Nitrogen fixation by Klebsiella grown in the presence of oxygen

Nitrogenase activity (acetylene reduction) of root-associated, cold-climate azospirillum, enterobacter, Klebsiella, and pseudomonas species during growth on various carbon sources and at various partial pressures of oxygen

A comprehensive view of the diazotrophic bacterial flora of plants requires that attention be paid to the appropriate carbon and oxygen requirements during isolation of the bacteria. Twenty compounds (monosaccharides, disaccharides, polyols, and organic acids) were therefore examined as carbon and energy sources for nitrogenase activity in semisolid stab cultures at pO(2) values of 0.21, 0.02, and </=0.002 with 12 strains of diazotrophic root-associated bacteria. With the facultatively anaerobic bacteria of the genera Klebsiella and Enterobacter, the best substrate was sucrose, followed by fructose and mannitol, whereas among the organic acids, only malic and fumaric acids supported any activity. With the obligately aerobic bacteria of the genera Azospirillum and Pseudomonas, disaccharides were not utilized for nitrogen fixation, but several organic acids were accepted in addition to monosaccharides and polyols; malate and glucose were the best substrates. The patterns of the carbon sources utilized for nitrogen fixation were coherent within the species, with the exception of one Klebsiella pneumoniae and one Enterobacter agglomerans strain, both isolated from the same individual grass plant, which were unable to utilize lactose. Anaerobic conditions (pO(2) value of </=0.002) were required for maximum nitrogenase activity with the facultatively anaerobic bacteria, with the exception of one strain of E. agglomerans, which required atmospheric oxygen (pO(2) value of 0.21). Also, the obligately aerobic diazotrophs required atmospheric oxygen for maximum nitrogenase activity. The maximum specific nitrogenase activities (expressed as micromoles of C(2)H(4) . milligram of bacterial protein . hour) noted during the exponential growth phase of the bacteria were the following: 2.68 with Azospirillum lipoferum on malate, 2.41 with K. pneumoniae and 1.58 with E. agglomerans on sucrose, and 0.95 with Pseudomonas sp. on malate.

Acetylene reduction (nitrogen fixation) by enterobacteriaceae isolated from paper mill process waters

Using selective media containing galactitol, over 130 Enterobacteriaceae have been isolated from paper mill process waters collected from different localities. These bacteria were extensively characterized and tested for acetylene-reducing (nitrogen-fixing) activity under anaerobic conditions. High activity was found in representatives of Klebsiella pneumoniae, Enterobacter aerogenes, Enterobacter cloacae, Erwinia herbicola, Citrobacter freundii, Citrobacter intermedius, and Escherichia coli. Under argon, nitrogenase synthesis was generally not repressed by 5 mM l-glutamate, l-aspartate, l-leucine or Casamino Acids (0.5 g/liter). In many strains, both the specific activities (nanomoles of C(2)H(4) per minute per milligram of protein) and the activities (nanomoles of C(2)H(4) per minute) had considerably declined after 24 h. In three selected strains, activity in intact cells grown under nitrogen was unaffected by the presence during assay of 10 mM l-amino acids or ammonium acetate. All of the strains examined were tolerant towards inactivation of nitrogen-fixing activity by 1.8% (vol/vol) oxygen during assay, and inactivation by up to 10% oxygen was partly reversible. Representatives of the six taxa synthesized nitrogenase in stirred aerobic cultures, though the protein concentrations attained were lower than under anaerobic conditions. It seems reasonable to suggest that under natural conditions, nitrogen fixation is able to contribute significantly to the nitrogen economy of the cells.

Oxygen relations of nitrogen fixation in cyanobacteria

The enigmatic coexistence of O2-sensitive nitrogenase and O2-evolving photosynthesis in diazotrophic cyanobacteria has fascinated researchers for over two decades. Research efforts in the past 10 years have revealed a range of O2 sensitivity of nitrogenase in different strains of cyanobacteria and a variety of adaptations for the protection of nitrogenase from damage by both atmospheric and photosynthetic sources of O2. The most complex and apparently most efficient mechanisms for the protection of nitrogenase are incorporated in the heterocysts, the N2-fixing cells of cyanobacteria. Genetic studies indicate that the controls of heterocyst development and nitrogenase synthesis are closely interrelated and that the expression of N2 fixation (nif) genes is regulated by pO2.

Effects of aerobiosis and nitrogen source on the proton motive force in growing Escherichia coli and Klebsiella pneumoniae cells

The electrochemical gradient of hydrogen ions, or proton motive force (PMF), was measured in growing Escherichia coli and Klebsiella pneumoniae in batch culture. The electrical component of the PMF (delta psi) and the chemical component (delta pH) were calculated from the cellular accumulation of radiolabeled tetraphenylphosphonium, thiocyanate, and benzoate ions. In both species, the PMF was constant during exponential phase and decreased as the cells entered stationary phase. Altering the growth rate with different energy substrates had no effect on the PMF. The delta pH (alkaline inside) varied with the pH of the culture medium, resulting in a constant internal pH. During aerobic growth in media at pH 6 to 7, the delta psi was constant at 160 mV (negative inside). The PMF, therefore, was 255 mV in cells growing at pH 6.3, and decreased progressively to 210 mV in pH 7.1 cultures. K. pneumoniae cells and two E. coli strains (K-12 and ML), including a mutant deficient in the H+-translocating ATPase and a pleiotropically energy-uncoupled mutant with a normal ATPase, had the same PMF during aerobic exponential phase. During anaerobic growth, however, both species had delta psi values equal to 0. Therefore, the PMF in anaerobic cells consisted only of the delta pH component, which was 75 mV or less in cells growing at pH 6.2 or greater. These data thus met the expectation that cells deriving metabolic energy from respiration have a PMF above a threshold value of about 200 mV when the ATPase functions in the direction of H+ influx and ATP synthesis; in fermenting cells, a PMF below a threshold value was expected since the enzyme functions in the direction of H+ extrusion and ATP hydrolysis. K. pneumoniae cells growing anaerobically had no delta psi whether the N source added was N2, NH+4 or one of several amino acids; the delta pH was unaffected. Therefore, any energy cost incurred by the process of nitrogen fixation could not be detected as an alteration of the proton gradient.

