ATP AND NITROGENASE ACTIVITY IN NITROGEN‐FIXING HETEROCYSTOUS BLUE‐GREEN ALGAE

Abstract

SUMMARYThe relationship between ATP and nitrogenase activity has been investigated using the obligate photoautotroph Anabaena cylindrica and Anabaenopsis circularis strain 6720, a photoautotroph which can also use exogenously supplied carbon. In the light photophosphorylation supports maximum observed rates of nitrogenase activity (0.60 and 1.2 μmoles C2H4 mg chl a−1 min−1 under aerobic and microaerobic conditions respectively). The ATP pools sustained under both conditions are similar at 160–200 μmoles mg chl a−1. Oxidative phosphorylation and substrate level phosphorylation are unimportant in supporting nitrogenase activity in the light. Cyclic photophosphorylation can supply all the necessary ATP provided fixed carbon compounds from photosynthetic CO2 fixation (Anabaena) or supplied exogenously (0.2%, w/v, fructose) (Anabaenopsis) are available. When the algae are placed in the dark nitrogenase activity declines. In Anabaenopsis, after an initial rapid drop, nitrogenase activity can be sustained in the presence of exogenous carbohydrate in the dark at up to 70% of the light rate under air, but not under anaerobic conditions. This decreased nitrogenase activity in the dark is not due to a lack of nitrogenase synthesis or of penetration of fixed carbon into the cells, but appears to be due to an inherent inability of dark metabolism to generate sufficient ATP and/or reductant for maximum nitrogenase activity. The ATP pool can be sustained at maximum levels when photophosphorylation, cyclic photophosphorylation, and oxidative phosphorylation operate individually, and to a lesser extent when substrate‐level phosphorylation operates. No consistent correlations were found between the ATP pool and nitrogenase activity, except under dark anaerobic conditions where there was a positive correlation. The data suggest either that the ATP pool which supports nitrogenase activity is distinct from that which supplies ATP for general filament metabolism, and/or that the supply of ATP is not the most critical short‐term factor in controlling nitrogenase activity in Anabaena and Anabaenopsis.