Nitrogen-fixing growth by nonheterocystous cyanobacterium Plectonema boryanum

Many oxygen-evolving nonheterocystous cyanobacteria can synthesise nitrogenase^1,2, but only a few are able to protect the enzyme from inhibition and denaturation by oxygen^3-5. It is difficult to reconcile concomitant oxygenic photosynthesis and the obligately anaerobic process of nitrogen fixation in the remaining strains. One possibility is a temporal separation of the processes: Plectonema boryanum (U. Tex strain 594) has been shown⁶ to fix nitrogen and then grow oxygenically to the limits of the fixed nitrogen when incubated without combined nitrogen under N₂/CO₂. Growth was not extensive. Here conditions are described in which the same strain of P. boryanum will simultaneously fix nitrogen and grow extensively, fully pigmented, apparently with sufficient metabolic energy to drive nitrogen fixation.