Though cyanobacteria do not have a great diversity of form, and though they are microscopic, they are rich in chemical diversity. Cyanobacteria get their name from the bluish pigment phycocyanin, which they use to capture light for photosynthesis. They also contain chlorophyll a, the same photosynthetic pigment that plants use. In fact the chloroplast in plants is a symbiotic cyanobacterium, taken up by a green algal ancestor of the plants sometime in the Precambrian. However, not all "blue-green" bacteria are blue; some common forms are red or pink from the pigment phycoerythrin. These bacteria are often found growing on greenhouse glass, or around sinks and drains. The Red Sea gets its name from occasional blooms of a reddish species of Oscillatoria, and African flamingos get their pink color from eating Spirulina.
Whatever their color, cyanobacteria are photosynthetic, and so can manufacture their own food. This has caused them to be dubbed "blue-green algae", though they have no relationship to any of the various eukayotic algae. The term "algae" merely refers to any aquatic organisms capable of photosynthesis, and so applies to several groups.
Cyanobacteria are very important organisms for the health and growth of many plants. They are one of very few groups of organisms that can convert inert atmospheric nitrogen into an organic form, such as nitrate or ammonia. It is these "fixed" forms of nitrogen which plants need for their growth, and must obtain from the soil. Fertilizers work the way they do in part because they contain additional fixed nitrogen which plants can then absorb throough their roots.
Nitrification cannot occur in the presence of oxygen, so nitrogen is fixed in specialized cells called heterocysts. These cells have an especially thickened wall that contains an anaerobic environment. You can see these larger cells among the filaments of Nostoc, shown at right.
Many plants, especially legumes, have formed symbiotic relations with nitrifying bacteria, providing specialized tissues in their roots or stems to house the bacteria, in return for organic nitrogen. This has been used to great advantage in the cultivation of rice, where the floating fern Azolla is actively distributed among the rice paddies. The fern houses colonies of the cyanobacterium Anabaena in its leaves, where it fixes nitrogen. The ferns then provide an inexpensive natural fertilizer and nitrogen source for the rice plants when they die at the end of the season.
Cyanobacteria also form symbiotic relationships with many fungi, forming complex symbiotic "organisms" known as lichens.
The cyanobacterium Spirulina, shown at right, has long been valued as a food source; it is high in protein, and can be cultivated in ponds quite easily. In tropical countries, it may be a very important part of the diet, and was eaten regularly by the Aztecs; it is also served in several Oriental dishes. In the US, the popularity of Spirulina is primarily as a "health food", being sold in stores as a dried powder or in tablet form.
Many other species of cyanobacteria produce populations that are toxic to humans and animals. Blue-green pond scums have been linked to the poisoning of cattle and dogs, and occasionally people. It is therefore not recommended that wild populations be gathered and eaten without some knowledge of the organisms involved.
Cyanobacteria may cause other problems as well; a species of Lyngbya is responsible for one of the skin irritations commonly known as "swimmer's itch."
Read more about toxic cyanobacteria at The Toxic Cyanobacteria Home Page.
Images of Nostoc and Spirulina provided by the University of Wisconsin Botanical Images Collection.