Showing papers on "Nitrogen fixation published in 2007"

How rhizobial symbionts invade plants: the Sinorhizobium – Medicago model

Abstract: Nitrogen-fixing rhizobial bacteria and leguminous plants have evolved complex signal exchange mechanisms that allow a specific bacterial species to induce its host plant to form invasion structures through which the bacteria can enter the plant root. Once the bacteria have been endocytosed within a host-membrane-bound compartment by root cells, the bacteria differentiate into a new form that can convert atmospheric nitrogen into ammonia. Bacterial differentiation and nitrogen fixation are dependent on the microaerobic environment and other support factors provided by the plant. In return, the plant receives nitrogen from the bacteria, which allows it to grow in the absence of an external nitrogen source. Here, we review recent discoveries about the mutual recognition process that allows the model rhizobial symbiont Sinorhizobium meliloti to invade and differentiate inside its host plant alfalfa (Medicago sativa) and the model host plant barrel medic (Medicago truncatula).

Spatial coupling of nitrogen inputs and losses in the ocean

Abstract: Nitrogen fixation is crucial for maintaining biological productivity in the oceans, because it replaces the biologically available nitrogen that is lost through denitrification. But, owing to its temporal and spatial variability, the global distribution of marine nitrogen fixation is difficult to determine from direct shipboard measurements. This uncertainty limits our understanding of the factors that influence nitrogen fixation, which may include iron, nitrogen-to-phosphorus ratios, and physical conditions such as temperature. Here we determine nitrogen fixation rates in the world's oceans through their impact on nitrate and phosphate concentrations in surface waters, using an ocean circulation model. Our results indicate that nitrogen fixation rates are highest in the Pacific Ocean, where water column denitrification rates are high but the rate of atmospheric iron deposition is low. We conclude that oceanic nitrogen fixation is closely tied to the generation of nitrogen-deficient waters in denitrification zones, supporting the view that nitrogen fixation stabilizes the oceanic inventory of fixed nitrogen over time.

Pesticides reduce symbiotic efficiency of nitrogen-fixing rhizobia and host plants

Abstract: Unprecedented agricultural intensification and increased crop yield will be necessary to feed the burgeoning world population, whose global food demand is projected to double in the next 50 years. Although grain production has doubled in the past four decades, largely because of the widespread use of synthetic nitrogenous fertilizers, pesticides, and irrigation promoted by the "Green Revolution," this rate of increased agricultural output is unsustainable because of declining crop yields and environmental impacts of modern agricultural practices. The last 20 years have seen diminishing returns in crop yield in response to increased application of fertilizers, which cannot be completely explained by current ecological models. A common strategy to reduce dependence on nitrogenous fertilizers is the production of leguminous crops, which fix atmospheric nitrogen via symbiosis with nitrogen-fixing rhizobia bacteria, in rotation with nonleguminous crops. Here we show previously undescribed in vivo evidence that a subset of organochlorine pesticides, agrichemicals, and environmental contaminants induces a symbiotic phenotype of inhibited or delayed recruitment of rhizobia bacteria to host plant roots, fewer root nodules produced, lower rates of nitrogenase activity, and a reduction in overall plant yield at time of harvest. The environmental consequences of synthetic chemicals compromising symbiotic nitrogen fixation are increased dependence on synthetic nitrogenous fertilizer, reduced soil fertility, and unsustainable long-term crop yields.

Quantitative imaging of nitrogen fixation by individual bacteria within animal cells

Abstract: Biological nitrogen fixation, the conversion of atmospheric nitrogen to ammonia for biosynthesis, is exclusively performed by a few bacteria and archaea. Despite the essential importance of biological nitrogen fixation, it has been impossible to quantify the incorporation of nitrogen by individual bacteria or to map the fate of fixed nitrogen in host cells. In this study, with multi-isotope imaging mass spectrometry we directly imaged and measured nitrogen fixation by individual bacteria within eukaryotic host cells and demonstrated that fixed nitrogen is used for host metabolism. This approach introduces a powerful way to study microbes and global nutrient cycles.

Nitrogen fixation by symbiotic cyanobacteria provides a source of nitrogen for the scleractinian coral Montastraea cavernosa

Abstract: Colonies of the Caribbean coral Montastraea cavernosa (Linnaeus) that harbor endosymbiotic cyanobacteria can fix nitrogen, whereas conspecifics without these symbionts cannot. The pattern of nitrogen fixation is diurnal and maximum rates occur in the early morning and evening. An analysis of delta N-15 stable isotope data showed that the zooxanthellae, but not the animal tissue, from colonies with cyanobacteria preferentially use the products derived from nitrogen fixation, and that these zooxanthellae also have a greater DNA content per cell, suggesting that these cells are in the DNA synthesis (S) and gap (G(2)) + Mitosis (M) phase of their cell cyle and are preparing to undergo cell division. Since nitrogen fixation did not occur during those times of the day when hyperoxia is known to occur, low oxygen concentrations might be required to support cyanobacterial respiration and provide the energy needed to fix nitrogen because the reaction centers of these cyanobacteria are uncoupled from light harvesting accessory pigments and the photosynthetic electron transport chain. Consistent with this were the depleted delta C-13 stable isotope signatures in all compartments of those corals with symbiotic cyanobacteria, which show an increase in heterotrophy compared with samples of M. cavernosa without cyanobacteria. Using modeled underwater light fields and measurements of photosynthesis, we show that the amount of time in which nitrogen fixation in these corals can take place increases with depth and that the distribution of corals with symbiotic cyanobacteria is positively correlated with increasing depth. The results presented here show that the zooxanthellae of M. cavernosa acquire nitrogen from cyanobacterial nitrogen fixation. Given that nitrogen limitation has long been proposed to contribute to the stability of these symbiotic associations, the mechanism by which zooxanthellae symbiosis in these corals is maintained remains an important question and the subject of future study.

