Hydrogen evolution: A major factor affecting the efficiency of nitrogen fixation in nodulated symbionts

Abstract

Nitrogenase-dependent hydrogen evolution from detached legume nodules and from reaction mixtures containing cell-free nitrogenase has been well established, but the overall effect of hydrogen evolution on the efficiency of nitrogen fixation in vivo has not been critically assessed. This paper describes a survey which revealed that hydrogen evolution is a general phenomenon associated with nitrogen fixation by many nodulated nitrogen-fixing symbionts. An evaluation of the magnitude of energy loss in terms of the efficiency of electron transfer to nitrogen, via nitirogenase, in excised nodules suggested that hydrogen production may severely reduce nitrogen fixation in many legumes where photosynthate supply is a factor limiting fixation. With most symbionts. including soybeans, only 40-60% of the electron fow to nitrogenase was transferred to nitrogen. The remainder was lost through hydrogen evolution. In situ measurements of hydrogen evolution and acetylene reduction by nodulated soybeans confirmed the results obtained with excised nodules. In an atmosphere of air, a major portion of the total electron flux available for the reduction of atmospheric nitrogen by either excised nodules or intact nodulated plants was utilized in the production of hydrogen gas. Some nonleguminous symbionts, such as Alnus rubra, and a few legumes (i.e., Vigns sinensis) apparently have evolved mechanisms of minimizing net hydrogen production, thus increasin their efficiency of electron. transfer to nitrogen. Our res ts indicate that the extent of hydrogen evolution during nitrogen reduction is a major factor affecting the efficiency of nitrogen fixation by many agronomically important legumes. The increasing world population and depletion of fossil fuel supplies have stimulated renewed interest in methods of increasing agricultural productivity while minimizing the consumption of fossil fuels. A major factor limiting agricultural production is nitrogen fertilizer, the synthesis of which consumes major quantities of energy. Approximately 3% (600 X 109 cubic feet; 16.8 X 109 meter3) of the natural gas consumed in the United States in 1973 was used for the synthesis of 17 X 106 U.S. tons (153 X 108 kg) of anhydrous ammonia (1, 2). About 10 X 106 U.S. tons of synthetic ammonia were used for nitrogen fertilizer to supply a portion of an annual agricultural nitrogen demand of about 18 X 106 U.S. tons (1, 2). A large part of the remaining need for nitrogen in agriculture was supplied by nitrogen-fixing organisms, such as legumes, which utilize photosynthetically stored solar energy to reduce atmospheric nitrogen to ammonia. Because the biological nitrogen-fixing process is not dependent upon nonrenewable energy resources, its use in agriculture should be maximized. Factors limiting biological nitrogen fixation therefore deserve thorough investigation (3,4).One characteristic of all cell-free nitrogenase preparations that might limit nitrogen fixation is the release of hydrogen gas concomitant with nitrogen reduction (5-11). During thi...

Azolla-Anabaena azollae Relationship

“…Applying the relative efficiency expression of Schubert and Evans i.e. H2 (Air) or H2 (Air) H2 (Ar) C2H2 (17), to our published data (11), the relative efficiency of the Azolla-Anabaena azollae symbiosis grown on N2 is 0.94 to 0.99, which is comparable to what they reported for nonlegume symbiotic associations. It is apparent that there should be a correlation between the C2H2/N2 ratio and the relative efficiency value obtained for any organism.…”

“…Since the reoxidation of H2 by the uptake hydrogenase provides additional reducing power, the efficiency of nitrogen fixation is presumably increased (6,17,18). Applying the relative efficiency expression of Schubert and Evans i.e.…”

Variation in Nitrogenase and Hydrogenase Activity of Alaska Pea Root Nodules

“…The first reproducible demonstration of H2 evolution from soybean root nodules (13) was later confirmed and extended to other legumes (7). Recent work (18) emphasized that H+ reduction by nitrogenase can result in 40 to 60% of the electron flow through this enzyme complex being lost as H2 and proposed an equation for the relative efficiency of N2 fixation, which related the total flow of electrons through nitrogenase (C2H2 reduction) to electron flow resulting in the reduction of H+ to H2. This equation incorporates data from net H2 metabolism by nitrogenase and any hydrogenase present.…”

“…Whole-plant studies revealed that plant ontogeny (5) and irradiance (4) during 4 weeks of growth markedly altered the relative efficiency of N2 fixation calculated according to Schubert and Evans (18). Although those experiments demonstrated that the host legume could affect the efficiency of bacteroid functioning, there were no data to indicate whether the altered levels of H2 evolution resulted from changes in uptake hydrogenase activity or from differences in H2 production by nitrogenase.…”

Symbiotic Hydrogenase Activity in Bradyrhizobium sp. ( Vigna ) Increases Nitrogen Content in Vigna unguiculata Plants

“…Certain rhizobial strains are able to oxidize this hydrogen through the expression of uptake hydrogenases. Hydrogen recycling has been shown to reduce energy losses associated with diazotrophy (19). Consequently, incorporation of the hydrogen oxidation capability into rhizobial strains has been proposed as a way to improve symbiotic nitrogen fixation (11).…”