as percentage of total biomass. The idea of complementarity among species in nitrogen acquisition was supported in both pure non-legume and mixed non-legume/ legume communities. In the latter, however, facilitation through increased nitrogen availability and retention, was probably dominating. Our results suggest that diversity effects on biomass and soil N pools through resource use complementarity depend on the functional traits of species, especially N2 fixation or high productivity.

Nitrogen Fixation in the High Arctic: Role of Vegetation and Environmental Conditions

Abstract: The course of nitrogen fixation by moss-associated cyanobacteria in Svalbard (78°N, 16°E), Norway, was studied using the acetylene reduction assay. In situ field measurements of nitrogen fixation activity were conducted in six different types of moss-dominated arctic vegetation from the beginning of the snowmelt in early June to the end of July 1998. Concurrently, the water content of the soil/vegetation layer was determined and correlated with the nitrogen fixation rates. At all sites with diminishing water content during the summer season, nitrogen fixation activity was positively correlated with the amount of available water in the vegetation. At two sites, where water content of the vegetation was constantly higher than 80% (w/w) throughout the season, nitrogen fixation activity was correlated with temperature. Depending on the type of vegetation, nitrogen fixation became limited when the water status fell below a minimum threshold level. The most desiccation-tolerant vegetation for nitrogen ...

Metabolic Reconstruction and Modeling of Nitrogen Fixation in Rhizobium etli

Abstract: Rhizobiaceas are bacteria that fix nitrogen during symbiosis with plants. This symbiotic relationship is crucial for the nitrogen cycle, and understanding symbiotic mechanisms is a scientific challenge with direct applications in agronomy and plant development. Rhizobium etli is a bacteria which provides legumes with ammonia (among other chemical compounds), thereby stimulating plant growth. A genome-scale approach, integrating the biochemical information available for R. etli, constitutes an important step toward understanding the symbiotic relationship and its possible improvement. In this work we present a genome-scale metabolic reconstruction (iOR363) for R. etli CFN42, which includes 387 metabolic and transport reactions across 26 metabolic pathways. This model was used to analyze the physiological capabilities of R. etli during stages of nitrogen fixation. To study the physiological capacities in silico, an objective function was formulated to simulate symbiotic nitrogen fixation. Flux balance analysis (FBA) was performed, and the predicted active metabolic pathways agreed qualitatively with experimental observations. In addition, predictions for the effects of gene deletions during nitrogen fixation in Rhizobia in silico also agreed with reported experimental data. Overall, we present some evidence supporting that FBA of the reconstructed metabolic network for R. etli provides results that are in agreement with physiological observations. Thus, as for other organisms, the reconstructed genome-scale metabolic network provides an important framework which allows us to compare model predictions with experimental measurements and eventually generate hypotheses on ways to improve nitrogen fixation.

Relevance of ammonium oxidation within biological soil crust communities

Abstract: Thin, vertically structured topsoil communities that become ecologically important in arid regions (biological soil crusts or BSCs) are responsible for much of the nitrogen inputs into pristine arid lands. We studied N(2) fixation and ammonium oxidation (AO) at subcentimetre resolution within BSCs from the Colorado Plateau. Pools of dissolved porewater nitrate/nitrite, ammonium and organic nitrogen in wetted BSCs were high in comparison with those typical of aridosoils. They remained stable during incubations, indicating that input and output processes were of similar magnitude. Areal N(2) fixation rates (6.5-48 micromol C(2)H(2) m(-2) h(-1)) were high, the vertical distribution of N(2) fixation peaking close to the surface if populations of heterocystous cyanobacteria were present, but in the subsurface if they were absent. Areal AO rates (19-46 micromol N m(-2) h(-1)) were commensurate with N(2) fixation inputs. When considering oxygen availability, AO activity invariably peaked 2-3 mm deep and was limited by oxygen (not ammonium) supply. Most probable number (MPN)-enumerated ammonia-oxidizing bacteria (6.7-7.9 x 10(3) cells g(-1) on average) clearly peaked at 2-3 mm depth. Thus, AO (hence nitrification) is a spatially restricted but important process in the nitrogen cycling of BSC, turning much of the biologically fixed nitrogen into oxidized forms, the fate of which remains to be determined.