Biology of the nitrogen cycle

Abstract: I. THE BIOLOGY OF THE NITROGEN CYCLE II. DENITRIFICATION III. BIOLOGICAL NITROGEN FIXATION IV. OTHER REACTIONS OF THE NITROGEN CYCLE

Nitrogen fixation in eukaryotes – New models for symbiosis

Abstract: 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. We have compared the morphological, physiological and molecular characteristics of nitrogen fixing symbiotic associations of bacteria and their diverse hosts. Special features of the interaction, e.g. vertical transmission of symbionts, grade of dependency of partners and physiological modifications have been considered in terms of extent of co-evolution and adaptation. Our findings are that, despite many adaptations enabling a beneficial partnership, most symbioses for molecular nitrogen fixation involve facultative interactions. However, some interactions, among them endosymbioses between cyanobacteria and diatoms, show characteristics that reveal a more obligate status of co-evolution. Our review emphasises that molecular nitrogen fixation, a driving force for interactions and co-evolution of different species, is a widespread phenomenon involving many different organisms and ecosystems. The diverse grades of symbioses, ranging from loose associations to highly specific intracellular interactions, might themselves reflect the range of potential evolutionary fates for symbiotic partnerships. These include the extreme evolutionary modifications and adaptations that have accompanied the formation of organelles in eukaryotic cells: plastids and mitochondria. However, age and extensive adaptation of plastids and mitochondria complicate the investigation of processes involved in the transition of symbionts to organelles. Extant lineages of symbiotic associations for nitrogen fixation show diverse grades of adaptation and co-evolution, thereby representing different stages of symbiont-host interaction. In particular cyanobacterial associations with protists, like the Rhopalodia gibba-spheroid body symbiosis, could serve as important model systems for the investigation of the complex mechanisms underlying organelle evolution.

Nitrogen‐fixation strategies and Fe requirements in cyanobacteria

Abstract: Diazotrophic (nitrogen-fixing) cyanobacteria are important contributors of new nitrogen to oligotrophic environments and greatly influence oceanic productivity. We investigated how iron availability influences the physiology of cyanobacterial diazotrophs with different strategies for segregating nitrogen fixation and photosynthesis. We examined growth, photosynthesis, nitrogen fixation, and Fe requirements of the filamentous nonheterocystous Trichodesmium, the filamentous heterocystous Anabaena, and the unicellular Cyanothece under a range of Fe concentrations. Under similar Fe concentrations the three species differed in N2-fixation rates, photosynthetic activity, the relative abundance of the photosynthetic units PSI : PSII, elemental stoichiometry, and Fe use efficiency. Complex colonial forms such as Trichodesmium and Anabaena are more likely to be Fe limited in their natural environments and are more efficient at utilizing Fe than unicellular diazotrophs such as Cyanothece. The varied physiological responses to Fe availability of the three cyanobacteria reflect their nitrogenfixation strategies, cell size, unicellular or colonial organization, and may explain, at least in part, the ecological distribution of these photosynthetic bacteria.

Dry and wet deposition of nutrients from the tropical Atlantic atmosphere: Links to primary productivity and nitrogen fixation

Abstract: Atmospheric deposition fluxes of soluble nutrients (N, P, Si, Fe, Co, Zn) to the tropical North Atlantic were determined during cruise M55 of the German SOLAS programme. Nutrient fluxes were highest in the east of the section along 10°N, owing to the proximity of source regions in West Africa and Europe, and lowest in the west, for both dry and wet deposition modes. In common with other recent studies, atmospheric P and Si inputs during M55 were strongly depleted relative to the stoichiometry of phytoplankton Fe, N, P and Si requirements. Atmospheric N inputs were equivalent to 0.1–4.7% of observed primary productivity during the cruise. Atmospheric nutrient supply was also compared to observed nitrogen fixation rates during M55. While atmospheric Fe supply may have been sufficient to support N fixation (depending on the relationship between our simple Fe leaching experiment and aerosol Fe dissolution in seawater), atmospheric P supply was well below the required rate. The stable nitrogen isotope composition of nitrate–N in aerosol and rain was also determined. Results of a simple model indicate that atmospheric deposition and nitrogen fixation introduce similar amounts of isotopically light nitrogen into surface waters of the study region. This implies that nitrogen isotope-based methods would overestimate nitrogen fixation here by a factor of 2, if atmospheric inputs were not taken into account.

Stable isotope probing with 15N2 reveals novel noncultivated diazotrophs in soil.

Abstract: Biological nitrogen fixation is a fundamental component of the nitrogen cycle and is the dominant natural process through which fixed nitrogen is made available to the biosphere While the process of nitrogen fixation has been studied extensively with a limited set of cultivated isolates, examinations of nifH gene diversity in natural systems reveal the existence of a wide range of noncultivated diazotrophs These noncultivated diazotrophs remain uncharacterized, as do their contributions to nitrogen fixation in natural systems We have employed a novel 15N2-DNA stable isotope probing (5N2-DNA-SIP) method to identify free-living diazotrophs in soil that are responsible for nitrogen fixation in situ Analyses of 16S rRNA genes from 15N-labeled DNA provide evidence for nitrogen fixation by three microbial groups, one of which belongs to the Rhizobiales while the other two represent deeply divergent lineages of noncultivated bacteria within the Betaproteobacteria and Actinobacteria, respectively Analysis of nifH genes from 15N-labeled DNA also revealed three microbial groups, one of which was associated with Alphaproteobacteria while the others were associated with two noncultivated groups that are deeply divergent within nifH cluster I These results reveal that noncultivated free-living diazotrophs can mediate nitrogen fixation in soils and that 15N2-DNA-SIP can be used to gain access to DNA from these organisms In addition, this research provides the first evidence for nitrogen fixation by Actinobacteria outside of the order Actinomycetales

Nitrogen fixation control under drought stress. Localized or systemic

Abstract: Legume-Rhizobium nitrogen fixation is dramatically affected under drought and other environmental constraints. However, it has yet to be established as to whether such regulation of nitrogen fixation is only exerted at the whole-plant level (e.g. by a systemic nitrogen feedback mechanism) or can also occur at a local nodule level. To address this question, nodulated pea (Pisum sativum) plants were grown in a split-root system, which allowed for half of the root system to be irrigated at field capacity, while the other half was water deprived, thus provoking changes in the nodule water potential. Nitrogen fixation only declined in the water-deprived, half-root system and this result was correlated with modifications in the activities of key nodule's enzymes such as sucrose synthase and isocitrate dehydrogenase and in nodular malate content. Furthermore, the decline in nodule water potential resulted in a cell redox imbalance. The results also indicate that systemic nitrogen feedback signaling was not operating in these water-stressed plants, since nitrogen fixation activity was maintained at control values in the watered half of the split-root plants. Thus, the use of a partially droughted split-root system provides evidence that nitrogen fixation activity under drought stress is mainly controlled at the local level rather than by a systemic nitrogen signal.

