Showing papers on "Nitrogen fixation published in 1993"

Biological Nitrogen Fixation

Abstract: Biological nitrogen fixation (BNF) is the process of the reduction of dinitrogen from the air to ammonia carried out by a large number of species of free-living and symbiotic microbes called diazotrophs. BNF presents an inexpensive and environmentally sound, sustainable approach to crop production and constitutes one of the most important Plant Growth Promotion (PGP) scenarios. Here I will summarize various aspects of BNF, including the dinitrogen reduction catalysed reaction carried out by “nitrogenase” and the enzymes/genes involved and their regulation, the inherent “oxygen paradox” , the identification of diazotrophs, sustainable agricultural uses of BNF, symbiotic plant-diazotroph interactions and endophytic diazotrophs, data from the field, and future prospects in BNF.

Gas Exchange of Legume Nodules and the Regulation of Nitrogenase Activity

Abstract: CONTENTS INTRODUCTION 483 METHODS FOR MEASURING NITROGENASE ACTIVITY IN LEGUME NODULES... 484 The Acetylene Reduction Assay (ARA) 485 Measurement of Nodule H2 Evolution . .... 487 Measured Values of Nitrogenase Activity: Past and Present 489 IN VIVO NITROGENASE ACTIVITY: POTENTIAL AND LIMITATIONS 490 H2 Inhibition of N2 Fixation 490 02 Limitation of Nitrogenase Activity 492 ENVIRONMENTAL FACTORS AFFECTING NITROGENASE ACTIVITy 498 Restriction of Phloem Sap Supply 498 Nitrate Fertilization 500 Drought Stress 502 DEVELOPMENTAL AND LONG-TERM ADAPTA nON TO RHIZOSPHERE p02 ......... 503 CONCLUDING REMARKS 504

Nitrogen isotope techniques.

Abstract: Introduction. Mass Spectrometry. Optical Emmission Spectrometry. Natural Abundance of 15N: Fractional Contribution of Two Sources to a Common Sink an Use of Isotope Discrimination. Nitrogen Fixation in Soil and Plant Systems. Nitrogen Fixation in Flooded Rice Soils, Aquatic and Sediment Systems. Nitrification and Dentrification. Mineralization and Assimilation in Soil and Plant Systems. Mineralization and Assimilation in Aquatic, Sediment, And Wet Land Systems. 13N Techniques. Chapter References. Index.

New Horizons in Nitrogen Fixation

Abstract: this volume) to become associated with apodinitrogenase. The role of general chaperonins in promoting accumulation of nif gene products (Govezensky et al.1991) will be discussed in chapters of this section and may suggest roles for some of the nif-specific gene products to which no role has yet been ascribed. It is hoped that by the time of the next meeting, the enzymatic or transfer reactions carried out by the various nif genes involved in metal cluster assembly will be known in detail. Acknowledgements. The author wishes to acknowledge the contributions of his collaborators, GP Roberts, VK Shah, RM Allen, MJ Homer, R Chatterjee, J Allen and J Roll at the University of WI. Work from the laboratories of the author and his collaborators has been generously supported by the NIH and NRI/CGP.

Nodule growth and activity may be regulated by a feedback mechanism involving phloem nitrogen

Abstract: We present a mechanism of regulation of growth and activity of legume root nodules which is consistent with published experimental observations. The concentration of reduced nitrogen compounds, probably amino acids, flowing into the nodules from the phloem, is sensed by the nodules; growth and activity of the nodules is adjusted accordingly. In many legumes this response may involve changes in the oxygen diffusion resistance of the nodule cortex. A straightforward feedback mechanism in which nodule activity is lowered when reduced N in the phloem is high and increased when it is low is envisaged. Almost all import into nodules is via the phloem sap originating in the lower leaves. As a plant develops, these mature leaves no longer utilize nitrogen delivered in the xylem and so export it in the phloem. In plants with an adequate nitrogen supply (from nodules or combined nitrogen in soil), a high concentration of nitrogen containing compounds in the phloem from the lower leaves may inhibit nodule growth as well as activity. This suggestion is an alternative to the hypotheses of carbohydrate deprivation or nitrate inhibition which are commonly used to explain the effects of combined nitrogen on nodule growth and activity.

