Nitrogen fixation

About: Nitrogen fixation is a research topic. Over the lifetime, 7940 publications have been published within this topic receiving 232921 citations. The topic is also known as: GO:0009399.

Significant N 2 fixation by heterotrophs, photoheterotrophs and heterocystous cyanobacteria in two temperate estuaries

Abstract: Nitrogen (N) fixation is fueling planktonic production in a multitude of aquatic environments. In meso- and poly-haline estuaries, however, the contribution of N by pelagic N2 fixation is believed to be insignificant due to the high input of N from land and the presumed absence of active N2-fixing organisms. Here we report N2 fixation rates, nifH gene composition and nifH gene transcript abundance for key diazotrophic groups over 1 year in two contrasting, temperate, estuarine systems: Roskilde Fjord (RF) and the Great Belt (GB) strait. Annual pelagic N2 fixation rates averaged 17 and 61 mmol N m−2 per year at the two sites, respectively. In RF, N2 fixation was mainly accompanied by transcripts related to heterotrophic (for example, Pseudomonas sp.) and photoheterotrophic bacteria (for example, unicellular diazotrophic cyanobacteria group A). In the GB, the first of two N2 fixation peaks coincided with a similar nifH-expressing community as in RF, whereas the second peak was synchronous with increased nifH expression by an array of diazotrophs, including heterotrophic organisms as well as the heterocystous cyanobacterium Anabaena. Thus, we show for the first time that significant planktonic N2 fixation takes place in mesohaline, temperate estuaries and that the importance of heterotrophic, photoheterotrophic and photosynthetic diazotrophs is clearly variable in space and time.

Phylogeny of nitrogenase sequences in Frankia and other nitrogen-fixing microorganisms.

Abstract: The complete nucleotide sequence of a nitrogenase (nifH) gene was determined from a second strain (HRN18a) ofFrankia, an aerobic soil bacterium. The open reading frame is 870 bp long and encodes a polypeptide of 290 amino acids. The amino acid and nucleotide sequences were compared with 21 other published sequences. The twoFrankia strains were 96% similar at the amino acid level and 93% similar at the nucleotide level. A number of methods were used to infer phylogenies of these nitrogen fixers, based onnifH amino acid and nucleotide sequences. The results obtained do not agree completely with other phylogenies for these bacteria and thus make probable occurrences of lateral transfer of thenif genes. The time of divergence of the twoFrankia strains could be estimated at about 100 million years. The vanadium-dependent (Type 2) nitrogenase present inAzotobacter spp. appears to be a recent derivation from the conventional molybdenum-dependent (Type 1) enzyme, whereas the iron-dependent (Type 3) alternative nitrogenase would have a much older origin.

What determines the efficiency of N(2)-fixing Rhizobium-legume symbioses?

Abstract: Biological nitrogen fixation is vital to nutrient cycling in the biosphere and is the major route by which atmospheric dinitrogen (N 2 ) is reduced to ammonia. The largest single contribution to biological N 2 fixation is carried out by rhizobia, which include a large group of both alpha and beta-proteobacteria, almost exclusively in association with legumes. Rhizobia must compete to infect roots of legumes and initiate a signaling dialog with host plants that leads to nodule formation. The most common form of infection involves the growth of rhizobia down infection threads which are laid down by the host plant. Legumes form either indeterminate or determinate types of nodules, with these groups differing widely in nodule morphology and often in the developmental program by which rhizobia form N 2 fixing bacteroids. In particular, indeterminate legumes from the inverted repeat-lacking clade (IRLC) (e.g., peas, vetch, alfalfa, medics) produce a cocktail of antimicrobial peptides which cause endoreduplication of the bacterial genome and force rhizobia into a nongrowing state. Bacteroids often become dependent on the plant for provision of key cofactors, such as homocitrate needed for nitrogenase activity or for branched chain amino acids. This has led to the suggestion that bacteroids at least from the IRLC can be considered as ammoniaplasts, where they are effectively facultative plant organelles. A low O 2 tension is critical both to induction of genes needed for N 2 fixation and to the subsequent exchange of nutrient between plants and bacteroids. To achieve high rates of N 2 fixation, the legume host and Rhizobium must be closely matched not only for infection, but for optimum development, nutrient exchange, and N 2 fixation. In this review, we consider the multiple steps of selection and bacteroid development and how these alter the overall efficiency of N 2 fixation.

Comparative study of nitrogen fixation and carbon metabolism in two chick-pea (Cicer arietinum L.) cultivars under salt stress

Abstract: salinity is one of the major environmental constraints on agriculture in many regions of the world (Boyer, 1982;