Survival of rhizobia in soil is sensitive to elevated zinc in the absence of the host plant

Abstract: The survival of free-living rhizobia in soil is sensitive to elevated heavy metals in soil and can explain adverse effects of metals on symbiotic nitrogen fixation in soils. A survival experiment was set-up to derive critical cadmium (Cd) and zinc (Zn) concentrations in a range of field-contaminated soils in the absence of their host plant (Trifolium repens L.). Soils applied with metal salts or sewage sludge >10 years ago were sampled and were inoculated with Rhizobium leguminosarum bv. trifolii (108 cells g−1 soil) and incubated outdoors for up to 6 months. The most probable number (MPN) decreased 1–2 orders of magnitude in uncontaminated soils during the incubation. There was no significant effect of total metal concentrations on rhizobia survival in soils contaminated with Cd salts or with high Ni/Cd sewage sludge with highest Cd concentrations between 18 and 118 mg Cd kg−1. In contrast, survival was strongly affected in soils contaminated by sewage sludge, where Zn was the principal metal contaminant. Neither total Cd nor soil solution Cd was large enough to attribute these effects to Cd when compared with the soil series, where Cd salts had been applied. The MPN decreased at least one order of magnitude above total Zn concentrations of 233 mg Zn kg−1 (soil pH 5.6) and 876 mg Zn kg−1 (soil pH 6.3). The EC50s of log MPN were 204 and 604 mg Zn kg−1, respectively, and were lower than those for the symbiotic nitrogen fixation measured in the pot trial on the same soils (respectively 602 and 737 mg Zn kg−1). This study corroborates the evidence that symbiotic nitrogen fixation is affected by Zn in the field when Zn decreases the free-living population of rhizobia to below a critical threshold.

Genetics and regulation of nitrogen fixation in free-living bacteria

Abstract: Series Preface Preface List of Contributors Dedication 1: Historical Perspective - Development of nif Genetics and Regulation in Klebsiella pneumoniae R Dixon 1 Introduction 2 The Early Years 3 Defining the K pneumoniae nif Genes 4 The Recombinant DNA Era 5 nif Gene Regulation 6 Coda References 2: Genetics of Nitrogen Fixation and Related Aspects of Metabolism in Species of Azotobacter: History and Current Status C Kennedy and P Bishop 1 Research on the Genus Azotobacter (1901-2003) 2 Application of the Tools of Genetics and Molecular Biology in Species of Azotobacter 3 The nif Genes encoding the Enzymes for Structure, Function, and Biosynthesis of Mo-containing Nitrogenase 4 Regulation of Expression of nif and Associated Genes by Ammonium and O2 5 Ancillary Properties of Azotobacter Species that Aid the Efficiency of Nitrogen Fixation 6 Discovery of Molybdenum-independent Nitrogenase Systems in A vinelandii 7 Molybdenum-independent Nitrogenase systems in other Azotobacter Species Acknowledgements References 3: Nitrogen Fixation in the Clostridia J-S Chen 1 Introduction 2 The Nitrogen-fixing Clostridia 3 Distinctive Features of the nif Genes of the Clostridia 4 Genes of Ammonia Assimilation 5 Regulation of Nitrogen Fixation and Ammonia Assimilation 6 Concluding Remarks References 4: Regulation of Nitrogen Fixation in Methanogenic Archaea JA Leigh 1 Introduction 2 History and Background 3 Transcriptional Regulation 4 Regulation of Nitrogenase Activity 5 Summary References 5: Nitrogen Fixation in Heterocyst-Forming Cyanobacteria T Thiel 1Introduction 2 Structure of Heterocysts 3 Nitrogenase Genes 4 Heterocyst Metabolism 5 Genes Important for Heterocyst Formation 6 heterocyst Pattern Formation 7 Regulation Acknowledgements References 6: N2 Fixation by Non-Heterocystous Cyanobacteria JR Gallon 1 Introduction 2 Non-heterocystous Cyanobacteria 3 Patterns of N2 Fixation Acknowledgements References 7: Nitrogen Fixation in the Photosynthetic Purple Bacterium Rhodobacter capsulatus B Masepohl, T Drepper and W Klipp 1 Introduction 2 Organization of Nitrogen-fixing Genes 3 The Nitrogen-fixation Regulon of R capsulatus 4 Ammonium Control of Synthesis and Activity of both Nitrogenases 5 Environmental Factors Controlling Nitrogen Fixation 6 Linkage of Nitrogen Fixation, Photosynthesis, and Carbon Dioxide Assimilation 7 Nitrogen Fixation in other Photosynthetic Purple Bacteria References 8: Post-translational Regulation of Nitorgenase in Photosynthetic Bacteria S Nordlund and PW Ludden 1 Introduction 2 Discovery of Nitrogen Fixation by Photosynthetic Bacteria 3 In vitro Studies of Nitrogenase in Photosynthetic Bacteria 4 The Protein Era 5 Evidence for the Drat/Drag System in other Organisms 6 Other ADP-Ribosylations 7 Genetics of the Drag/Drat System 8 Signal Transduction to Drat and Drat 9 Conclustions Acknowledgement References 9: Regulation of Nitrogen Fixation in Free-Living Diazotrophs MJ Merrick 1 Introduction 2 General Nitrogen Control Systems 3 nif-specific Nitrogen Control 4 Nitrogen Control of Nitrogenase Activity 5 Conclusions References 10: Molybdenum Uptake and Homeostatis RN Pau 1 Molybdenum Outside Cells 2

