EFFECT OF TEMPERATURE ON THE SENSITIVITY OF NITROGENASE TO OXYGEN IN TWO HETEROCYSTOUS CYANOBACTERIA

The effect of temperature and oxygen on nitrogenase activity in two heterocystous cyanobacteria, Anabaena variabilis Kutz. ATCC29413 and Nostoc sp. PCC7120, was investigated. The cyanobacteria were grown under a 12:12 light:dark (L:D) cycle at 27 degrees C and were subsequently exposed to different temperatures (27, 36, 39, and 42 degrees C) at different steady-state O-2 concentrations (20, 10, 5, 0%). Light response curves of nitrogenase activity were recorded under each of these conditions using an online acetylene reduction assay combined with a sensitive laser photoacoustic ethylene detection method. The light response curves were fitted with the rectangular hyperbola model from which the model parameters N-m, N-d, and alpha were derived. In both strains, nitrogenase activity (N-tot = N-m + N-d) was the highest at 39 degrees C-42 degrees C and at 0% O-2. The ratio N-tot/N-d was 4.1 and 3.1 for Anabaena and Nostoc, respectively, indicating that respectively 25% and 33% of nitrogenase activity was supported by respiration (N-d). N-tot/N-d increased with decreasing O-2 concentration and with increasing temperature. Hence, each of these factors caused a relative increase in the light-driven nitrogenase activity (N-m). These results demonstrate that photosynthesis and respiration both contribute to nitrogenase activity in Anabaena and Nostoc and that their individual contributions depend on both O-2 concentration and temperature as the latter may dynamically alter the flux of O-2 into the heterocyst.