Redirection of Metabolism for Biological Hydrogen Production

Abstract: A major route for hydrogen production by purple photosynthetic bacteria is biological nitrogen fixation. Nitrogenases reduce atmospheric nitrogen to ammonia with the concomitant obligate production of molecular hydrogen. However, hydrogen production in the context of nitrogen fixation is a rather inefficient process because about 75% of the reductant consumed by the nitrogenase is used to generate ammonia. In this study we describe a selection strategy to isolate strains of purple photosynthetic bacteria in which hydrogen production is necessary for growth and independent of nitrogen fixation. We obtained four mutant strains of the photosynthetic bacterium Rhodopseudomonas palustris that produce hydrogen constitutively, even in the presence of ammonium, a condition where wild-type cells do not accumulate detectable amounts of hydrogen. Some of these strains produced up to five times more hydrogen than did wild-type cells growing under nitrogen-fixing conditions. Transcriptome analyses of the hydrogen-producing mutant strains revealed that in addition to the nitrogenase genes, 18 other genes are potentially required to produce hydrogen. The mutations that caused constitutive hydrogen production mapped to four different sites in the NifA transcriptional regulator in the four different strains. The strategy presented here can be applied to the large number of diverse species of anoxygenic photosynthetic bacteria that are known to exist in nature to identify strains for which there are fitness incentives to produce hydrogen.

Associative and endophytic nitrogen-fixing bacteria and cyanobacterial associations

Abstract: Preface to the Series. Preface. List of Contributors. Dedication. 1. Historical Perspective: From Bacterization to Endophytes C. Elmerich 1. The Nitrogen Cycle: Heritage from the 19th Century 2. Nutritional Interactions between Bacteria and Plants 3. Associative Nitrogen-fixing Bacteria 4. Discovery of Nitrogen-fixing Endophytes 5. Cyanobacterial Associations 6. Concluding Remarks Acknowledgement References 2. Molecular Phylogeny and Ecology of Root-Associated Diazotrophic a- and ss-Protobacteria M. Schmid and A. Hartmann 1. Introduction 2. Tools for Molecular Phylogeny and in situ Localizationof Bacterial Isolates and Communities 3. Molecular Phylogeny and Ecology of Azospirillum and Other Nitrogen-fixing a-Subclass Protobacteria 4. Molecular Phylogeny and Ecology of Herbaspirillum, Diazotrophic Burkholderia spp., and Other Nitrogen-fixing ss-Protobacteria 5. Conclusions and Prospects for Future Studies Acknowledgements References 3. Regulation of Nitrogen Fixation and Ammonium Assimilation in Associative and Endophytic Nitrogen-fixing Bacteria F. O. Pedrosa and C. Elmerich 1. Introduction 2. Rhizospheric and Endophytic Bacteria: General Features 3. Structural Organization of nif Genes 4. Identification of RpoN and Its Involvement in Nitrogen Fixation 5. Thr Ntr System and Control of Nitrogen Metabolism and Nitrogen Fixation 6. Regulation of Nitrogen Fixation 7. Conclusions Acknowledgements References 4. Chemotaxis in Soil Diazotrophs: Survival and Adaptive Response G. Alexandre and I. B. Zhulin 1. Introduction 2. Gene-Expression Regulation and Chemotaxis as Adaptive Responses to Environmental changes 3. Molecular Mechanism of the Chemotactic Response: Learning from Escherichia coli 4. Directed Motility in Soil Diazotrophs 5. Future Studies References 5. Molecular Genetics of Rhizosphere and Plant-Root Colonization E. Vanbleu and J. Vanderleyden 1. Introduction 2. Motility of Associative Diazotrophs 3. Attachment to Plant Roots 4. Rhizosphere Competence 5. Conclusions Acknowledgement References 6. Microbial Production of Plant Hormones B. E. Baca and C. Elmerich 1. Discovery of Phytohormones 2. Production and Role of Phytohormones 3. Pathways for Plant Hormone Biosynthesis: Common Routes in Plants, Bacteria and Fungi 4. Major Routes for IAA synthesis in Pathogenic and Beneficial Nitrogen-fixing Bacteria Associated with Plants 5. Multiple Routes for IAA Synthesis in Azospirillum 6. Other Phytohormones Produced by Plant Pathogenic and Nitrogen-fixing Associated and Endophytic Bacteria 7. Plant Growth Promotion (PGP): Role of Bacterial Phytohormone Production, ACC-Deaminase, and the Use of Synthetic Auxins 8. Concluding Remarks Acknowledgement References 7. The Plant Growth-Promoting Effect and Plant Responses S. Dobbelaere and Y. Okon 1. N2 Fixation vs. "Hormonal" Effects: Historical Perspectives 2. Effects of Azospirillum and Other Diazotrophs on Root Morphology 3. Effects on Root Function 4. Effects on Plant Growth 5. Future Studies References 8. Biocontrol of Plant Diseases by Associative and Endophytic Nitrogen-fixing Bacteria R. Bally and C. Elmerich 1. Beneficial Plant-Associated Nitrogen-fixing Bacteria and Biocontrol of Plant Disease 2. Interactions within Microbial Communities: Competition 3. Biological Control against Soil-Borne Diseases 4. Regulation of Biocontrol Properties and Cell-Cell 5. Plant Response to Pathogens and Biological Control in the Rhizosphere 6. Concluding Remarks Acknowledgements References 9. Endophytic Associations of Azoarcus spp B. Reinhold-Hurek and T. Hurek 1. Introduction 2. The Rise of Interest in Diazotrophic Endophytes 3. Azoarcus spp. and related Genera: Strictly Plant-Associated vs. Soil Bacteria 4. Habitats and Ecophysiology 5. Interactions with Fungi 6. Infection of Roots by Endophytic Diazotrophs: An Active Specific Process? 7. Concluding Remarks References 10. Biological Nitrogen Fixation in Sugarcane V. Reis, S. Lee and C. Kennedy 1. Short History of the Sugarcane-Cropping System 2. Nitrogen-fixing Bacteria Colonizing Sugarcane: New Phylogenetic Data, Properties, and Endophytic Status 3. Contribution of BNF to the Sugarcane Crop 4. Effect of N Fertilization on BNF 5. Genes for Nitrogen Fixation and Their Regulation in G. diazotrophicus and H. seropedicae 6. Is Indole Acetic Acid Production an Important Factor in the Ability of G. diazotrophicus to Enhance Growth of Sugarcane? 7. Concluding Remarks Acknowledgements References 11. Heterocyst Differentiation and Nitrogen Fixation in Cyanobacteria R. Haselkorn 1. Early History of the Association of Nitrogen Fixation with Heterocysts 2. Cyanobacterial Nitrogenase and nif-Genes Organization 3. Pathway of N Assimilation 4. Carbon Metabolism in Heterocysts 5. Genetic Tools for Studying Cyanobacterial Nitrogen Fixation 6. Regulatory Genes Required for Heterocyst Differentiation 7. Prospects Acknowledgement References 12. Cyanobacterial Associations B. Bergman, A. N. Rai and U. Rasmussen 1. Introduction 2. Historical Aspects and Landmarks 3. Symbioses with Diatoms (Algae) 4. Symbioses with Fungi 5. Symbioses with Bryophytes 6. Symbioses with Pteridophytes 7. Symbioses with Cycads 8. Symbiosis with Gunnera 9. Creation of New Symbioses and Prospects Acknowledgements References 13. Prospects for Significant Nitrogen Fixation in Grasses from Bacterial Endophytes E. W. Triplett 1. Ultimate Objective of Nitrogen-fixation Research - Nitrogen Fixation in Maize, Wheat and Rice 2. Understanding the Basic Biology of Endophytic Colonization: Using K. pneumoniae 342 as the Model Diazotrophic Endophyute 3. Attributes Needed for a Model Diazotrophic Endophyte 4. Future Work Needed to Replace Nitrogen Fertilizer with Diazotrophic Endophytes References Subject Index

