Abstract
Unicellular diazotrophic cyanobacteria fix N2 during the night so as to protect nitrogenase from O2 that is evolved by photosynthesis. However, N2-fixation and photosynthesis of many unicellular species occur simultaneously when they were cultured under continuous illumination (CL). Changes in the abundance of nitrogenase (Fe-protein)-containing cells and the activities of nitrogenase and photosynthesis were studied during the acclimation process to CL with Gloeothece sp. 68DGA grown in a continuous culture system. Nitrogenase activity was completely suppressed in the first subjective dark phase upon transferring from 12h light/12h dark cycles (12L/12D) to CL, though about 70% cells expressed nitrogenase. Activities of nitrogenase and photosynthesis became almost constant, and more than 92% cells expressed nitrogenase constitutively after cells were proliferated about 10 generations under CL. Diurnal oscillation of nitrogenase activity recovered from the second dark phase upon transferring from CL to 12L/12D, though about 60% cells still expressed nitrogenase constitutively.