Unicellular, aerobic nitrogen-fixing cyanobacteria of the genus Cyanothece.

Abstract: Two marine, unicellular aerobic nitrogen-fixing cyanobacteria, Cyanothece strain BH63 and Cyanothece strain BH68, were isolated from the intertidal sands of the Texas Gulf coast in enrichment conditions designed to favor rapid growth. By cell morphology, ultrastructure, a GC content of 40%, and aerobic nitrogen fixation ability, these strains were assigned to the genus Cyanothece. These strains can use molecular nitrogen as the sole nitrogen source and are capable of photoheterotrophic growth in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea and glycerol. The strains demonstrated a doubling time of 10 to 14 h in the presence of nitrate and 16 to 20 h under nitrogen-fixing conditions. Rapid growth of nitrogen-fixing cultures can be obtained in continuous light even when the cultures are continuously shaken or bubbled with air. Under 12-h alternating light and dark cycles, the aerobic nitrogenase activity was confined to the dark phase. The typical rates of aerobic nitrogenase activity in Cyanothece strains BH63 and BH68 were 1,140 and 1,097 nmol of C2H2 reduced per mg (dry weight) per h, respectively, and nitrogenase activity was stimulated twofold by light. Ultrastructural observations revealed that numerous inclusion granules formed between the photosynthetic membranes in cells grown under nitrogen-fixing conditions. These Cyanothece strains posses many characteristics that make them particularly attractive for a detailed analysis of the interaction of nitrogen fixation and photosynthesis in an aerobic diazotroph.

Roles of urease in plant cells

Abstract: Publisher Summary This chapter discusses the role of urease in plant cells Urease is important for efficient nitrogen assimilation Considerable amounts of plant nitrogen flow through urea (urease substrate), which can be recycled only by urease action This recapture can have significant quality impact on protein rich crops It appears to have an important role in germination of protein poor seeds The urease substrate urea is derived from arginine and ureides Arginine is the richest nitrogen repository among the amino acids of seed storage proteins On the other hand, ureides are not only significant sources of nitrogen in nucleic acid turnover but are also a predominant transport from of fixed nitrogen in soybean and other tropical legumes Urease-negative plants accumulate substantial, nonutilizable urea in both maternal and embryonic tissue During germination of urease-negative seeds, further urea accumulates as a dead end in nitrogen metabolism Abundant seed ureases, such as, Sumner's jackbean urease, may play a chemical defense role All of the ureases, both from bacteria and plants, resemble each other in primary structure and in their requirement for accessory genes

Methods for enhancing symbiotic nitrogen fixation

Abstract: Biological nitrogen fixation of leguminous crops is becoming increasingly important in attempts to develop sustainable agricultural production. However, these crops are quite variable in their effectiveness in fixing nitrogen. By the use of the 15N isotope dilution method some species have been found to fix large proportions of their nitrogen, while others like common bean have been considered rather inefficient. Methods for increasing N2 fixation are therefore of great importance in any legume work. Attempts to enhance nitrogen fixation of grain legumes has been mainly the domain of microbiologists who have selected rhizobial strains with superior effectiveness or competitive ability. Few projects have focused on the plant symbiont with the objective of improving N2 fixation as done in the FAO/IAEA Co-ordinated Research Programme which is being reported in this volume. The objective of the present paper is to discuss some possibilities available for scientists interested in enhancing symbiotic nitrogen fixation in grain legumes. Examples will be presented on work performed using agronomic methods, as well as work on the plant and microbial symbionts. There are several methods available to scientists working on enhancement of N2 fixation. No one approach is better than the others; rather work on the legume/Rhizobium symbiosis combining experience from various disciplines in inter-disciplinary research programmes should be pursued.