Nitrogen fixation by unicellular diazotrophic cyanobacteria in the temperate oligotrophic North Pacific Ocean

Abstract: N2 fixation has been understudied in marine environments outside of the subtropical and tropical oceans and where water temperatures are typically below 20–25uC. We identified nifH phylotypes and measured N2 fixation rates under ambient conditions (maximum of 19uC) in water collected 750 km off the coast of California in oligotrophic waters of the North Pacific Ocean (34uN, 129uW). Near-surface N2 fixation rates averaged 0.25 6 0.05 nmol N L21 d21 for 24 incubation bottles. Despite low ambient concentrations of iron (,0.1 nmol L21) and phosphorus (,0.3 mmol L21), N2 fixation rates were unaffected by iron and phosphorus amendments. Using reverse transcription–quantitative polymerase chain reaction (RT-QPCR) methodology, we estimated transcript abundance and patterns of expression for several unicellular diazotrophs, including the group A phylotype, which showed the highest daily mRNA abundances. The N2-fixing assemblage extended to 60–80 m depth, well below the seasonal thermocline (40 m). The calculated areal N2 fixation rate (15 m mol Nm 22 d21) was small compared with estimates from other regions of the Pacific; however, the estimated fixation rate was similar to other published results, suggesting that processes other than cellular growth rate may determine the abundance of unicellular diazotrophs. Despite the low N2 fixation rates, the new nitrogen added to the euphotic zone by N2 fixation could account for at least 10% of new production during the study period.

Symbiotic nitrogen fixation in legume nodules: process and signaling. A review

Abstract: The Green Revolution was accompanied by a huge increase in the application of fertilizers, particularly nitrogen. Recent studies indicate that a sizeable proportion of the human population depends on synthetic nitrogen (N) fertilizers to provide the 53 million t N that is harvested globally in food crops each year. Nitrogen fertilizers affect the balance of the global nitrogen cycle, pollute groundwater and increase atmospheric nitrous oxide (N2O), a potent “greenhouse” gas. The production of nitrogen fertilizer by industrial nitrogen fixation not only depletes our finite reserves of fossil fuels, but also generates large quantities of carbon dioxide, contributing to global warming. The process of biological nitrogen fixation offers an economically attractive and ecologically sound means of reducing external nitrogen input and improving the quality and quantity of internal resources. Recent studies show that in irrigated cropping systems, legume N is generally less susceptible to loss processes than fertilizers. Biological nitrogen fixation (BNF) has provided a number of useful paradigms for both basic and applied research. Establishing a fully functional symbiosis requires a successful completion of numerous steps that lead from recognition signals exchanged between the plant and bacteria to the differentiation and operation of root nodules, the plant organ in which nitrogen fixation takes place. The initial sensing of the two organisms by each other starts with the release of root exudates by the plant that include flavonoids and nutrients such as organic acids and amino acids. Flavonoids secreted by the host plant into the rhizosphere function as inducers of the rhizobial nod genes. nod gene induction results in the secretion of lipochitin oligosaccharides that are thought to bind to specific plant receptor kinases that contain LysM motifs, such as NFR1 and NFR5 in Lotus japonicus and LYK3 and LTK4 in Medicago truncatula. This initiates a complex signaling pathway involving calcium spiking in root hairs. The result is that the root hairs curl and trap the rhizobia, which then enter the root hair through tubular structures known as infection threads that are formed by the plant. The infection threads then grow into the developed nodule tissue. Ultimately, the invading bacteria are taken into the plant cell by a type of endocytosis in which they are surrounded by a plant-derived peribacteroid membrane (PBM). The resulting symbiosomes fill the plant cell cytoplasm and as plant and bacterial metabolism develops, the bacteria become mature bacteroids able to convert atmospheric nitrogen to ammonium. To increase knowledge of this system of particular importance in sustainable agriculture, major emphasis should be laid on the basic research. More work is needed on the genes responsible in rhizobia and legumes, the structural chemical bases of rhizobia/legume communication, and signal transduction pathways responsible for the finely orchestrated induction of the symbiosis-specific genes involved in nodule development and nitrogen fixation. This review unfolds the various events involved in the progression of symbiosis.

In situ lateral transfer of symbiosis islands results in rapid evolution of diverse competitive strains of mesorhizobia suboptimal in symbiotic nitrogen fixation on the pasture legume Biserrula pelecinus L.

Abstract: The multi-billion dollar asset attributed to symbiotic nitrogen fixation is often threatened by the nodulation of legumes by rhizobia that are ineffective or poorly effective in N(2) fixation. This study investigated the development of rhizobial diversity for the pasture legume Biserrula pelecinus L., 6 years after its introduction, and inoculation with Mesorhizobium ciceri bv. biserrulae strain WSM1271, to Western Australia. Molecular fingerprinting of 88 nodule isolates indicated seven were distinctive. Two of these were ineffective while five were poorly effective in N(2) fixation on B. pelecinus. Three novel isolates had wider host ranges for nodulation than WSM1271, and four had distinct carbon utilization patterns. Novel isolates were identified as Mesorhizobium sp. using 16S rRNA, dnaK and GSII phylogenies. In a second study, a large number of nodules were collected from commercially grown B. pelecinus from a broader geographical area. These plants were originally inoculated with M. c bv. biserrulae WSM1497 5-6 years prior to isolation of strains for this study. Nearly 50% of isolates from these nodules had distinct molecular fingerprints. At two sites diverse strains dominated nodule occupancy indicating recently evolved strains are highly competitive. All isolates tested were less effective and six were ineffective in N(2) fixation. Twelve randomly selected diverse isolates clustered together, based on dnaK sequences, within Mesorhizobium and distantly to M. c bv. biserrulae. All 12 had identical sequences for the symbiosis island insertion region with WSM1497. This study shows the rapid evolution of competitive, yet suboptimal strains for N(2) fixation on B. pelecinus following the lateral transfer of a symbiosis island from inoculants to other soil bacteria.

