Summary: is a marine filamentous cyanobacterium with the exceptional ability to fix atmospheric nitrogen during the day without differentiating the specialized oxygen-protective cells known as heterocysts. The localization of the Fe-protein of nitrogenase (dinitrogenase reductase) was examined in cross-sectioned colonies of three species: and , using immunogold/transmission electron microscopy and immunofluorescence/light microscopy. The enzyme was confined to a limited subset of cells (on average 14%) in the sections, randomly distributed within the colonies in all three species. The frequency of nitrogenase-containing cells varied on a diurnal basis, being highest and comparatively constant during the day, the period of active nitrogen fixation. The percentage of nitrogenase-labelled cells decreased in the evening and approached zero just before dawn. After sunrise a period of rapid synthesis of nitrogenase followed. Although the frequency of nitrogenase-containing cells was constant during the day, the relative concentration of nitrogenase within the labelled cells clearly increased until noon and then successively decreased in the afternoon/night. The data imply that in three species of the non-heterocystous cyanobacterium there are a limited number of cells, or possibly whole trichomes (filaments), which are specialized for nitrogen fixation. Most likely the same set of cells is active throughout the day, with the variation in activity being due to a variation in the quantity of nitrogenase within each cell. Comparisons between nitrogenase-containing cells and heterocysts are discussed.


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