Showing papers on "Nitrogen fixation published in 1998"

New Nitrogen-Fixing Microorganisms Detected in Oligotrophic Oceans by Amplification of Nitrogenase (nifH) Genes.

Abstract: Oligotrophic oceanic waters of the central ocean gyres typically have extremely low dissolved fixed inorganic nitrogen concentrations, but few nitrogen-fixing microorganisms from the oceanic environment have been cultivated. Nitrogenase gene (nifH) sequences amplified directly from oceanic waters showed that the open ocean contains more diverse diazotrophic microbial populations and more diverse habitats for nitrogen fixers than previously observed by classical microbiological techniques. Nitrogenase genes derived from unicellular and filamentous cyanobacteria, as well as from the α and γ subdivisions of the class Proteobacteria, were found in both the Atlantic and Pacific oceans. nifH sequences that cluster phylogenetically with sequences from sulfate reducers or clostridia were found associated with planktonic crustaceans. Nitrogenase sequence types obtained from invertebrates represented phylotypes distinct from the phylotypes detected in the picoplankton size fraction. The results indicate that there are in the oceanic environment several distinct potentially nitrogen-fixing microbial assemblages that include representatives of diverse phylotypes.

Effects of salt stress on growth, photosynthesis and nitrogen fixation in chick-pea (Cicer arietinum L.)

Abstract: utes is more a consequence of damage produced by salt stress than of a protective strategy. Plants of chick-pea (Cicer arietinum L. cv. ILC1919) inoculated with Mesorhizobium ciceri strain ch‐191 Key words: Chick-pea, salt stress, nitrogen fixation, photowere grown in a controlled environmental chamber, synthesis, ammonium assimilation. and were administered salt (0, 50, 75, and 100 mM NaCl) during the vegetative period. Four harvests (4, 7, 11, and 14 d after treatment) were analysed. The Introduction aim was to ascertain whether the negative effect of Chick-pea (Cicer arietinum L.) is an important crop in saline stress on nitrogen fixation is due to a limitation

Nitrogen Fixation with Non-Legumes

Abstract: Acetobacter diazotrophicus, a diazotrophic endophyte of sugarcane, carries a cluster of nifgenes similar to those characterized in other Proteobacterial diazotrophs. These include, in order, nifA, nifB, nifHDKEN, nifUSVW andfixABC. This order of nifand fix gene organization is most similar to that found in Azospirillum brasilense, although the deduced gene products of each gene are generally more similar to those characterized in species of Rhizobium, Bradyrhizobium or Azorhizobium. Downstream of fixABC is a gene encoding a protein with similarity to MCPA, a methyl-accepting chemotaxis protein which is involved in chemotatic responses to extracellular signals. mcpA gene may play an important role in symbiosis and plant-microbe interaction by allowing colonization of sugarcane tissues far distant from the site of A. diazotrophicus infection or entry.

An assessment of human influences on fluxes of nitrogen from the terrestrial landscape to the estuaries and continental shelves of the North Atlantic Ocean