Competition and facilitation between unicellular nitrogen-fixing cyanobacteria and non—nitrogen-fixing phytoplankton species

Abstract: Recent discoveries show that small unicellular nitrogen-fixing cyanobacteria are more widespread than previously thought and can make major contributions to the nitrogen budget of the oceans. We combined theory and experiments to investigate competition for nitrogen and light between these small unicellular diazotrophs and other phytoplankton species. We developed a competition model that incorporates several physiological processes, including the light dependence of nitrogen fixation, the switch between nitrate assimilation and nitrogen fixation, and the release of fixed nitrogen. Model predictions were tested in nitrogen-limited and lightlimited chemostat experiments using the unicellular nitrogen-fixing cyanobacterium Cyanothece sp. Miami BG 043511, the picocyanobacterium Synechococcus bacillaris CCMP 1333, and the small green alga Chlorella_cf sp. CCMP 1227. Parameter values of the species were estimated by calibration of the model in monoculture experiments. The model predictions were subsequently tested in a series of competition experiments at different nitrate levels. The model predictions were generally in good agreement with observed population dynamics. As predicted, in experiments with high nitrate input concentrations, the species with lowest critical light intensity (S. bacillaris) competitively excluded the other species. At low nitrate input concentration, nitrogen release by Cyanothece enabled stable coexistence of Cyanothece and S. bacillaris. More specifically, model simulations predicted that fixed nitrogen release by Cyanothece enabled S. bacillaris to become four times more abundant in the species mixture than it would have been in monoculture. This intricate interplay between competition and facilitation is likely to be a major determinant of the relative abundances of unicellular nitrogen-fixing cyanobacteria and non–nitrogen-fixing phytoplankton species in the oligotrophic ocean.

The potential use of the legume-rhizobium symbiosis for the remediation of arsenic contaminated sites

Abstract: The sustainable remediation of arsenic (As) contaminated sites requires an understanding of how As alters the biogeochemical processes in soil. Leguminous species are often used in the remediation of contaminated sites because of their capacity to fix nitrogen and enhance site fertility. While excess As is known to reduce the formation of root nodules in legumes, currently, little is known about how the legume–rhizobium symbiosis is affected by high As concentrations. Soybean ( Glycine max ) cv. Curringa and its rhizobial symbiont, Bradyrhizobium japonicum strain CB1809, were studied in dilute solution culture at As concentrations of 0, 1, 5 and 10 μM. As the As concentration of the nutrient solution increased, greater time was required for inoculated plants to produce root nodules ( P =0.001) and the number of root nodules per plant at harvest decreased ( P =0.007). Inspection of the soybean roots showed the number of root hairs decreased as the As concentration in the solution increased. The dry weight of soybean roots and shoots decreased significantly as the As concentration of the nutrient solution increased ( P P B. japonicum stimulated the growth of soybean via the production of growth-promoting hormones. This is the first reported evidence of rhizobial bacteria promoting the growth of plants at elevated concentrations of a heavy metal via a mechanism other than improved nitrogen nutrition. The potential use of rhizobia as growth-promoting bacteria for the remediation of heavy metal contaminated sites is an exciting new area of research.

Isolation and characterization of hydrogen-oxidizing bacteria induced following exposure of soil to hydrogen gas and their impact on plant growth

Abstract: In many legumes, the nitrogen fixing root nodules produce H2 gas that diffuses into soil. It has been demonstrated that such exposure of soil to H2 can promote plant growth. To assess whether this may be due to H2-oxidizing microorganisms, bacteria were isolated from soil treated with H2 under laboratory conditions and from soils collected adjacent to H2 producing soybean nodules. Nineteen isolates of H2-oxidizing bacteria were obtained and all exhibited a half-saturation coefficient (Ks) for H2 of about 1 ml l(-1). The isolates were identified as Variovorax paradoxus, Flavobacterium johnsoniae and Burkholderia spp. using conventional microbiological tests and 16S rRNA gene sequence analysis. Seventeen of the isolates enhanced (57-254%) root elongation of spring wheat seedlings. Using an Arabidopsis thaliana bioassay, plant biomass was increased by 11-27% when inoculated by one of four isolates of V. paradoxus or one isolate of Burkholderia that were selected for evaluation. The isolates of V. paradoxus found in both H2-treated soil and in soil adjacent to soybean nodules had the greatest impact on plant growth. The results are consistent with the hypothesis that H2-oxidizing bacteria in soils have plant growth promoting properties.