Nitrogenase activity in cultured Rhizobium sp. strain 32H1: nutritional and physical considerations

Nutritional and physical conditions affecting nitrogenase activity in the strain of "cowpea" rhizobia, 32H1, were examined using cultures grown on agar medium. Arabinose in the basic medium (CS7) could be replaced by ribose, xylose, or glycerol, but mannitol, glucose, sucrose, or galactose only supported low nitrogenase (C2H2 reduction) activity. Succinate could be replaced by pyruvate, fumarate, malate, or 2-oxoglutarate, but without any carboxylic acid, nitrogenase activity was low or undetectable unless a high level of arabinose was provided. Inositol was not essential. Several nitrogen sources could replace glutamine including glutamate, urea, (NH4)2SO4 and asparagine. The maximum nitrogenase activity of cultures grown in air at 30 degrees C was observed under assay conditions of pO2=0.20-0.25 atm and 30 degrees C incubation. Greatest activity occurred after a period of rapid bacterial growth, when viable cell count was relatively constant. Compared with results obtained on the CS7 medium, nitrogenase activity could be substantially increased and/or sustained for longer periods of time by using 12.5 MM succinate and 100 mM arabinose, by increasing phosphate concentration from 2 to 30-50 mM, or by culturing the bacteria at 25 degrees C.

Arrangement and regulation of the nitrogen fixation genes in Klebsiella pneumoniae studied by depression kinetics

Events underlying depression of the nitrogen fixation (nif) genes in Klebsiella pneumoniae M5A1 were analyzed in vivo by comparing the effects of selective inhibitors of transcription and translation on subsequent nitrogenase activity (rate of acetylene reduction). When batch cultures were induced for depression, an 87-min lag separated ammonium ion/oxygen removal and the appearance of activity.

Acetylene reduction (nitrogen fixation) by pulp and paper mill effluents and by Klebsiella isolated from effluents and environmental situations

High rates of acetylene (C(2)H(2)) reduction (nitrogenase activity) were observed in woodroom effluent from a neutral sulfite semi-chemical mill under aerobic (up to 644 nmol of C(2)H(4) produced per ml per h) and under anaerobic (up to 135 nmol of C(2)H(4) produced per ml per h) conditions. Pasteurized effluent developed C(2)H(2) reduction activity when incubated under anaerobic but not under aerobic conditions. Activities were increased by addition of 0.5 to 3.0% glucose or xylose. Enrichment and enumeration studies showed that N(2)-fixing Azotobacter and Klebsiella were abundant, and N(2)-fixing Bacillus was present. Of 129 isolates of Klebsiella from pulp mills, lakes, rivers, and drainage and sewage systems, 32% possessed nitrogen-fixing ability.

Regulation of nitrogenase synthesis by oxygen in Klebsiella pneumoniae

Klebsiella pneumoniae does not fix N(2) under aerobic conditions. The two protein components required for nitrogenase activity were studied during aeration of cells in nitrogen-free media. Component II of nitrogenase was inactivated more slowly in vivo than component I during aeration. The rate of loss of component II was less than the rate of component II synthesis during derepression. No inactive components were detected in cells that had been growing on NH(4) (+) and then aerated in nitrogen-free medium. This supports the hypothesis that O(2) somehow represses the formation of nitrogenase.

Nitrogenase activity and photosynthesis in Plectonema boryanum

Nitrogen-starved Plectonema boryanum 594 cultures flushed with N(2)/CO(2) or A/CO(2) (99.7%/0.3%, vol/vol) exhibited nitrogenase activity when assayed either by acetylene reduction or hydrogen evolution. Oxygen evolution activities and phycocyanin pigments decreased sharply before and during the development of nitrogenase activity, but recovered in the N(2)/CO(2) cultures after a period of active nitrogen fixation. Under high illumination, the onset of nitrogenase activity was delayed; however, the presence of 3-(3, 4-dichlorophenyl)-1, 1-dimethylurea (DCMU) eliminated this lag. Oxygen was a strong and irreversible inhibitor of nitrogenase activity at low (>0.5%) concentrations. In the dark, low oxygen tensions (0.5%) stimulated nitrogenase activity (up to 60% of that in the light), suggesting a limited but significant respiratory protection of nitrogenase at low oxygen tensions. DCMU was not a strong inhibitor of nitrogenase activity. A decrease in nitrogenase activity after a period of active nitrogen fixation was observed in the N(2)/CO(2-), but not in the A/CO(2-), flushed cultures. We suggest that this decrease in nitrogenase activity is due to exhaustion of stored substrate reserves as well as inhibition by the renewed oxygen evolution of the cultures. Repeated peaks of alternating nitrogenase activity and oxygen evolution were observed in some experiments. Our results indicate a temporal separation of these basically incompatible reactions in P. boryanum.