Abstract: Our analysis for the International SCOPE Nitrogen Project shows that the fluxes of nitrogen in rivers to the coast of the North Atlantic Ocean vary markedly among regions, with the lowest fluxes found in northern Canada (76 kg N km−2 yr−1) and the highest fluxes found in the watersheds of the North Sea (1450 kg N km−2 yr−1). Non-point sources of nitrogen dominate the flux in all regions. The flux of nitrogen from the various regions surrounding the North Atlantic is correlated (r2 = 0.73) with human-controlled inputs of nitrogen to the regions (defined as net inputs of nitrogen in food, nitrogen fertilizer, nitrogen fixation by agricultural crops, and atmospheric deposition of oxidized nitrogen), and human activity has clearly increased these nitrogen flows in rivers. On average, only 20% of the human-controlled inputs of nitrogen to a region are exported to the ocean in riverine flows; the majority (80%) of these regional nitrogen inputs is stored in the landscape or denitrified. Of all the nitrogen inputs to regions, atmospheric deposition of NOy is the best predictor of riverine export of nitrogen from non-point sources (r2 = 0.81). Atmospheric deposition of this oxidized nitrogen, most of which derives from fossil-fuel combustion, may be more mobile in the landscape than are regional inputs of nitrogen from fertilizer, nitrogen fixation in agriculture, and nitrogen in foods and feedstocks. Agricultural sources of nitrogen, although larger total inputs to most temperate regions surrounding the North Atlantic Ocean, appear to be more tightly held in the landscape. Deposition of ammonium from the atmosphere appears to be a very good surrogate measure of the leakiness of nitrogen from agricultural sources to surface waters. This suggests a management approach for controlling ‘surplus’ nitrogen used in agricultural systems. The sum of NOy and ammonium deposition proves to be an amazingly powerful predictor of nitrogen fluxes from non-point sources to the coastal North Atlantic Ocean for temperate-zone regions (r2 = 0.92; p = 0.001). By comparing fluxes with some estimates of what occurs in watersheds with minimal human impact, it appears that human activity has increased riverine nitrogen inputs to the ocean by some 11-fold in the North Sea region, by 6-fold for all of Europe, and by 3-fold for all of North America. These increased flows of nitrogen have clearly led to severe eutrophication in many estuaries, and have probably contributed to some eutrophication on the continental shelf in the North Sea and in the Gulf of Mexico. In other regions, however, the input of nitrogen to continental shelves is dominated by cross-shelf advection from deep-Atlantic waters, and the increased inputs from rivers are relatively minor.

Plant growth promoting rhizobacteria accelerate nodulation and increase nitrogen fixation activity by field grown soybean [Glycine max (L.) Merr.] under short season conditions

Abstract: A 3 × 2 × 2 factorial field experiment, organized in a randomized complete block split-plot with four replications, was conducted in 1994 to evaluate the effect of two plant growth-promoting rhizobacteria (PGPR) strains (Serratia liquefaciens 2-68 or Serratia proteamaculans 1-102) on nodulation, nitrogen fixation, and total nitrogen yield by two soybean cultivars in a short season area. The experiments were conducted at the Emile A. Lods Research Centre, McGill University, Macdonald Campus, Montreal, Canada, and performed at two adjacent sites. One site was fumigated with methyl bromide (50 g m-2). Another site was kept unfumigated. Co-inoculation of soybean with B. japonicum and PGPR increased soybean nodulation and hastened the onset of nitrogen fixation, when the soils were still cool. Total fixed N, fixed N as a percentage of total plant N, and protein and N yield were also increased by PGPR inoculation. AC Bravor tended to be more responsive to both PGPR treatments for total fixed N and N yields than Maple Glen, suggesting that inoculation with PGPR was more effective for cultivars with higher yield potentials.

Symbiosis between nitrogen-fixing cyanobacteria and plants

Abstract: T he cyanobacteria (blue-green algae) are a morphologically diverse group of Gram-negative prokaryotes within the domain Bacteria (Giavannoni et al. 1988). This group includes unicellular forms, filamentous forms that contain only vegetative cells, and filamentous forms that differentiate specialized cells (Castenholz and Waterbury 1989). Cyanobacteria are photoautotrophs with an oxygenevolving photosynthetic mechanism that is identical to that of chloroplasts of algae and higher plants. Indeed, molecular genetic evidence strongly supports the idea that a recent ancestor(s) in the cyanobacterial evolutionary line gave rise to chloroplasts (Douglas 1994). Many cyanobacteria fix atmospheric nitrogen, which is an oxygen-sensitive process; they fix nitrogen either under low-oxygen growth conditions or, when in air, in specialized cells called heterocysts (Fay 1992). As a group, oxygenic photoautotrophic cyanobacteria, especially those species that fix nitrogen, are the most nutritionally independent organisms in the biosphere. Because of this nutritional independence, as well as their tolerance to physicochemical extremes,

N2-Fixing Microbial Consortia Associated with the Ice Cover of Lake Bonney, Antarctica.

