Beschreibung:
<jats:p>The effect of temperature and oxygen on nitrogenase activity in two heterocystous cyanobacteria, <jats:italic>Anabaena variabilis</jats:italic> Kütz. ATCC29413 and <jats:italic>Nostoc</jats:italic> 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 O<jats:sub>2</jats:sub> 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 <jats:italic>N</jats:italic><jats:sub>m</jats:sub>, <jats:italic>N</jats:italic><jats:sub>d</jats:sub>, and α were derived. In both strains, nitrogenase activity (<jats:italic>N</jats:italic><jats:sub>tot</jats:sub> = <jats:italic>N</jats:italic><jats:sub>m</jats:sub> + <jats:italic>N</jats:italic><jats:sub>d</jats:sub>) was the highest at 39°C–42°C and at 0% O<jats:sub>2</jats:sub>. The ratio <jats:italic>N</jats:italic><jats:sub>tot</jats:sub>/<jats:italic>N</jats:italic><jats:sub>d</jats:sub> was 4.1 and 3.1 for <jats:italic>Anabaena</jats:italic> and <jats:italic>Nostoc</jats:italic>, respectively, indicating that respectively 25% and 33% of nitrogenase activity was supported by respiration (<jats:italic>N</jats:italic><jats:sub>d</jats:sub>). <jats:italic>N</jats:italic><jats:sub>tot</jats:sub>/<jats:italic>N</jats:italic><jats:sub>d</jats:sub> increased with decreasing O<jats:sub>2</jats:sub> concentration and with increasing temperature. Hence, each of these factors caused a relative increase in the light‐driven nitrogenase activity (<jats:italic>N</jats:italic><jats:sub>m</jats:sub>). These results demonstrate that photosynthesis and respiration both contribute to nitrogenase activity in <jats:italic>Anabaena</jats:italic> and <jats:italic>Nostoc</jats:italic> and that their individual contributions depend on both O<jats:sub>2</jats:sub> concentration and temperature as the latter may dynamically alter the flux of O<jats:sub>2</jats:sub> into the heterocyst.</jats:p>