Comparison of nitrogen fixation rates determined with different methods: a study in the Baltic Proper

Abstract: Nitrogen fixation rates were determined at approximately monthly intervals during spring/summer 2001 in the eastern Gotland Sea by means of the 15 N2 method. Additionally, a com- prehensive set of variables were measured in order to apply 3 different budget approaches for the determination of integrated nitrogen fixation: (1) the nutrient concept based on excess PO4 owing to the low DIN/PO4 ratios (6.8) in the winter surface water, (2) the CO2 concept using a combined car- bon/nitrogen mass balance and (3) the biomass concept based on the increase in total nitrogen dur- ing the cyanobacteria bloom. For the spring season (March to May) the budget approaches gave rise to postulating an additional nitrogen input of about 100 to 200 mmol m -2 . However, this was not attributed to nitrogen fixation because 15 N2 rate measurements from several years did not show any significant nitrogen fixation during spring. Rate measurements integrated from May to August yielded a nitrogen fixation of 138 mmol m -2 , which was consistent with the estimate based on the bio- mass concept. The CO2 concept resulted in a nitrogen input of 268 mmol m -2 from May to August. The nutrient concept was questionable because the excess PO4 was consumed during spring and not transferred to the summer bloom. High PON/POP ratios in summer indicated that PO4 release from particulate organic matter provided some of the PO4 necessary for the primary production based on nitrogen fixation. The estimates obtained from the rate measurements and from the CO2 concept are considered to represent the lower and upper limit for the nitrogen fixation in the Baltic Proper and correspond to a total nitrogen input of 434 000 and 792 000 t yr -1 respectively.

Physiological and molecular diversity of feather moss associative N2-fixing cyanobacteria.

Abstract: Cyanobacteria colonizing the feather moss Pleurozium schreberi were isolated from moss samples collected in northern Sweden and subjected to physiological and molecular characterization. Morphological studies of isolated and moss-associated cyanobacteria were carried out by light microscopy. Molecular tools were used for cyanobacteria identification, and a reconstitution experiment of the association between non-associative mosses and cyanobacteria was conducted. The influence of temperature on N2 fixation in the different cyanobacterial isolates and the influence of light and temperature on N2-fixation rates in the moss were studied using the acetylene reduction assay. Two different cyanobacteria were effectively isolated from P. schreberi: Nostoc sp. and Calothrix sp. A third genus, Stigonema sp. was identified by microscopy, but could not be isolated. The Nostoc sp. was found to fix N2 at lower temperatures than Calothrix sp. Nostoc sp. and Stigonema sp. were the predominant cyanobacteria colonizing the moss. The attempt to reconstitute the association between the moss and cyanobacteria was successful. The two isolated genera of cyanobacteria in feather moss samples collected in northern Sweden differ in their temperature optima, which may have important ecological implications.