Abstract: Nitrogen (N) availability is a key nutritional factor controlling microbial production in Antarctic freshwater and soil habitats Since there are no significant sources of biologically available N entering these ecosystems, nitrogen fixation may be a major source of ``new'' N supporting primary and secondary production The role of N2 fixation was examined in cyanobacteria-dominated microbial aggregates embedded in the permanent ice cover of Lake Bonney, McMurdo Dry Valley (Victoria Land) lakes area, and in cyanobacterial mats found in soils adjacent to the ice edge Nitrogenase activity was extremely low compared to temperate and tropical systems, but N2 fixation was found at all study sites N2 fixation occurred under both dark and light conditions, indicating the potential involvement of both phototrophic and heterotrophic diazotrophs Nitrogenase activity measurements (acetylene reduction assay) and molecular characterization (PCR amplification of nifH fragments) demonstrated a diverse and periodically active (when liquid water is present) diazotrophic community in this arid, nutrient-limited environment As a result of the close proximity to other microorganisms and the nutritional constraints of this environment, these diazotrophs may be involved in mutually beneficial consortial relationships that enhance their growth when water is available

Alanine, not ammonia, is excreted from N2-fixing soybean nodule bacteroids

Abstract: Symbiotic nitrogen fixation, the process whereby nitrogen-fixing bacteria enter into associations with plants, provides the major source of nitrogen for the biosphere. Nitrogenase, a bacterial enzyme, catalyzes the reduction of atmospheric dinitrogen to ammonium. In rhizobia-leguminous plant symbioses, the current model of nitrogen transfer from the symbiotic form of the bacteria, called a bacteroid, to the plant is that nitrogenase-generated ammonia diffuses across the bacteroid membrane and is assimilated into amino acids outside of the bacteroid. We purified soybean nodule bacteroids by a procedure that removed contaminating plant proteins and found that alanine was the major nitrogen-containing compound excreted. Bacteroids incubated in the presence of 15N2 excreted alanine highly enriched in 15N. The ammonium in these assays neither accumulated significantly nor was enriched in 15N. The results demonstrate that a transport mechanism rather than diffusion functions at this critical step of nitrogen transfer from the bacteroids to the plant host. Alanine may serve only as a transport species, but this would permit physiological separation of the transport of fixed nitrogen from other nitrogen metabolic functions commonly mediated through glutamate.

Microclimatic parameters influencing nitrogen fixation in the phyllosphere in a Costa Rican premontane rain forest.

Abstract: The acetylene reduction method was used to measure nitrogen fixation in the phyllosphere of attached leaves of different phorophytes under natural conditions in a premontane rain forest in Costa Rica. Maximum rates of nitrogen fixation (26 ng N · cm−2 leaf area · h−1) – mainly due to the activity of two species of Scytonema (Cyanobacteria) – were measured in the rainy season in bright sunlight. Rates of nitrogen fixation were correlated with the leaf area covered by Scytonema. In periods without precipitation the fixation activity decreased to zero within 2–3 days. As long as the epiphylls were sufficiently supplied with water, other microclimatic factors like temperature and light intensity also influenced nitrogen fixation rates, but to a lesser extent. Relative humidity and species of phorophyte showed no direct influence. It was concluded that the most important factor for nitrogen fixation in the phyllosphere was the availability of liquid water. Linking these results to meteorological data, the input of nitrogen by biological nitrogen fixation in the phyllosphere in the investigation area was estimated to be as much as 1.6 ± 0.8 kg N · ha−1 · year−1 per unit of leaf area index (LAI). For an LAI of 2 for the understory the nitrogen input would vary between 2 and 5 kg N · ha−1 · year−1. This work also demonstrates the importance of detailed knowledge of variation in microclimate throughout the year as a basis for extrapolation of the annual nitrogen input.

Does nitrogen fixation of commercial, dryland chickpea and faba bean crops in north-west New South Wales maintain or enhance soil nitrogen?