Genotypic variation in biological nitrogen fixation by common bean

Abstract: Field experiments were performed in Austria, Brazil, Chile, Colombia, Guatemala, Mexico and Peru as part of an FAO/IAEA Co-ordinated Research Programme to investigate the nitrogen fixing potential of cultivars and breeding lines of common bean (Phaseolus vulgaris L.). Each experiment included approximately 20 bean genotypes which were compared using the 15N isotope dilution method. Great differences in nitrogen fixation were observed between and within experiments, with average values of 35% N derived from atmosphere (% Ndfa) and highest values of 70% Ndfa being observed. These values which were larger than had been reported previously for common bean, were observed only when environmental factors were favorable. Therefore, common bean lines are available, which can support high biological nitrogen fixation. These can be used either directly as cultivars for production or in breeding programmes to enhance nitrogen fixation in other cultivars.

15N natural abundance of plant and soil components of a Banksia woodland ecosystem in relation to nitrate utilization, life form, mycorrhizal status and N2-fixing abilities of component species.

Abstract: Studies of the variation in δ15N values for plants from a fire-prone Banksia woodland in South West Australia showed that pioneer herbaceous, non-mycorrhizal species which were active in nitrate reduction and storage, had the highest values (1.81%c). A detailed study of one such species Ptilotus polystachus demonstrated a close correspondence between the δ15N values of soil nitrate, xylem nitrate and leaf total nitrogen, suggesting an exclusive reliance on nitrate ions as nitrogen source. These pioneer species also showed a preponderance of the chloroplastic isoform of glutamine synthetase while woody species generally had higher activity associated with the cytosolic isoform. The group comprising monocotyledonous hemicryptophytes and geophytes contained species with slightly positive δ15N values and moderately active in nitrate reduction and storage. Nitrogen-fixing species had the lowest δ15N values (–0.36‰), irrespective of their apparent utilisation of nitrate. However, woody resprouter species which had low levels of nitrate reduction and storage had δ15N values which fell within the range of values obtained for the miscellaneous assemblage of N2-fixing species. Consequently, 15N abundance values failed to distinguish N2 fixing from non-fixing woody species, and therefore, could not be used in the ecosystem to determine the dependence of putative nitrogen fixing species on N2 fixation. The study demonstrated complex patterns of nitrogen utilization in the ecosystem in which exploitation of different nitrogen resources related to plant life form and the physiological attributes of nitrogen assimilation by component species.

New sources of high-temperature tolerant rhizobia for Phaseolus vulgaris L.

Abstract: Common bean (Phaseolus vulgaris L.) represents an important crop in tropics, but previous screenings of Rhizobium leguminosarum bv. phaseoli did not show strains that could fix N2 in symbiosis with bean at temperatures higher than 35°C (Hungria and Franco, 1993). However, there are other rhizobia and bradyrhizobia species that nodulate some tropical leguminous trees and can fix N2 at high temperatures. In a trial of rhizobial strains isolated from leguminous trees, we found that 14 out of 21 isolates from Gliricidia, Lonchocarpus and Leucaena were also able to nodulate common beans at optimal temperatures (28/23°C, day/night). When we exposed beans inoculated with these strains to high temperature conditions, 40°C/8 h/day, some of them accumulated at flowering time as much or more N as bean plants receiving mineral N. These broad host-range sources of rhizobia capable of fixing nitrogen with bean at high temperature seem to have the potential to improve yields in tropical soils.

