Photorespiration and nitrogenase activity in the blue-green alga, <i>Anabaena cylindrica</i>

Abstract

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 pO2 while dark respiration is saturated at a pO2 near 0.05 atm. Photorespiration is inhibited rapidly and completely by DCMU (3 x 10-5 m) but KCN (10-4m) has little effect. Exogenously supplied hydroxyethane sulphonate (10-5m), an inhibitor of glycollate oxidase activity, and glycollate do not affect respiration, although 14C-labelled glycollate is assimilated in the light and in the dark. Photorespiration is highly sensitive to p CO2 and to NaHCO3 concentration and approaches true photosynthetic oxygen production at the CO2 compensation point of 10 parts/106. A CO2 concentration of 0.02 atm completely inhibits photorespiration whereas true photosynthesis is scarcely affected. Conditions which stimulate photorespiration (low p CO2 and high pO2) 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.