Abstract: Summary. Nitrogen (N 2 ) fixation accords pulse crops the potential to sustain or enhance total soil nitrogen (N) fertility. However, regional field experiments have shown that this potential is often not realised because N 2 fixation is inhibited by the supply of nitrate N in the root zone (0‐90 cm) coupled with a low demand for N during plant growth. The objectives of this study were to establish whether commercially grown chickpea and faba bean crops in the northern grain belt of New South Wales were depleting, maintaining or enhancing soil N fertility, and whether current farm management practices were maximising the N 2 fixation potential of the crops. Fifty-one rainfed crops of chickpea (Cicer arietinumL.) and faba bean (Vicia faba L.) were surveyed in the Moree, Walgett and Gunnedah districts of north-west New South Wales during the winters of 1994 and 1995. Nitrogen fixation was measured using the natural 15 N abundance technique. Net N balance was calculated for each crop by subtracting grain N harvested from fixed N 2 . Soil, plant and fallow conditions with potential to influence N 2 fixation were also documented. The percentage of crop N derived from N 2 fixation (P fix ) ranged from 0 to 81% for chickpea and 19 to 79% for faba bean. Nitrogen fixation of chickpea was uniformly low in the 1994 drought. Total N 2 fixed ranged from 0 to 99 kg/ha for chickpea and 15 to 171 kg/ha for faba bean. Net N balance ranged from ‐ 47 to +46 kg N/ha for chickpea crops, and ‐12 to +94 kg N/ha for faba bean crops. About 60% of the difference in P fix between chickpea and faba bean at the average level of soil nitrate (65 kg/ha) was explained by the higher N demand of the latter. The remaining 40% could be due to greater tolerance of the faba bean symbiosis to nitrate effects. In addition, faba bean had a lower N harvest index than chickpea, which meant that proportionally less N needed to be fixed by faba bean to offset removal of grain N. On average, P fix needed to exceed 35% for chickpea and 19% for faba bean to balance soil N. The equivalent soil nitrate levels were 43 kg nitrate N/ha for chickpea and 280 kg/ha for faba bean (extrapolated from the relationship between measured P fix and soil nitrate). Double-cropping chickpea into summer cereal or grass pasture stubble provided the most consistent strategy for achieving the low levels of soil nitrate.

Whole-Cell Immunolocalization of Nitrogenase in Marine Diazotrophic Cyanobacteria, Trichodesmium spp.

Abstract: The mechanism by which planktonic marine cyanobacteria of the genusTrichodesmium fix N2 aerobically during photosynthesis without heterocysts is unknown. As an aid in understanding how these species protect nitrogenase, we have developed an immunofluorescence technique coupled to light microscopy (IF-LM) with which intact cyanobacteria can be immunolabeled and the distribution patterns of nitrogenase and other proteins can be described and semiquantified. Chilled ethanol was used to fix the cells, which were subsequently made permeable to antibodies by using dimethyl sulfoxide. Use of this technique demonstrated that about 3 to 20 cells (mean ± standard deviation, 9 ± 4) consecutively arranged in a Trichodesmium trichome were labeled with the nitrogenase antibody. The nitrogenase-containing cells were distributed more frequently around the center of the trichome and were rarely found at the ends. On average 15% of over 300 randomly encountered cells examined contained nitrogenase. The percentage of nitrogenase-containing cells (nitrogenase index [NI]) in an exponential culture was higher early in the light period than during the rest of the light-dark cycle, while that for a stationary culture was somewhat constant at a lower level throughout the light-dark cycle. The NI was not affected by treatment of the cultures with the photosynthetic inhibitor dichloro 1,3′-dimethyl urea or with low concentrations of ammonium (NH4Cl). However, incubation of cultures with 0.5 μM NH4Cl over 2 days reduced the NI. The IF technique combined with 14C autoradiography showed that the CO2 fixation rate was lower in nitrogenase-containing cells. The results of the present study suggest that (i) the IF-LM technique may be a useful tool for in situ protein localization in cyanobacteria, (ii) cell differentiation occurs inTrichodesmium and only a small fraction of cells in a colony have the potential to fix nitrogen, (iii) the photosynthetic activity (CO2 uptake) is reduced if not absent in N2-fixing cells, and (iv) variation in the NI may be a modulator of nitrogen-fixing activity.