Nitrogen-fixing leguminous symbioses

Abstract: Preface to the Series, Preface, List of Contributors, Dedication 1. Evolution and Diversity of Legume Symbiosis: J. I. Sprent 1. Introduction 2. The Diversity of Legume Nodules 3. The First 700 Million Years 4. The Last 60 Million Years 5. The Present 6. Conclusions References 2. Ecology of Root-nodule Bacteria of Legumes: P. H. Graham 1. Introduction 2. Taxonomy of Root-nodule Bacteria 3. Population Structure of Rhizobia in Soil and Rhizosphere 4. Above and Below Ground Diversity and Symbiotic Function 5. Strain Competitiveness, Rhizosphere Colonization, and Persistence 6. Edaphic Factors Affecting Rhizobia 7. Future Dimensions of Rhizobial Ecology References 3. Maintaining Cooperation in the Legume-Rhizobia Symbiosis: Identifying Selection Pressures and Mechanisms: E. T. Kiers, S. A. West and R. F. Denison 1. Introduction 2. Explaining Cooperation: The Problem 3. Explaining Cooperation: The Hypothesis 4. Cheating and Mixed Nodules 5. Future Directions 6. Conclusions Acknowledgements References 4. Inoculation Technology for Legumes: D. F. Herridge 1. Introduction 2. The Need to Inoculate 3. Selection of Rhizobial Strains for Use in Inoculants 4. Inoculants in the Market Place 5. Pre-inoculated and Custom-inoculated Seed 6. Co-inoculation of Legumes with Rhizobia and Other Beneficial Microorganisms 7. Quality Control of Legume Inoculants 8. Constraints to Inoculant Use and Future Prospects 9. Concluding Statements References 5. Fine-tuning of Symbiotic Genes in Rhizobia: Flavonoid Signal Transduction Cascade: H. Kobayashi and W. J. Broughton 1. Introduction 2. NODD and NOD-boxes: Central Elements in Transduction of Flanovoid Signals 3. Functions of Genes Controlled by NOD-boxes4. Fine-tuning Expression of Symbiotic Genes in Restricted Host-Range Rhizobia 5. Post-genomic Studies: Do Flavonoids Regulate Other Genes? 6. Conclusion and Perspectives: Roles of Flavonoid-inducible Regulons in Symbiosis, Signaling, and Adaptation References 6. Cell Biology of Nodule Infection and Development: N. Maunoury, A. Kondorosi, E. Kondorosi and P. Mergaert 1. Introduction 2. Nod-factor Signalling 3. Cortical-cell Activation Leading to Primordium Formation and Infection 4. Secondary Signals for Nod Factor-induced Cell Activation 5. Differentiation of N2-fixing Cells: The Role of Endoreduplication 6. Bacteroid Differentiation 7. Nitrogen Fixation 8. Senescence of Nodules References 7. Genetics, A Way to Unravel Molecular Mechanisms Controlling the Rhizobial-Legume Symbiosis: P. Smit and T. Bisseling 1. Introduction 2. Model Legumes 3. Genetic Dissection of the Nod Factor-Signalling Pathway 4. Nature of the Nod-factor Receptors 5. The DMI Proteins 6. NSPs are Nod Factor-Response Factors 7. NIN is a Nod Factor-Response Factor 8. Autoregulation of Nodule Number 9. Concluding Remarks References 8. Legume Genomics Relevant to N2 Fixation: L. Schauser, M. Udvardi, S. Tabata and J. Stougaard 1. Introduction 2. Genomes 3. Transcriptome Analysis 4. Proteomic 5. Metabolomics 6. Genetic Analysis Using Genomics 7. Comparative Genomics 8. Conclusions References 9. Physiology of Root-nodule Bacteria: P. S. Poole, M. F. Hynes, A. W. B. Johnston, R. P. Tiwari, W. G. Reeve and J. A. Downie 1. Introduction 2. Introduction to Central Metabolism 3. Metabolism and the Environment 4. Micronutrition, Metals, and Vitamins 5. Environmental Responses of Rhizobia 6. Changes in Gene Expression in Bacteroids 7. Stress Responses in Rhizobia References 10. Carbon and Nitrogen Metabolism in Legume Nodules: C. P. Vance 1. Legume Root Nodules are Carbon and Nitrogen Factories 2. Nodule Carbon Metabolism 3. Sucrose Synthase 4. Phosphoenolpyruvate Carboxylase 5. Carbonic Anhydrase 6. Malate Dehydrogenase 7. Initial Assimilation of Fixed-N 8. Glutamine Synthetase 9. Glutamate Synthase 10. Aspartate Aminotransferase 11. Asparagine Synthetase 12. Ureide Biosynthesis 13. Genomic Insights 14. Overview References 11. Oxygen Diffusion, Production of Reactive Oxygen and Nitrogen Species, and Antioxidants in Legume Nodules: F. R. Minchin, E. K. James and M. Becana 1. Introduction 2. Physiological Evidence for a Variable Oxygen-diffusion Barrier 3. Structure of the Cortical Oxygen-diffusion Barrier 4. Development of the Cortical Oxygen-diffusion Barrier 5. Regulation of the Cortical Oxygen-diffusion Barrier 6. Infected Zone Control 7. Reactive Oxygen and Nitrogen Species in Nodules 8. Antioxidants in Nodules 9. Concluding Remarks References 12. Prospects for the Future Use of Legumes: J. G. Howieson, R. J. Yates, K, Foster, D. Real and B. Besier 1. Introduction 2. Current and Past Legume-Usage Patterns 3. New Uses for Legumes 4. Matching Legumes and the Symbiosis to Edaphic and Economic Factors 5. Utilising the Basic Advances 6. Conclusions References Subject Index

In vitro synthesis of the iron–molybdenum cofactor of nitrogenase from iron, sulfur, molybdenum, and homocitrate using purified proteins

Abstract: Biological nitrogen fixation, the conversion of atmospheric N2 to NH3, is an essential process in the global biogeochemical cycle of nitrogen that supports life on Earth. Most of the biological nitrogen fixation is catalyzed by the molybdenum nitrogenase, which contains at its active site one of the most complex metal cofactors known to date, the iron–molybdenum cofactor (FeMo-co). FeMo-co is composed of 7Fe, 9S, Mo, R-homocitrate, and one unidentified light atom. Here we demonstrate the complete in vitro synthesis of FeMo-co from Fe2+, S2−, MoO42−, and R-homocitrate using only purified Nif proteins. This synthesis provides direct biochemical support to the current model of FeMo-co biosynthesis. A minimal in vitro system, containing NifB, NifEN, and NifH proteins, together with Fe2+, S2−, MoO42−, R-homocitrate, S-adenosyl methionine, and Mg-ATP, is sufficient for the synthesis of FeMo-co and the activation of apo-dinitrogenase under anaerobic-reducing conditions. This in vitro system also provides a biochemical approach to further study the function of accessory proteins involved in nitrogenase maturation (as shown here for NifX and NafY). The significance of these findings in the understanding of the complete FeMo-co biosynthetic pathway and in the study of other complex Fe-S cluster biosyntheses is discussed.

Biological nitrogen fixation with non-legumes: An achievable target or a dogma?