Phosphorus regulation of nitrogen fixation in a traditional Mexican agroecosystem

Abstract: Although nitrogen is considered to be the nutrient that most commonly limits production of natural and managed terrestrial ecosystems, I propose that phosphorus may regulate productivity in many continuously cultivated agroecosystems that do not receive applications of synthetic fertilizers. One way P may limit agroecosystem productivity is by controlling nitrogen fixation of legume crops, thus affecting nitrogen availability in the overall agroecosystem. I tested this hypothesis in two studies by examining the effect of phosphorus nutrition on nitrogen fixation of alfalfa in traditional Mexican agroecosystems. All farms used in the research relied on alfalfa as the primary nitrogen source for maize cultivation and other crops, and had minimal or no reliance on synthetic fertilizers. In one study, I used the natural abundance of15N to estimate nitrogen fixation in five alfalfa plots with soils representing a wide range of P fertility. I found a correlation of r = 0.85 between foliage P concentrations and nitrogen fixation in the alfalfa plots. Mean nitrogen fixation in alfalfa plots ranged between 232–555 kg ha−1 yr−1 as estimated by the15N-natural abundance method. In a second study, I sampled soils from alfalfa plots on traditional farms located in 5 different physiographic regions of Mexico. Half of each soil sample was augmented with phosphorus in a greenhouse experiment. I grew alfalfa on the fertilized and unfertilized soils from each site and then determined nitrogenase activity (acetylene reduction) of the Rhizobium on the plant roots. Nitrogenase activity increased in the alfalfa grown on all soils with added phosphorus, with two of the five differences being statistically significant at P < 0.01, 0 and one at P < 0.05. Foliage P concentrations and nitrogenase activity were 0 positively correlated (r = 0.81,P < 0.01).0

Symbiotic dinitrogen fixation and host‐plant growth during development of and recovery from phosphorus deficiency

Abstract: Characterization of nodule growth and function, phosphorus and nitrogen status of plant tissues and host-plant growth of nodulated soybean (Glycine max L. Merr.) plants developing and recovering from phosphorus deficiency was used to evaluate the role of phosphorus in symbiotic dinitrogen fixation. The sequence of physiological responses during recovery from phosphorus deficiency was; (1) rapid uptake of phosphorus, (2) rapid increases in the phosphorus concentration of leaves and nodules, (3) enhanced growth and function of nodules, (4) increased nitrogen concentrations in all plant organs and (5) enhanced plant growth. The sequence of physiological responses to onset of phosphorus deficiency was; (1) decreased phosphorus uptake, (2) decreased phosphorus concentrations in leaves and nodules, (3) decreased nodule function, (4) decreased nitrogen concentration in plant organs and (5) decreased plant growth. These results, in conjunction with previously published data (Sa and Israel, Plant Physiol. 97: 928–935, 1991), support an interpretation that the total response of symbiotic dinitrogen fixation in soybean plants to altered phosphorus supply is a function of both indirect effects on host-plant growth and more direct effects on the metabolic function of nodules.

Nitrogen fixation by the thermophilic green sulfur bacterium Chlorobium tepidum.

Abstract: The thermophilic green sulfur bacterium Chlorobium tepidum grew with N2, NH4+, or glutamine as the sole nitrogen source under phototrophic (anaerobic-light) conditions. Growth on N2 required increased buffering capacity to stabilize uncharacterized pH changes that occurred during diazotrophic growth. Increased sulfide levels were stimulatory for growth on N2. Levels of nitrogenase activity (acetylene reduction) in N2-grown C. tepidum cells were very high, among the highest ever reported for anoxygenic phototrophic bacteria. Maximal acetylene reduction rates in C. tepidum cells were observed at 48 to 50 degrees C, which is about 15 degrees C higher than the optimum temperature for nitrogenase activity in mesophilic chlorobia, and nitrogenase activity in C. tepidum responded to addition of ammonia by a "switch-off/switch-on" mechanism like that in phototrophic purple bacteria. C. tepidum cells assimilated ammonia mainly via the glutamine synthetase-glutamate synthase pathway, elevated levels of both of these enzymes being present in cells grown on N2. These results show that N2 fixation can occur in green sulfur bacteria up to at least 60 degrees C and that regulatory mechanisms important in control of nitrogenase activity in mesophilic anoxygenic phototrophs also appear to regulate thermally active forms of the enzyme.

