Nitrogen Fixation in Cultures of the Cyanobacterium Gloeocapsa (Gloeothece) sp. 1430/3 Incubated in the Dark

SUMMARY: Gloeocapsa sp. 1430/3 fixed N₂ in the dark, but at a lower rate than in the light. Following the transfer of exponentially growing cultures to the dark, the rate of N₂ fixation increased for between 2 and 5 h and then decreased, becoming negligible after 12 h. No substantial increase in activity occurred for about 10 h following re-illumination. It is suggested that the decrease in nitrogenase activity which occurred about 5 h after transfer to the dark was not caused by exhaustion of carbon reserves but by a cessation of nitrogenase synthesis coupled with an irreversible inactivation of the enzyme, probably by O₂. Subsequent recovery of activity apparently depended upon resynthesis of nitrogenase.