Biological nitrogen fixation is not a major contributor to the nitrogen demand of a commercially grown South African sugarcane cultivar

Abstract: It has previously been reported that endophytic diazotrophic bacteria contribute significantly to the nitrogen budgets of some graminaceous species. In this study the contribution of biological nitrogen fixation to the N-budget of a South African sugarcane cultivar was evaluated using 15N natural abundance, acetylene reduction and 15N incorporation. Plants were also screened for the presence of endophytic diazotrophic bacteria using acetylene reduction and nifH-gene targeted PCR with the pure bacterial strains. 15N natural abundance studies on field-grown sugarcane indicated that the plants did not rely extensively on biological nitrogen fixation. Furthermore, no evidence was found for significant N2-fixation or nitrogenase activity in field-grown or glasshouse-grown plants using 15N incorporation measurements and acetylene reduction assays. Seven endophytic bacterial strains were isolated from glasshouse-grown and field-grown plants and cultured on N-free medium. The diazotrophic character of these seven strains could not be confirmed using acetylene reduction and PCR screening for nifH. Thus, although biological nitrogen fixation may occur in South African sugarcane varieties, the contribution of this N-source in the tested cultivar was not significant.

Liberation of amino acids by heterotrophic nitrogen fixing bacteria.

Abstract: Large amounts of amino acids are produced by nitrogen-fixing bacteria such as Azotobacter, Azospirillum, Rhizobium, Mesorhizobium and Sinorhizobium when growing in culture media amended with different carbon and nitrogen sources. This kind of bacteria live in close association with plant roots enhanced plant growth mainly as a result of their ability to fix nitrogen, improving shoot and root development suppression of pathogenic bacteria and fungi, and increase of available P concentration. Also, it has been strongly evidenced that production of biologically substances such as amino acids by these rhizobacteria are involved in many of the processes that explain plant-grown promotion. This paper reviews literature concerning amino acids production by nitrogen-fixing bacteria. The role of amino acids in microbial interactions in the rhizosphere and establishment of plant bacterial association is also discussed.

Endophytic colonization and in planta nitrogen fixation by a diazotrophic Serratia sp. in rice.

Abstract: Nitrogen fixing endophytic Serratia sp. was isolated from rice and characterized. Re-colonization ability of Serratia sp. in the rice seedlings as endophyte was studied under laboratory condition. For detecting the re-colonization potential in the rice seedlings, Serratia sp. was marked with reporter genes (egfp and Kmr) using transposon mutagenesis. The conjugants were screened for re-colonization ability and presence of nif genes using PCR. Further, the influence of flavonoids and growth hormones on the endophytic colonization and in planta nitrogen fixation of Serratia was also investigated. The flavonoids, quercetin (3 microg/ml) and diadzein (2 microg/ml) significantly increased the re-colonization ability of the endophytic Serratia, whereas the growth hormones like IAA and NAA (5 microg/ml) reduced the endophytic colonization ability of Serratia sp. Similarly, the in planta nitrogen fixation by Serratia sp. in rice was significantly increased due to flavonoids. The inoculation of endophytic diazotrophs increased the plant biomass and biochemical constituents.

Respiratory/Carbon Costs of Symbiotic Nitrogen Fixation in Legumes

Abstract: This chapter presents an overview of the respiratory/carbon costs of symbiotic nitrogen fixation. The various theoretical costings for nitrogen fixation suggest that respiration directly associated with nitrogenase activity will require between 1.77 and 3.01 g C g−1-N (4.35–7.00 mol CO2 mol−1 N2), while respiration of the entire nitrogen-fixing nodules will require between 2.78 and 4.81 g C g−1-N (6.51–11.19 mol CO2 mol−1 N2). Early attempts to measure these costs were beset by methodological problems, but some reliable approaches were developed. Measured values based on root respiration during the period of active nitrogen fixation are in the range of 5–10 g C g−1-N (11.6–23.4 mol CO2 mol−1 N2), with an average value of 6.5 g C g−1-N (15.1 mol CO2 mol−1 N2). On a nodule basis, values in the range of 3–5 g C g−1-N (7–12 mol CO2 mol−1 N2) appear to represent the ‘normal’ for legume nodules, while values below about 2.5 g C g−1-N are likely to be erroneous. The implications of these costings are considered in terms of the need for legumes to carefully regulate nitrogen fixation and the requirement for such regulation systems to be operational in any novel nitrogen-fixing plants.