Minimizing the effect of mineral nitrogen on biological nitrogen fixation in common bean by increasing nutrient levels

Abstract: Although common bean (Phaseolus vulgaris L.) has good potential for N2 fixation, some additional N provided through fertilizer usually is required for a maximum yield. In this study the suppressive effect of N on nodulation and N2 fixation was evaluated in an unfertile soil under greenhouse conditions with different levels of soil fertility (low = no P, K and S additions; medium = 50, 63 and 10 mg kg-1 soil and high = 200, 256 and 40 mg kg-1 soil, respectively) and combined with 5, 15, 60 and 120 mg N kg-1 soil of 15N-labelled urea. The overall average nodule number and weight increased under high fertility levels. At low N applications, nitrogen had a synergistic effect on N2 fixation, by stimulating nodule formation, nitrogenase activity and plant growth. At high fertility and at the highest N rate (120 mg kg-1 soil), the stimulatory effect of N fertilizer on N2 fixation was still observed, increasing the amounts of N2 fixed from 88 up to 375 mg N plant-1. These results indicate that a suitable balance of soil nutrients is essential to obtain high N2 fixation rates and yield in common beans.

Nitrogen fixation in chickpea. I. Influence of prior cropping or fallow, nitrogen fertilizer and tillage

Abstract: N fixation was measured in chickpea in a field experiment on the Darling Downs, Queensland. The 15N natural abundance method was used with barley as a non-fixing control crop. Chickpea was grown with 0, 50 and 100 kg/ha of applied N after pretreatments of either fallow or grain sorghum where sorghum stubble was either incorporated by cultivation, removed or retained on the surface with zero tillage. When chickpea was grown after sorghum both the percentage and amount of N in the tops derived from fixation were larger than after fallow. N fertilizer additions reduced per cent but not total N2fixation. Stubble management had no influence on total N2 fixed. There was a close inverse relationship (R2 = 0.95) between soil nitrate measured at establishment of chickpea and per cent of N derived from fixation. All treatment differences in per cent Nz fixation whether due to prior cropping, fallowing, N fertilizer or tillage could be explained through treatment influences on soil nitrate. Chickpea that had a plentiful supply of soil mineral N for growth accumulated significantly more plant N than plants dependent on N2 fixation. Chickpea grain yields were, however, unaffected by the source or quantity of plant N accumulated. Chickpea provided a positive soil N balance at high fixation rates and a negative balance at low fixation rates.

Effect of low static nitrate concentrations on mineral nitrogen uptake, nodulation, and nitrogen fixation in field pea

Abstract: Combined nitrogen [nitrate (NO3‐), ammonium (NH4+), and urea] will inhibit all components of symbiotic nitrogen (N2) fixation if present in sufficient concentrations. It is generally accepted that nitrate is particularly inhibitory to nodule growth and nitrogenase activity, and somewhat less inhibitory to the infection process. This project examined whether providing low (0.1 ‐ 0.5 mM) static concentrations of NO3‐ to pea (Pisum sativum L. cv. Express), seedlings could avoid the period of N hunger experienced prior to the establishment of N2 fixation, without delaying or reducing the symbiotic N2 fixation. All concentrations of NO3 − tested significantly inhibited all measured components of N2 fixation. The nodulation process as measured by nodule number was inhibited to a similar degree as the other parameters. A concentration dependent response was evident, with 0.1 mM NO3 − causing less inhibition than the 0.2 or 0.5 mM concentrations. Our results indicate the within the concentrations of 0.1 ...

