Anaerobic Nitrogen Fixation Research Articles

Pathways of Energy Metabolism Required for Phenotypic Expression of Nif+Kp Genes in Escherichia Coli

In E. coli K12 (F'nif+Kp) hybrids, electron-transport-dependent phosphorylation is not necessary for anaerobic nitrogen fixation, and substrate level phosphorylation can provide sufficient ATP from glucose for nitrogenase activity. The fumarate-reduction system, however, is essential in these hybrids for the transfer of electrons to nitrogenase. This system is probably also involved in maintaining the membrane in the energized state, thereby allowing nitrogen fixation to occur. The nitrate-reduction system, which can energize the membrane like the fumarate-reduction system, is not necessary for nitrogenase activity in the E. coli K12(F'nif+Kp) hybrids. However, two nitrate reductase genes, chlA, and chlB, are essential for inhibition of nitrogen fixation by nitrate. Moreover, nitrate inhibits nitrogenase activity and this inhibition is most probably effected through a regulator factor coded by chlA and chlB.

Nitrogen fixation in seawater.

Summary. The acetylene reduction technique was used for a 3‐year period to monitor potential nitrogen fixation by aerobic heterotrophic bacteria in the sea 2 miles offshore in Cardigan Bay. Samples from depths down to 15 m were membrane‐filtered and the residues incubated aerobically or anaerobically in acetylene‐containing gas mixtures in sealed Millipore Field Monitors. Pure cultures of aerobic heterotrophs isolated from spread‐plates or monitor membranes supplied with ‘nitrogen‐free’media, were tested for ability to reduce acetylene. Glucose was the best of 14 substrates tested to support both growth and acetylene reduction by marine bacteria. The results suggested the presence of a few aerobic or facultatively anaerobic nitrogen fixers among much more numerous and efficient ‘fixed‐nitrogen‐scavengers’. The acetylene‐reducing capacity of pure cultures was inexplicably variable, even under closely‐standardized conditions: this is discussed. The more consistent acetylene reducers included various‐sized rods (Gram positive and negative), coccobacilli and yeasts, which latter may have had non‐culturable bacteria associated with them. No recognizable Azotobacter sp. was isolated, despite the strongly‐selective conditions imposed. Similar results were obtained for seawater samples from the Irish Sea and an Iceland fjord.

Acetylene‐Reduction Assay of Anaerobic Nitrogen Fixation by Sediments of Selected Wisconsin Lakes

AbstractSediments from two hardwater and six softwater lakes of varying trophic levels from Wisconsin were incubated under conditions which permitted continued synthesis of methane. Rates of acetylene reduction indicated that, on the average, sediments from hardwater and softwater lakes may fix 37 and 7.7 ng of N, respectively, per gram (dry weight) per day. The rate of acetylene reduction diminished in the presence of nitrogenase inhibitors.

NON-SYMBIOTIC NITROGEN FIXATION IN SOME QUEBEC SOILS

The occurrence of free-living nitrogen fixers, the potential for nitrogen fixation, and the correlation between the nitrogen-fixing capacities of the soils and bacterial counts were studied using representative Quebec soils.Clostridium occurred more frequently than did Azotobacter. Studies with N15showed that nitrogen fixation was more frequent under anaerobic than under aerobic conditions in all the soil types studied in their unamended state. The addition of glucose stimulated nitrogen fixation. During anaerobic incubation, nitrogen fixation was found to be correlated significantly with the increase in numbers of both total aerobes and Clostridia. The results suggested that facultatively anaerobic nitrogen fixers, and aerobic nitrogen fixers other than Azotobacter, were present.