Prelabeled Anabaena variabilis Ktitz. evolves ~4CO2 in the light with KCN and DCMU (2,1dimethylurea) present, comparable to the dark control without inhibitors added. Double-reciprocal plots of CO2 release vs. light intensity with either KCN or KCN + DCMU present result in two straight lines intersecting at the ordinate. Apparently, reducing equivalents originating from carbohydrate catabolism are channeled into the photosynthetic electron-transport chain, competing for electrons from photosystem II. Under these conditions, the CO2 release is accompanied by a light-dependent oxygen uptake, presumably due to oxygen-reducing photosystem-I activity while ribulose-bisphosphate carboxylase is inhibited by KCN.
Comparing nine blue-green algae it was shown that only nitrogen-fixing species release substantial amounts of CO2 in the light with KCN or KCN + DCMU present. This release is particularly obvious with Anabaena variabiIis Kfitz. under nitrogen-fixing conditions, but small when the alga is grown with combined nitrogen.
We conclude that nitrogen-fixing species share a common link between respiratory and photosynthetic electron transport. The physiological role may be electron supply of nitrogenase by photosystem I.