PMC

The fit results of the four different models, given in Table , to a measured light-response curve of nitrogenase activity (acetylene reduction assay [ARA]). chla, chlorophyll a.

Photosynthesis (O₂ evolution) (A) and nitrogenase acetylene reduction (ARA) (B) light-response curves for N. spumigena. Both curves were fitted with the model proposed by Webb et al. (). The parameters fitted by this model are indicated (P_m or N_m, R_d or N_d, and α) as well as the parameters I_k and N_tot. chl-a, chlorophyll a.

The average I_k (A) (open circles) and α (B) (open triangles) values from batch cultures of N. spumigena (n = 19) as well as the conversion factors (left y axes) for I_k (closed squares) (A) and α (closed triangles) (B) (see Table ) plotted against the theoretical /N_m ratio. This ratio is used to specify the different models for fitting the nitrogenase light-response curves (acetylene reduction assay [ARA]).

Two theoretical nitrogenase activity-versus-irradiance curves (acetylene reduction assay [ARA]) generated with a Michaelis-Menten equation. The curve of open triangles is the curve at anoxic conditions when only light-mediated ATP generation occurs. The curve of the closed circles represents the situation that ATP originates both from oxidative phosphorylation and photophosphorylation (respiration and photosynthesis). The parameters N_m, N_d, and α were fitted by the rectangular hyperbola model. N_tot (N_m + N_d) was the same in both curves. The parameter I_k was calculated by N_m/α. Both curves gave the same I_k, but N_m and α were different. The substrate concentration is given in arbitrary units (A.U.).