Tricarboxylic acid cycle and anaplerotic enzymes in rhizobia

Abstract: Rhizobia are a diverse group of Gram-negative bacteria comprised of the genera Rhizobium, Bradyrhizobium, Mesorhizobium, Sinorhizobium and Azorhizobium. A unifying characteristic of the rhizobia is their capacity to reduce (fix) atmospheric nitrogen in symbiotic association with a compatible plant host. Symbiotic nitrogen fixation requires a substantial input of energy from the rhizobial symbiont. This review focuses on recent studies of rhizobial carbon metabolism which have demonstrated the importance of a functional tricarboxylic acid (TCA) cycle in allowing rhizobia to efficiently colonize the plant host and/or develop an effective nitrogen fixing symbiosis. Several anaplerotic pathways have also been shown to maintain TCA cycle activity under specific conditions. Biochemical and physiological characterization of carbon metabolic mutants, along with the analysis of cloned genes and their corresponding gene products, have greatly advanced our understanding of the function of enzymes such as citrate synthase, oxoglutarate dehydrogenase, pyruvate carboxylase and malic enzymes. However, much remains to be learned about the control and function of these and other key metabolic enzymes in rhizobia.

Microbial Phototrophic, Heterotrophic, and Diazotrophic Activities Associated with Aggregates in the Permanent Ice Cover of Lake Bonney, Antarctica

Abstract: The McMurdo Dry Valley lakes, Antarctica, one of the Earth's southernmost ecosystems containing liquid water, harbor some of the most environmentally extreme (cold, nutrient-deprived) conditions on the planet. Lake Bonney has a permanent ice cover that supports a unique microbial habitat, provided by soil particles blown onto the lake surface from the surrounding, ice-free valley floor. During continuous sunlight summers (Nov.–Feb.), the dark soil particles are heated by solar radiation and melt their way into the ice matrix. Layers and patches of aggregates and liquid water are formed. Aggregates contain a complex cyanobacterial–bacterial community, concurrently conducting photosynthesis (CO2 fixation), nitrogen (N2) fixation, decomposition, and biogeochemical zonation needed to complete essential nutrient cycles. Aggregate-associated CO2- and N2-fixation rates were low and confined to liquid water (i.e., no detectable activities in the ice phase). CO2 fixation was mediated by cyanobacteria; both cyanobacteria and eubacteria appeared responsible for N2 fixation. CO2 fixation was stimulated primarily by nitrogen (NO3 −), but also by phosphorus (PO4 3−). PO4 3− and iron (FeCl3+ EDTA) enrichment stimulated of N2 fixation. Microautoradiographic and physiological studies indicate a morphologically and metabolically diverse microbial community, exhibiting different cell-specific photosynthetic and heterotrophic activities. The microbial community is involved in physical (particle aggregation) and chemical (establishing redox gradients) modification of a nutrient- and organic matter-enriched microbial ``oasis,'' embedded in the desertlike (i.e., nutrient depleted) lake ice cover. Aggregate-associated production and nutrient cycling represent microbial self-sustenance in a microenvironment supporting ``life at the edge,'' as it is known on Earth.

Foliar application of molybdenum in common beans. I. Nitrogenase and reductase activities in a soil of high fertility

Abstract: Although common bean (Phaseolus vulgaris L.) has a good potential for N2 fixation, poor nodulation following inoculation, principally under field conditions, has led to increased nitrogen (N) fertilizer use in this crop. In the face of the negative environmental effects of N fertilizer, alternative methods have been studied to minimize the amount to be applied. In this sense, foliar application of molybdenum (Mo) has been cited as a promising method. Several papers show that high bean yields (1,500–2,500 kg ha‐1), may be obtained in the southeasten region of Brazil, when there is an application of N as side dressing or Mo spray 25 days after plant emergence. A field experiment was carried out to verify the effect of Mo foliar application on nitrogenase and nitrate reductase activities and on bean yield. Treatments included Rhizobium inoculation (with and without), foliar application of Mo (0 and 40 g ha‐1), N at planting (0 and 20 kg ha‐1) and N applied as side dressing (0 and 30 kg ha‐1). Molybd...

