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The Cycad Pages
CORALLOID ROOTS AND NITROGEN FIXATION
Juliana Medeiros and Dennis Stevenson

Physiology of Nitrogen Fixation

Nitrogenase is very sensitive to oxygen, which may cause temporary or permanent inhibition by binding with the iron portion of the enzyme. Nitrogenase activity is also completely suppressed by the presence of ammonia, which eliminates the need for cyanobacteria to produce fixed nitrogen. Conversely, the presence of the ammonia assimilation enzymes glutamine synthetase and glutamate synthase stimulates nitrogenase synthesis. When ammonia is abundant, these compounds are in constant use, so they are not free to affect nitrogenase production. When ammonia is in short supply, however, they are not occupied and are available to stimulate activity in compounds responsible for the production of nitrogenase (Carr and Whitton, 1982).

A side reaction occurring with the fixation of nitrogen by nitrogenase is the evolution of hydrogen gas. Dihydrogen evolves concomitantly with the fixation of dinitrogen to ammonia at the rate of at least 25% of the electron flow through nitrogenase. In order to recapture energy lost by this process, many organisms that utilize nitrogenase employ another enzyme, uptake hydrogenase (Lindblad and Sellstedt, 1990). Uptake hydrogenase serves several purposes; it recycles dihydrogen to form ATP, protects nitrogenase from oxygen through an oxyhydrogen reaction, and it removes dihydrogen which is a competitive inhibitor of nitrogen fixation. This is a membrane bound enzyme system that does not, however, appear to be functional in cycad/cyanobacteria symbiosis which were found to evolve dihydrogen at rates at least equivalent to those of ammonium production. These results are consistent with observations that cycad symbionts appear to have lost uptake hydrogenase activity (Lindblad et al., 1991; Kumar et al., 1986).

Several methods are employed in order to investigate the rate of production of biologically fixed nitrogen. The most common methods of measuring nitrogen fixation are acetylene reduction and nitrogen isotope enrichment. Nitrogenase catalyzes several reactions in addition to the reduction of nitrogen to ammonia. The reduction of acetylene to ethylene is useful for measurement by gas chromatography, and is the simplest, most commonly employed method for measuring nitrogen fixation by nitrogenase. Use of nitrogen isotope studies is less common because of the extremely short half-life of heavy nitrogen (radioactive N15).



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