Diazotrophy under continuous light in a marine unicellular diazotrophic cyanobacterium, Gloeothece sp 68DGA

Nitrogenase is extremely sensitive to molecular oxygen (O-2), and unicellular diazotrophic cyanobacteria separate nitrogen (N-2)-fixation and photosynthesis to protect nitrogenase from O-2 produced by photosynthesis. When grown under 12 In light/12 h dark cycles (LID), the marine unicellular diazotrophic cyanobacterium Gloeothece sp. 68DGA expressed the nitrogenase protein and its activity (acetylene reduction activity) only during the dark phase. However, this strain was able to grow diazotrophically under continuous light (CL). To determine whether nitrogenase synthesis and N-2-fixation are temporally separated from photosynthesis in the Gloeothece cells that have fully acclimated to CL, the proportion of cells containing nitrogenase (the Fe-protein of nitrogenase) in the culture was measured using an immunocytochemical technique. Cells were grown in a continuous-culture device to maintain constant cell density. Under LID, the cells showed diurnal oscillation of nitrogenase activity, photosynthesis, respiration and the expression and the abundance of the Fe-protein. The oscillation was gradually reduced after the transfer of the cells to CL, and was lost after 23-25 days of cultivation under CL. In CL-acclimated cultures, the Fe-protein was always detected in about 94% of the cells, although the nitrogenase activity was about one-third of the maximum activity in LD-acclimated cultures. These results suggest that synthesis of nitrogenase proceeds without diurnal oscillation in the CL-acclimated cells of Gloeothece sp. 68DGA. As the respiration rate in CL-acclimated culture was as high as the maximum rate observed in LD-acclimated culture, O-2-uptake mechanism(s) may have been upregulated to maintain low intracellular PO2.