Variation in nitrogen fixing ability among natural isolates of Azospirillum

Abstract: A total of 285 strains of Azospirillum were isolated from soils from seven geographic regions in New South Wales, Australia, using an immunomagnetic separation procedure which does not select strains according to their nitrogen-fixing ability. By combining amplification and restriction analysis of 16S rDNA (ARDRA) patterns with serological, morphological and biochemical results, we found that almost all isolates were A. brasilense and A. lipoferum. There was wide variation in the nitrogenase (acetylene reduction) activity of isolates grown in nitrogen-free, semisolid medium, with differences in average activities between regions. Isolates with zero or negligible nitrogenase activity were found in samples from only two regions, one of which had two out of 26 strains with no activity. Representative isolates, having the highest, the lowest, and intermediate nitrogen fixation rates for each site, were used to inoculate the roots of wheat plants in a model system. Most of the isolates, in association with wheat roots, reduced between 1 and 5 nmol C2H4· mg dry root−1· day−1, but certain strains gave considerably higher activities. The rank order of nitrogen fixation activity on wheat roots did not correlate well with that of nitrogen fixation in pure culture; some strains that fixed nitrogen vigorously in pure culture had low rates of fixation on roots, and vice versa. This inconsistency could not be explained by variations in the root colonizing ability of different strains. However, isolates of A. lipoferum had a higher average nitrogenase activity than A. brasilense, both in Nfb medium and in association with wheat roots. The majority of the most active nitrogen fixers were A. lipoferum. When wheat plants were inoculated with mixtures of two or four strains, nitrogen fixation rates were generally between the rates for the component strains when inoculated individually. There was no benefit from using mixtures of different strains.

A Survey of Symbiotic Nitrogen Fixation by Rhizobia

Abstract: Nitrogen fixation, that is reduction of nitrogen gas into ammonia, is not restricted to rhizobia. In fact, this property, although unique to prokaryotic organisms, is widely spread among the prokaryotic phylogenetic tree (Young, 1992). In addition, the properties of the enzymatic complex which allows the saturation of the very stable triple bond of dinitrogen are remarkably conserved. A. recent illustration has been provided by the resolution of the three dimensional structure of the two components of the nitrogenase complex in as different organisms as Azotobacter vinelandii and Clostridium pasteurianum (reviewed in Smith et al., 1995).

Effects of salinity and nitrogen source on growth and nitrogen fixation in alfalfa

Abstract: The Interaction between the effects of nitrate (NO3) and sodium chloride (NaCl) concentration on growth) water relations, nitrogen (N) contents and N fixation were investigated in alfalfa (Medicago sativa L. cv. Magali). The plants were grown hydroponically in a growth chamber, in the presence or absence of 3 mM potassium nitrate (KNO3) and exposed to various concentrations of NaCl. Increased salinity resulted in a significant decrease in shoot and root biomass, relative water content and water potential. Shoot growth was more inhibited by NaCl than root biomass. The plants grown in the presence of NO3 were slightly less affected by NaCl than the plants dependent on N fixation for their N nutrition. Nitrogenase activity measured by acetylene reduction activity was substantially inhibited by NaCl, and this inhibition was significantly correlated to the inhibition of shoot growth and total N contents. The comparison of the curves of ARA response to oxygen (O2) partial pressure showed that the salt‐...

Improvement of biological nitrogen fixation in Egyptian winter legumes through better management of Rhizobium

Abstract: The diversity of rhizobia nodulating common bean ( Phaseolus vulgaris), berseem clover (Trifolium alexanderinum) and lentil (Lens culinaris) was assessed using several characterization techniques, including nitrogen fixation efficiency, intrinsic antibiotic-resistance patterns (IAR), plasmid profiles, serological markers and rep-PCR fingerprinting. Wide diversity among indigenous rhizobial populations of the isolates from lentil, bean and clover was found. Strikingly, a large percentage of the indigenous rhizobial population was extremely poor at fixing nitrogen. This emphasizes the need to increase the balance of highly efficient strains within the rhizobial population. Use of high-quality inocula strains that survive and compete with other less-desired and less-efficient N2-fixing rhizobia represents the best approach to increase biological nitrogen fixation of the target legume. In field-grown lentils, the inoculant strains were not able to outcompete the indigenous rhizobia and the native lentil rhizobia occupied 76–88% of the total nodules formed on inoculated plants. Nitrogen fixation by lentils, estimated using the 15N isotope dilution technique, ranged between 127 to 139 kg ha-1 in both inoculated and un-inoculated plants. With berseem clover, the inoculant strains were highly competitive against indigenous rhizobia and occupied 52–79% of all nodules. Inoculation with selected inocula improved N2 fixation by clover from 162 to 205 kg ha-1 in the three cuts as compared with 118 kg ha-1 in the un-inoculated treatment. The results also indicated the potential for improvement of N2 fixation by beans through the application of efficient N2-fixing rhizobia.

