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Azolla-Anabaena azollae Relationship

V. ¹⁵N₂ Fixation, Acetylene Reduction, and H₂ Production ^1,2

^a Charles F. Kettering Research Laboratory, 150 East South College Street, Yellow Springs, Ohio 45387

In order to characterize the reactions catalyzed by nitrogenase in the Azolla-Anabaena association, ¹⁵N₂ fixation, C₂H₂ reduction, and ATP-dependent H₂ production were measured in both the Azolla-Anabaena complex and in the alga isolated from the complex.

The rate of reduction of substrates and of ATP-dependent H₂ evolution was determined at various partial pressures of C₂H₂ and N₂. A pC₂H₂ of 0.1 atm was nearly optimal for C₂H₄ production and inhibited H₂ production by 95%. The ratio of C₂H₂ reduced to N₂ fixed was determined as a function of constant pC₂H₂ (0.1 atm) and variable pN₂. This ratio decreased with increasing pN₂ and the decrease was correlated with less H₂ production. Ratios obtained at N₂ partial pressures of approximately 0.3, 0.6, and 0.8 atm, respectively, were 3.2, 2.0, and 1.7 for the association and 4.4, 3.0, and 2.5 for the isolated symbiont. Rates obtained for C₂H₂ reduction, N₂ fixation, and H₂ production were used to obtain an expression of the electron balance in vivo.

² Contribution No. 582 from Charles F. Kettering Research Laboratory.