Bacterial heterogeneity in deep subsurface tunnels at Rainier Mesa, Nevada test site

Abstract: To characterize the deep subsurface environment of Rainier Mesa, Nevada Test Site, rock samples were taken from tunnels U 12b, U12g, U12p, and U 12n, which varied in depth from 50 m to 450 m and in gravimetric moisture content from 4% to 27%. Values for total count, viable count, biomass, Simpson diversity, equitability, similarity coefficient, and number of distinct colony types indicated microbiological variability between samples. Viable counts ranged from less than 1 × 101 to 2.4 × 105 CFU g dry wt−1 of rock. Direct counts and enumeration based on phospholipid determination indicated larger numbers of cells g dry wt-1 of rock than viable counts. Simpson diversity indices, equitability, and numbers of distinct colony types varied from 3.00 to 8.05, 0.21 to 0.89, and 7 to 19, respectively, and indicated heterogeneity between samples. Each distinct morphotype was purified and characterized. Gram reaction, morphology, metal and antibiotic resistances, and metabolic activities of each isolate confirmed spatial variability among microbiota isolated from different locations. Most probable numbers of nitrifying, sulfur oxidizing, and sulfur-reducing bacteria were below the limit of detection in all samples, while the numbers of nitrogen fixing bacteria ranged from below the level of detection to 7.8 × 102 cells g dry wt−1 of rock sample, and the numbers of dentrifying bacteria ranged from below the level of detection to greater than 1.6 × 103 cells g dry wt−1 of rock sample.

Partitioning of Nitrogen Derived from N2 Fixation and Reserves in Nodulated Medicago sativa L. During Regrowth

Abstract: Nodulated alfalfa plants were grown hydroponically. In order to quantify N 2 fixation and remobilization of N reserves during regrowth the plants were pulse-chase-labelled with 15 N. Starch and ethanol-soluble sugar contents were analysed to examine changes associated with those of N compounds. Shoot removal caused a severe decline in N 2 fixation and starch reserves within 6 d after cutting. The tap root was the major storage site for metabolizable carbohydrate compounds used for regrowth; initially its starch content decreased and after 14 d started to recover reaching 50% of the initial value on day 24

Enhancement of symbiotic nitrogen fixation by vitamin-secreting fluorescent Pseudomonas

Abstract: Fluorescent Pseudomonas sp. strain 267 promotes growth of nodulated clover plants under gnotobiotic conditions. In the growth conditions (60 μM FeCl3), the production of siderophores of the pseudobactin-pyoverdin group was repressed. Plant growth enhancement results from secretion of B vitamins by Pseudomonas sp. strain 267. This was proven by stimulation of clover growth by naturally auxotrophic strains of Rhizobium leguminosarum bv. trifolii and marker strains E. coli thi- and R. meliloti pan- in the presence of the supernatant of Pseudomonas sp. strain 267. The addition of vitamins to the plant medium increased symbiotic nitrogen fixation by the clover plants.

Plant symbiotic mutants as a tool to analyse nitrogen nutrition and yield relationship in field-growth peas (Pisum sativum L.)

Abstract: Pisum sativum L. is known for high seed and protein yields but also for.yield instability. Because legumes utilize two sources of nitrogen (atmospheric N2 fixed in nodules and assimilation of soil mineral N), studies on their nitrogen nutrition is more complex than in other plants. In this work, pea symbiotic mutants (with no nodules at all ([Nod-]), with inefficient nodules ([Nod+Fix-]) or showing an hypernodulating and a ‘nitrate-tolerant symbiosis’ character ([Nod++Nts]), their semi-leafless isogenic homologues and the parental control line cv Frisson were fertilized with three levels of mineral nitrogen (0, 25 or 50 g N m-2) to generate a range of mineral nitrogen regimes in the same genetic background. Impact of the source and level of nitrogen nutrition was measured on reproductive development, growth, nitrogen accumulation and seed yield. It was shown that a N deficiency induced flowering termination. It also led to a large decrease in the number of seeds produced and the amount of N accumulated in forage and in seeds, when little effect was observed on the progression rates of reproductive stages along the stem. The single seed weight and the amount of dry matter accumulated in forage neither responded strongly to N deficiency. The source of nitrogen was shown to be of little importance to yield but the application of about 50 g N m-2 was necessary to reach the yield of the control cv Frisson when exclusive assimilation was ensuring the N requirements of the plant. Despite the fact that the nitrate-tolerant and hypernodulating mutant P64 used in this study did not yield as well as the parent cv Frisson, it is proposed that [Nod++Nts] characters could act as a yield regulating factor.