rRNA based identification and detection systems for rhizobia and other bacteria

Abstract: Ribosomal ribonucleic acids are excellent marker molecules for the elucidation of bacterial phylogeny; they also provide useful target sites for identification and detection with nucleic acid probes. Based on the currently available 16S rRNA sequence data, bacteria of the rhizobial phenotype (plant nodulation, nitrogen fixation) are members of three moderately related phylogenetic sub-groups of the α-subclass of the Proteobacteria: i.e. the rhizobia group, the bradyrhizobia group, and the azorhizobia group. All rhizobia, azo-, brady-, meso-and sinorhizobia are closely related to and in some cases phylogenetically intermixed with, non-symbiotic and/or non-nitrogen-fixing bacteria. Especially in the case of Bradyrhizobium japonicum strains, the 16S rRNA sequence data indicate substantial heterogeneity. Specific probe design and evaluation are discussed. A multiprobe concept for resolving specificity problems with group specific probes is presented. In situ identification with group specific probes of rhizobia in cultures as well as rhizobia and cyanobacteria within plant material is shown.

Modified combined carbon N-deficient medium for isolation, enumeration and biomass production of diazotrophs

Abstract: Dinitrogen fixation associated with non-legumes has been well documented (Dobereiner, 1983; Hegazi, 1983; Fayez, 1990) and is now generally considered to make a significant contribution to the nitrogen economy of several ecosystems. This intensifies the use of field inoculation with various diazotrophs. Therefore, the need arises to monitor the populations and individuals of several associative diazotrophs inhabiting the plant-soil system. The ability of several diazotrophs to multiply and reduce acetylene on numerous carbon sources encourages the search for a single medium suitable for collective growth and biomass production of a number of diazotrophs. The present study evaluates a new medium modification based on basal salts and a mixture of five carbon sources (glucose, malic acid, mannitol, sodium lactate and sucrose) to fulfil the requirements for a single medium on which the majority of associative diazotrophs would exhibit good growth and biomass production.

Potential salinity limitations on nitrogen fixation in sediments from Mono Lake, California

Abstract: Mono Lake is a hypersaline alkaline lake in the high altitude Great Basin desert of eastern California. Algal productivity of the lake is nitrogen-limited, and a contributing source is derived from benthic nitrogen fixation. Lake level and salinity have fluctuated with natural climatic variations but have also been affected by the diversion of tributary streams. This research examines the influence of varied salinity and lake level on the potential for benthic nitrogen fixation in Mono Lake. A sediment-surface microbial mat community was exposed directly, and in acclimated cultures, to a range of Mono Lake salinities under anaerobic incubations and the activity of nitrogenase assayed by acetylene reduction. Activity was stimulated in light, but also occurred in darkness. Over an experimental salinity range from 50 to 150 g L-1 TDS, nitrogenase activity was reduced by 90 per crnt, with the activity persisting at the highest salinity being attributable to dark fixation alone. Between a salinity of 50 g L-1, occurring in Mono Lake over 50 years ago, and 100 g L-1, nitrogenase activity was reduced by nearly half. Changes in the area of the littoral zone at varied lake levels also affect the total amount of potential benthic nitrogen fixation in the lake. An accounting of yearly inputs of nitrogen to Mono Lake suggests N2-fixation could contribute as much as 76–81 percent of the total. Inhibition of nitrogen fixation rates by increased salinity could limit the long-term nutrient supply and benthic primary productivity of this ecosystem.