EFFECT OF TEMPERATURE ON THE SENSITIVITY OF NITROGENASE TO OXYGEN IN TWO HETEROCYSTOUS CYANOBACTERIA1

EFFECT OF TEMPERATURE ON THE SENSITIVITY OF NITROGENASE TO OXYGEN IN TWO HETEROCYSTOUS CYANOBACTERIA1: "

The effect of temperature and oxygen on nitrogenase activity in two heterocystous cyanobacteria, Anabaena variabilis Kütz. ATCC29413 and Nostoc sp. PCC7120, was investigated. The cyanobacteria were grown under a 12:12 light:dark (L:D) cycle at 27°C and were subsequently exposed to different temperatures (27, 36, 39, and 42°C) at different steady-state O₂ 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 α were derived. In both strains, nitrogenase activity (N_tot = N_m + N_d) was the highest at 39°C–42°C and at 0% O₂. 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 O₂ 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 O₂ concentration and temperature as the latter may dynamically alter the flux of O₂ into the heterocyst.

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