Abstract: Nitrogen is most often the limiting nutrient for crop production, since only a fraction of atmospheric ni- trogen is made available to the plants through biologi- cal nitrogen fixation (BNF). Extending the BNF ability to non-legumes would be a useful technology for in- creased crop yields among resource-poor farmers. The idea that genetic manipulation techniques might be used to engineer crop plants to fix nitrogen is, of course, not new. However, the more we understand about the biochemistry and physiology of BNF, the less likely it seems that this goal will be achieved by 'simply' transferring the genes for nitrogen fixation to suitable crop species. Induction of nodulation has therefore been the main target of researchers over the past few years. This review briefly describes the pro- gress made towards nodular symbiosis between rhizo- bia and other free-living atmospheric nitrogen fixers and non-legume crops. NITROGEN is an essential plant nutrient. It is the nutrient that is most commonly deficient, contributing to reduced agricultural yields throughout the world. Molecular nitrogen or dinitrogen (N2) makes up four-fifths of the atmosphere, but is metabolically unavailable directly to higher plants or animals. It is available to some microorganisms through biological nitrogen fixation (BNF) in which atmospheric nitrogen is converted to ammonia by the enzyme nitro- genase. According to statistics by FAO (2001), about 42 million tons of fertilizer N is being used annually on a global scale for the production of three major cereal crops, i.e. wheat, rice and maize (17, 9 and 16 million tons res- pectively). Crop plants are able to use about 50% of the applied fertilizer N, while 25% is lost from the soil-plant system through leaching, volatilization, denitrification and due to many other factors causing not only an annual economic loss of US$ 3 billion but also cause pollution to the environment. Some of the adverse environmental effects of excessive use of nitrogenous fertilizers are: (i) metheamoglobinemia in infants due to NO3 and NO2 in waters and food, (ii) cancer due to secondary amines, (iii) respiratory illness due to NO 3, aerosols, NO2 and HNO3, (iv) eutrophication due to N in surface water, (v) material and ecosystem damage due to HNO3 in rainwater, (vi) plant toxicity due to high levels of NO 2 and NH4 in soils, and (vii) excessive plant growth due to more available N, depletion of stratospheric ozone due to NO and N2O. If a BNF system could be assembled in the non-legume plants, it could increase the potential for nitrogen supply because fixed nitrogen would be available to the plants directly, with little or no loss. Such a system could also enhance resource conservation and environmental security, besides freeing farmers from the economic burden of purchasing fertilizer nitrogen for crop production. Thus, a significant reduction in the relative use of fertilizer N can be achieved if atmospheric N is made available to non-legumes directly through an effective associative system with some of the characteristics of legume symbiosis. Recently, several ap- proaches using techniques developed in the area of bio- technology have raised new hopes that success in this secondary objective may yet be realized. It is the authors opinion that there are now sound reasons to anticipate that at least some non-leguminous field crops may also become independent of soil nitrogen. We intend to ex- plain the reasons for this renewed optimism, against the background of knowledge accumulated in the past century that will be relevant to any ultimate su ccess in exploiting these new approaches.

Biological nitrogen fixation with non-legumes : An achievable target or a dogma?

Abstract: Nitrogen is most often the limiting nutrient for crop production, since only a fraction of atmospheric ni- trogen is made available to the plants through biologi- cal nitrogen fixation (BNF). Extending the BNF ability to non-legumes would be a useful technology for in- creased crop yields among resource-poor farmers. The idea that genetic manipulation techniques might be used to engineer crop plants to fix nitrogen is, of course, not new. However, the more we understand about the biochemistry and physiology of BNF, the less likely it seems that this goal will be achieved by 'simply' transferring the genes for nitrogen fixation to suitable crop species. Induction of nodulation has therefore been the main target of researchers over the past few years. This review briefly describes the pro- gress made towards nodular symbiosis between rhizo- bia and other free-living atmospheric nitrogen fixers and non-legume crops. NITROGEN is an essential plant nutrient. It is the nutrient that is most commonly deficient, contributing to reduced agricultural yields throughout the world. Molecular nitrogen or dinitrogen (N2) makes up four-fifths of the atmosphere, but is metabolically unavailable directly to higher plants or animals. It is available to some microorganisms through biological nitrogen fixation (BNF) in which atmospheric nitrogen is converted to ammonia by the enzyme nitro- genase. According to statistics by FAO (2001), about 42 million tons of fertilizer N is being used annually on a global scale for the production of three major cereal crops, i.e. wheat, rice and maize (17, 9 and 16 million tons res- pectively). Crop plants are able to use about 50% of the applied fertilizer N, while 25% is lost from the soil-plant system through leaching, volatilization, denitrification and due to many other factors causing not only an annual economic loss of US$ 3 billion but also cause pollution to the environment. Some of the adverse environmental effects of excessive use of nitrogenous fertilizers are: (i) metheamoglobinemia in infants due to NO3 and NO2 in waters and food, (ii) cancer due to secondary amines, (iii) respiratory illness due to NO 3, aerosols, NO2 and HNO3, (iv) eutrophication due to N in surface water, (v) material and ecosystem damage due to HNO3 in rainwater, (vi) plant toxicity due to high levels of NO 2 and NH4 in soils, and (vii) excessive plant growth due to more available N, depletion of stratospheric ozone due to NO and N2O. If a BNF system could be assembled in the non-legume plants, it could increase the potential for nitrogen supply because fixed nitrogen would be available to the plants directly, with little or no loss. Such a system could also enhance resource conservation and environmental security, besides freeing farmers from the economic burden of purchasing fertilizer nitrogen for crop production. Thus, a significant reduction in the relative use of fertilizer N can be achieved if atmospheric N is made available to non-legumes directly through an effective associative system with some of the characteristics of legume symbiosis. Recently, several ap- proaches using techniques developed in the area of bio- technology have raised new hopes that success in this secondary objective may yet be realized. It is the authors opinion that there are now sound reasons to anticipate that at least some non-leguminous field crops may also become independent of soil nitrogen. We intend to ex- plain the reasons for this renewed optimism, against the background of knowledge accumulated in the past century that will be relevant to any ultimate su ccess in exploiting these new approaches.

Symbiotic nitrogen fixation in a tropical rainforest: 15N natural abundance measurements supported by experimental isotopic enrichment.

Abstract: Summary • Leguminous trees are very common in the tropical rainforests of Guyana. Here, species-specific differences in N2 fixation capability among nodulating legumes growing on different soils and a possible limitation of N2 fixation by a relatively high nitrogen (N) and low phosphorus (P) availability in the forest were investigated. • Leaves of 17 nodulating species and 17 non-nodulating reference trees were sampled and their δ15N values measured. Estimates of N2 fixation rates were calculated using the 15N natural abundance method. Pot experiments were conducted on the effect of N and P availability on N2 fixation using the 15N-enriched isotope dilution method. • Nine species showed estimates of > 33% leaf N derived from N2 fixation, while the others had low or undetectable N2 fixation rates. High N and low P availability reduced N2 fixation substantially. • The results suggest that a high N and low P availability in the forest limit N2 fixation. At the forest ecosystem level, N2 fixation was estimated at c. 6% of total N uptake by the tree community. We conclude that symbiotic N2 fixation plays an important role in maintaining high amounts of soil available N in undisturbed forest.

