Blue-greens are considered to be a special class of prokaryote because their photosynthetic mechanism is similar to that of plants and algae. In all other respects, however, they resemble the bacteria very closely. They are clearly prokaryotic, having no mitochondria, chloroplasts, or membrane bound nucleus. Typical blue-greens exist in the form of long columns of cells called filaments.
An unusual characteristic of blue-greens is their ability to fix nitrogen gas. This characteristic, combined with their ability to fix CO2 gas by photosynthesis makes them the life forms with the simplest nutrient requirements on earth. They literally live on air, although some minerals are also required. Paradoxically, nitrogen fixation cannot occur in the presence of oxygen, a product of blue-green photosynthesis. To solve this problem blue-greens such as Anabaena have the ability to allow certain cells in a filament to develop into a heterocyst, a specialized cell with thick walls that excludes oxygen from the cell so that nitrogen fixation can occur. Other blue-greens, including Oscillatoria, cannot produce heterocysts and thus can only fix nitrogen in an anaerobic habitat.
View a wet mount from each culture
