50] Nitrogen fixation by synchronously growing unicellular aerobic nitrogen-fixing cyanobacteria

Abstract

This chapter focuses on the nitrogen fixation by synchronously growing unicellular aerobic nitrogen-fixing cyanobacteria. Unicellular aerobic nitrogen-fixing cyanobacteria are unique in that both oxygenic photosynthesis and anoxygenic nitrogen fixation are carded out in a single cell. Nitrogenase in nitrogen-fixing cyanobacteria must be protected from both internal photosynthetically produced oxygen and external atmospheric oxygen diffusing into the cells. To elucidate the mechanism by which apparently incompatible reactions are carried out within a single cell, cultures grown under several different conditions are examined. These include batch cultures under continuous illumination or light-dark cycles and synchronous cultures under continuous illumination or light-dark cycles. It has been shown with Gloeothece (Gloeocapsa) sp. that the changes in activities of photosynthesis and nitrogen fixation are inversely correlated during the batch growth with continuous illumination. The activity of nitrogenase can be measured by the acetylene reduction method without concentration of the culture material. As nitrogenase activity appears in a segregated manner within the cell cycle, correlation between the changes in activities of nitrogenase and hydrogenase can be examined with synchronously growing cultures.