Photorespiration and nitrogenase activity in the blue-green alga, Anabaena cylindrica

Abstract

Oxygen uptake in the light (photorespiration) by the nitrogen-fixing blue-green alga Anabaena cylindrica may be up to twenty times the dark respiration rate. The rate of uptake in the light increases linearly with increasing p O 2 while dark respiration is saturated at a p O 2 near 0.05 atm. Photorespiration is inhibited rapidly and completely by DCMU (3 x 10 -5 m) but KCN (10 -4 m) has little effect. Exogenously supplied hydroxyethane sulphonate (10 -5 m), an inhibitor of glycollate oxidase activity, and glycollate do not affect respiration, although 14 C-labelled glycollate is assimilated in the light and in the dark. Photorespiration is highly sensitive to p CO 2 and to NaHCO 3 concentration and approaches true photosynthetic oxygen production at the CO 2 compensation point of 10 parts/10 6 . A CO 2 concentration of 0.02 atm completely inhibits photorespiration whereas true photosynthesis is scarcely affected. Conditions which stimulate photorespiration (low p CO 2 and high p O 2 ) progressively inhibit acetylene reduction. In short-term studies DCMU inhibits acetylene reduction under condi­tions which stimulate photorespiration but has little effect under conditions which inhibit photorespiration. The results suggest that photorespiration and nitrogenase activity com­pete indirectly for reducing power and that at least one mechanism of oxygen inhibition of nitrogenase activity is via a stimulation of photorespiration.