Effects of upstream lakes and nutrient limitation on periphytic biomass and nitrogen fixation in oligotrophic, subalpine streams

Abstract: SUMMARY 1. We conducted bioassays of nutrient limitation to understand how macronutrients and the position of streams relative to lakes control nitrogen (N2) fixation and periphytic biomass in three oligotrophic Rocky Mountain catchments. We measured periphytic chlorophyll-a (chl-a) and nitrogen-fixation responses to nitrogen (N) and phosphorus (P) additions using nutrient-diffusing substrata at 19 stream study sites, located above and below lakes within the study catchments. 2. We found that periphytic chl-a was significantly co-limited by N and P at 13 of the 19 sites, with sole limitation by P observed at another four sites, and no nutrient response at the final two sites. On average, the addition of N, P and N + P stimulated chl-a 35%, 114% and 700% above control values respectively. The addition of P alone stimulated nitrogen fixation by 2500% at five of the 19 sites. The addition of N, either with or without simultaneous P addition, suppressed nitrogen fixation by 73% at nine of the 19 sites. 3. Lake outlet streams were warmer and had higher dissolved organic carbon concentrations than inlet streams and those further upstream, but position relative to lakes did not affect chl-a and nitrogen fixation in the absence of nutrient additions. Chl-a response to nutrient additions did not change along the length of the study streams, but nitrogen fixation was suppressed more strongly by N, and stimulated more strongly by P, at lower altitude sites. The responses of chl-a and nitrogen fixation to nutrients were not affected by location relative to lakes. Some variation in responses to nutrients could be explained by nitrate and/or total N concentration. 4. Periphytic chl-a and nitrogen fixation were affected by nutrient supply, but responses to nutrients were independent of stream position in the landscape relative to lakes. Understanding interactions between nutrient supply, nitrogen fixation and chl-a may help predict periphytic responses to future perturbations of oligotrophic streams, such as the deposition of atmospheric N.

Genetic Diversity of Acacia seyal Del. Rhizobial Populations Indigenous to Senegalese Soils in Relation to Salinity and pH of the Sampling Sites

Abstract: The occurrence and the distribution of rhizobial populations naturally associated to Acacia seyal Del. were characterized in 42 soils from Senegal. The diversity of rhizobial genotypes, as characterized by polymerase chain reaction restriction fragment length polymorphism (RFLP) analysis of 16S–23S rDNA, performed on DNA extracted from 138 nodules resulted in 15 clusters. Results indicated the widespread occurrence of compatible rhizobia associated to A. seyal in various ecogeographic areas. However, the clustering of rhizobial populations based on intergenic spacer (IGS) RFLP profiles did not reflect their geographic origin. Four genera were discriminated on the basis of 16S rRNA gene sequences of the strains representative for the IGS-RFLP profiles. The majority of rhizobia associated to A. seyal were affiliated to Mesorhizobium and Sinorhizobium 64 and 29%, respectively, of the different IGS-RFLP profiles. Our results demonstrate the coexistence inside the nodule of plant-pathogenic non-N2-fixing Agrobacterium and Burkholderia strains, which induced the formation of ineffective nodules, with symbiotic rhizobia. Nodulation was recorded in saline soils and/or at low pH values or in alkaline soils, suggesting adaptability of natural rhizobial populations to major ecological environmental stress and their ability to establish symbiotic associations within these soil environments. These results contribute to the progressing research efforts to uncover the biodiversity of rhizobia and to improve nitrogen fixation in agroforestry systems in sub-Saharan Africa.

Interactive effect of Rhizobium strains and P on soybean yield, nitrogen fixation and soil fertility

Abstract: Pot studies under natural conditions were undertaken to determine the effect of various exotic Bradyrhizobium japonicum strains viz., TAL 377, 379, 102 used alone or in mixture with and without phosphorus on soybean growth, yield and nitrogen fixation parameters. Surface sterilized soybean seeds (NARC-4 var.) coated with Rhizobium strains were sown in earthen pots. Phosphorus (P) was applied as single super phosphate (SSP) at the time of sowing in the soil. Nitrogenase activity, pink bacteriodal tissue volume and specific nitrogenase activity was determined at flowering stage. These parameters were relatively higher when mixed rhizobial strains were applied in combination with P. However, efficiency of different rhizobial strains for specific nitrogenase activity and other parameters was TAL102>TAL379>TAL377 either alone or in combination with P. Application of Rhizobium strain and P also increased the growth and yield of soybean and also improved soil fertility and NPK uptake by plant tissues. It is concluded that increase in soybean yield can be achieved by applying Rhizobium mixed culture with phosphorus, which also improved soil fertility for sustainable agriculture system.

Benthic nitrogen fixation in the SW New Caledonia lagoon

Abstract: Various types of microbial mats are widespread in the SW New Caledonia lagoon. Both heterocystous (Nodularia harveyana) and non-heterocystous (Hydrocoleum cantharidosmum, H. lyngbyaceum) cyanobacteria dominate these mats. Using the acetylene reduction technique, nitro- genase activity was observed at all sites. Heterocystous cyanobacteria fix N2 during the daytime, whereas non-heterocystous cyanobacteria fix N2 during the night. The intensity of nitrogenase activ- ity depended on the level of light energy received during daylight. An estimation of nitrogen fixation by benthic cyanobacteria (16.4 ± 5.4 mg N2 m -2 d -1 ) at 21 m depth (average depth of the lagoon) represented 19% of the nitrogen requirement for benthic primary production.

Seasonal nitrogen fixation and primary production in the Southwest Pacific: nanoplankton diazotrophy and transfer of nitrogen to picoplankton organisms

Abstract: We applied a high-sensitivity dual isotopic tracer technique ( 13 C: 15 N) to measure N2 fix- ation and primary production in the total phytoplanktonic community and in 3 size fractions (>10, 10 µm, i.e. Trichodesmium) often fixed the bulk of available nitrogen at very high rates (up to 1.8 nmol l -1 h -1 ). Elevated 15 N2 accumulation (up to 0.83 nmol l -1 h -1 ) was always observed in the <10 µm fraction, representing a mean of 31 ± 20% of total N2 fixation, up to 92% in the lagoon and 98% in the oceanic region. Direct fixation was detected in the <10 µm fraction during the day as well as during the night in the New Caledonia lagoon, indicating that unicellular nanoplanktonic cyanobacteria could be a significant source of new nitrogen. Some accumulation of 15 N2 was also detectable in the <3 µm fraction, especially in surface samples. The rates of this nitrogen accumulation were generally very low (<0.17 nmol l -1 h -1 ), repre- senting ~10% of total fixation. However, in August 2002, this 15 N accumulation in the <3 µm fraction contributed nearly 50% of the total nitrogen fixation. However, with the post-size-fractionation experiments it was not possible to distinguish direct N2 fixation from picoplanktonic assimilation of organic compounds released by large cyanobacteria. Nevertheless, the results demonstrate a close coupling between larger diazotrophs and picoplanktonic populations, and show that new nitrogen could rapidly be provided for the pelagic microbial food web.