NITROGENASE QUANTITY VARIES DIURNALLY IN A SUBSET OF CELLS WITHIN COLONIES OF THE NONHETEROCYSTOUS CYANOBACTERIA TRICHODESMIUM SPP

Trichodesmium is a marine filamentous cyanobacterium with the exceptio nal Stockholm University, ability to fix atmospheric nitrogen during t he day without differentiating the specialized oxygen-protective cells known as heterocysts. The localization of the Fe-protein of nitrogena se (dinitrogenase reductase) was examined in cross-sectioned colonies of three species: Trichodesmium thiebautii, T. tenue and T. erythraeum , using immunogold/transmission electron microscopy and immunofluoresc ence/light microscopy. The enzyme was confined to a limited subset of cells (on average 14%) in the sections, randomly distributed within th e colonies in all three species. The frequency of nitrogenase-containi ng cells varied on a diurnal basis, being highest and comparatively co nstant during the day, the period of active nitrogen fixation. The per centage of nitrogenase-labelled cells decreased in the evening and app roached zero just before dawn. After sunrise a period of rapid synthes is of nitrogenase followed. Although the frequency of nitrogenase-cont aining cells was constant during the day, the relative concentration o f nitrogenase within the labelled cells clearly increased until noon a nd then successively decreased in the afternoon/night. The data imply that in three species of the non-heterocystous cyanobacterium Trichode smium 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 variatio n in activity being due to a variation in the quantity of nitrogenase within each cell. Comparisons between nitrogenase-containing Trichodes mium cells and heterocysts are discussed.