Translocation ‐ A key factor limiting the efficiency of nitrogen fixation in legume nodules

Abstract: A hypothesis is presented that the availability of water for export of nitrogenous products from legume nodules is a major factor limiting the efficiency of symbiotic nitrogen fixation. Water for export of solutes in the xylem probably depends largely on the import of water and reduced carbon in the phloeum, and one function of respiration may be to dispose of reduced carbon in order to increase the supply of water. A second hypothesis presented is that control of gas diffusion in soybean nodules is largely restricted to the cortex nearby the vascular bundles, thus making possible the linkage of solute balances in xylem and phloem with resistance to diffusion. These concepts are used in a re-examination of literature on manipulations of nodules and nodulated plants such as lowering of light levels, water stress, defoliation, stem girdling, and alteration of oxygen supply. The concept of translocation as a major factor limiting efficiency of symbiotic fixation is consistent with the failure of superior rhizobial isolates to improve N input significantly, and this limitation could also prevent exploitation of superior bacterial symbionts in the future

Effects of root-associated N2-fixing cyanobacteria on the growth and nitrogen content of wheat (Triticum vulgare L.) seedlings

Abstract: Following co-cultivation of wheat with N2-fixing cyanobacterial isolates capable of forming associations, Nostoc 2S6B, 2S9B or Anabaena C5, for 15 days in the presence or absence of combined N a large stimulation of root length was observed without any increase in root dry weight. Increases in the N concentrations of both roots and shoots occurred following co-cultivation with most cyanobacteria tested. The increase in plant N concentrations appeared to be dependent on the wheat cultivar and the cyanobacterial isolate used. Nostoc isolates had similar nitrogenase activities when associated with roots and when grown in shake-flask cultures. The nitrogenase activity of roots colonized by Anabaena C5 or Nostoc 2S6B was higher following removal of loosely associated cyanobacteria.

Estimation of N2 fixation based on differences in the natural abundance of 15N among freshwater N2-fixing and non-N2-fixing algae.

Abstract: The dynamics of nitrogen supply was investigated for blue-green and green algae from Smith Lake and other freshwaters of subarctic and arctic Alska. The natural abundance of 15N (defined as δ15N) of six N2-fixing blue-green algae was 1.0±%o(X±SE), indicating supply of metabolic nitrogen from atmospheric N2 (δ15N=0.0). The δ15N of six green algae showed an average of 6.6±4.5%o, which is significantly higher than δ15N of N2-fixing blue-green algae from the same waters, reflecting the utilization of dissolved inorganic nitrogen (DIN). Nitrogen-fixing algae also showed higher nitrogen content (7.1±2.1%) than non-N2-fixing algae (2.9±1.5%). The δ15N of a bloom-forming species, Anabaena flos-aquae (Lyngb.) Breb. in Smith Lake showed no significant interannual variations during a three-year study period. The changes in δ15N during each bloom were probably due to variations in the 15N composition of DIN and in the proportional uptake of DIN and N2 fixation. An estimation of the fractional contribution of atmosphere-derived nitrogen (ADN) from δ15N indicated that A. flos-aquae obtained 58-75% of its nitrogen by N2 fixation. This technique agreed with the result obtained using a 15N2 enrichment method. The δ15N of the presumed N2-fixing terrestrial plant was similar to that of the atmospheric N2, whereas the δ15N of the presumed non-N2-fixing terrestrial plants reflected their nitrogen sources.