Cyanobacteria in biological soil crusts of India

Abstract: Species of filamentous, sheath-forming cyanobacteria were the major component in the blackish-brown crusts on the upper millimetre of soils in different regions of India. Chlorophyll a density of these biological crusts on lateritic soils of Bhubaneswar, Orissa, brown forest soils of Salbani, West Bengal, arid soils of Tiruchirappalli, Tamil Nadu and sandy soils of old Goa ranged between 248 and 282 mg m - 2 , which is of the same order as in the leaves of higher plants. The species composition of cyanobacterial community in these soil crusts of India has been documented. Organisms in the crust absorbed water rapidly, regained photosynthesis and nitrogenase activity, which were stabilized within 48 to 72 h of wetting. The dominant cyanobacteria in the crust were rich in carotenoid pigment, absorbing at 507 nm and mycosporine amino acid-like substances absorbing in UV only when desiccated and simultaneously exposed to bright sunlight. It is concluded that the highly active upper layers of arid soils contain certain sheathed cyanobacteria that bind with soil particles forming a matrix protecting them from wind erosion. In addition, they are finely tuned in their physiology to the natural environmental conditions contributing organic matter and nitrogen through carbon and nitrogen fixation, thus increasing soil fertility.

Utilization of nitrogen from green manure and mineral fertilizer by sugarcane

Abstract: Given their potential for biological nitrogen fixation, legumes used as green manure are an alternative source of nitrogen to crops, and can supplement or even replace mineral nitrogen fertilization. The utilization of nitrogen by sugarcane (Saccharum spp.) fertilized with sunn hemp (Crotalaria juncea L.) and ammonium sulphate (AS) was evaluated using the 15N tracer technique. Amounts of 195.8 kg and 70 kg N per hectare, respectively, of sunn hemp and AS were added in the following treatments: without green manure and without AS; without green manure, with AS -15N; with green manure-15N and with AS; with green manure-15N, without AS; with green manure and with AS-15N. Four samples from the leaves +3 were collected and 2 m of the sugar cane row were harvested to estimate crop yield. The results for N contents (g kg-1), isotopic abundance of N (atoms % 15N) in leaf +3 samples, and sugarcane productivity were used to calculate cumulative N, nitrogen in the plant derived from the fertilizer-Ndff (% and kg ha-1), as well as percent recovery of fertilizer N (R%). Sugarcane was analysed and pol and total recovered sugar calculated. The highest Ndff percentages were observed eight months after sugarcane planting for treatments containing green manure without mineral N, and green manure with mineral N, at 15.3 and 18.4%, respectively. The best nitrogen recovery was observed during harvest, 18 months after planting; the treatment containing mineral fertilizer showed 34.4% recovery, while the sum between mineral N plus green manure N showed 40.0%. Treatments containing green manure plus mineral N changed soil attributes, by increasing Ca and Mg contents, sum of bases, pH, and base saturation, and decreasing potential acidity. In the plant, those treatments increased Ca and K contents.

Effect of Sodium Chloride on Growth, Nutrient Accumulation, and Nitrogen Fixation of Common Bean Plants in Symbiosis with Isogenic Strains

Abstract: The effect of sodium chloride (NaCl)-salinity on growth responses and tissues organic solutes and mineral content was investigated in common bean plants inoculated with salt-tolerant Rhizobium tropici wild-type strain CIAT899 and four mutant derivatives having decreased salt-tolerance (DST). Under non-saline conditions two mutants formed partially effective (HB10, HB12) and another two almost ineffective (HB8, HB13) nodules. A great variation of NaCl tolerance in the different symbiosis tested was observed at harvest, 32 day after planting. Common bean plant responded to salinity by decreasing the content of dry plant biomass, nodule number and the nitrogen fixation, and increasing the root to shoot ratio. The salt dose of 25 mM produced an increase of total soluble sugar and free amino acids content. This result suggest that these metabolites might be related with a nodule osmotic adjustment response under saline conditions, however cannot be excluded that the increase of amino acids content cou...