Azolla-Anabaena azollae Relationship: V. N(2) Fixation, Acetylene Reduction, and H(2) Production.

In order to characterize the reactions catalyzed by nitrogenase in the Azolla-Anabaena association, (15)N(2) fixation, C(2)H(2) reduction, and ATP-dependent H(2) 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 H(2) evolution was determined at various partial pressures of C(2)H(2) and N(2). A pC(2)H(2) of 0.1 atm was nearly optimal for C(2)H(4) production and inhibited H(2) production by 95%. The ratio of C(2)H(2) reduced to N(2) fixed was determined as a function of constant pC(2)H(2) (0.1 atm) and variable pN(2). This ratio decreased with increasing pN(2) and the decrease was correlated with less H(2) production. Ratios obtained at N(2) 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 C(2)H(2) reduction, N(2) fixation, and H(2) production were used to obtain an expression of the electron balance in vivo.