Characteristics of the Blue-green Algae

Being in the kingdom Monera, blue-green algae lack membrane-enclosed nuclei. They are, thus, a cellular, and closely related to bacteria. In fact, they have been given the name cyanobacteria. Monera lack not only nuclei, but also mitochondria, Golgi bodies, and plastids.

The prokaryotic blue-greens can be characterized by their cell walls and their pigments. As they are in bacteria, the cell walls of blue-green algae are composed of several layers of mucoprotein and other polysaccharides including lipopolysaccharides. Cellulose, which is found in the cells of flowering plants, is mostly absent.

The pigments in blue-green algae are chlorophyll a, carotenoids, phycocyanin (blue), and phycoerythrin. These organisms often are not blue-green in color; they are sometimes yellowish, red, purple, violet, or nearly black. Trichodesmium erythraeum, a reddish blue-green alga that occasionally grows in great abundance, imparts a red color to water. The Red Sea is so named for this. The pigment chlorophyll a is directly responsible for photosynthesis, the carbohydrate product of which is glycogen (the same carbohydrate produced in animal livers).

The colored portion of the cell lies around the periphery, and the central part of the cell (called the incipient nucleus) contains the chromatin. Vacuoles do not occur in healthy young cells. Some forms of blue-green alga produce endospores.

The blue-greens are well known for their capacity to thrive in adverse conditions. Some can grow on snow and ice, while others can be found in hot springs where the temperature approaches boiling. Blue-greens can also grow in desert soils when water is present. Some forms of blue-green alga grow in nodules in the roots of Cycads in the same manner as nitrogen-fixing bacteria grow on the roots of legumes. Some blue-greens serve as significant sources of food for fish, an arrangement that sometimes results in the fish themselves becoming poisonous. (Cattle have, in fact, died from drinking blue-green infested water.) Blue-green algae sometimes grow inside amoebas, diatoms, and other algal cells. A form of Anabaena (figure 12-1c) grows in the hollow leaves of Azolla, and Nostoc (figure 12-lb) grows in the thallus of Anthoceros, a bryophyte. Both Anabaena and Nostoc grow in the shells of turtles and snails, the intestines of some animals, and the hair follicles of the three-toed sloth.

Reproduction among the blue-greens is entirely asexual. They reproduce only by fission or a form of fragmentation into hormogonia. Sterols are important to sexual reproduction, and these compounds do not occur in the blue-green algae.