Fig.21.3 Cylindrospermum

Cyanobacteria are included among the many aquatic and photosynthetic organisms at the bottom of various food chains. They store energy in the sugar they produce through photosynthesis. Small fish and crustaceans feed on them, only to be eaten by larger fish, which then become food for other aquatic organisms or humans. During warmer months, various algae and cyanobacteria may become abundant in bodies of fresh water, especially if the water is polluted. A floating scum or mat called a bloom may extend across a square meter or two of small bodies of water to 2,000 or more square kilometers (800 square miles) of the surfaces of larger lakes in late summer. Cyanobacteria, in particular, tend to become abundant in blooms when agricultural runoffs are high in nitrogen and phosphorus. The water in the vicinity of a bloom often acquires a “fishy” or otherwise objectionable odor or taste. Anabaena, Microcystis, and other genera of cyanobacteria produce toxic substances that can kill both domestic and wild animals. Cattle, hogs, horses, sheep, rabbits, dogs, and even poultry have been poisoned through drinking water that had an algal-bacterial bloom.

Many poisonous fish that are immune to the toxins of certain cyanobacteria become poisonous to their predators only after feeding on the bacteria. While the cyanobacteria are alive, they produce oxygen, and the oxygen content of the water is temporarily increased. Later, however, decay bacteria decomposing the bodies of organisms that have died may deplete the available oxygen so much that fish and other organisms are killed. Cyanobacteria can be attacked by specific viruses called cyanophages, and there has been considerable interest in using the viruses to control blooms in lakes and ponds. Many of the cyanobacteria responsible for the blooms, however, have thick, mucilaginous sheaths that may prevent viruses (and, incidentally, fungi) from penetrating the cells. Blooms that develop in calm ocean waters in the tropics can be even more extensive than those of fresh waters. A marine bloom once reported between the shore and the Great Barrier Reef of Australia was 1,600 kilometers (almost 1,000 miles) long and covered 52,000 square kilometers (20,000 square miles) of ocean. Most cyanobacteria are not palatable to humans, but there are exceptions.

Species of Spirulina have been used for food by the natives of the Lake Chad region of central Africa and areas around Mexico City for centuries. Spirulina, which has a signficant vitamin content, is now cultured commercially and sold in health-food stores. The Japanese use several cyanobacteria, including two species of Nostoc, as side dishes, and the Chinese treat Nostoc commune as a delicacy. A few colorless forms are mild parasites in humans and animals. Some strains of certain Nostoc species produce antibiotics that kill related strains of the same species. Scytonema hofmannii and other cyanobacteria produce antibiotics called cyanobacterins that kill many different forms of both cyanobacteria and eukaryotic algae. These particular cyanobacteria undoubtedly play a role in their own survival by inhibiting growth of competing organisms. Swimmers in Hawaii occasionally suffer from “swimmers’ itch,” a severe skin inflammation that is caused by a species of Lyngbya that sometimes becomes abundant. Ironically, the toxin produced by these organisms has been demonstrated to suppress leukemia and several other types of cancer. In human water supplies, cyanobacteria frequently clog filters, corrode steel and concrete, cause natural softening of water, and produce undesirable odors or coloration in the water. Many communities control cyanobacteria in reservoirs through the addition of very dilute amounts of copper sulfate. They may be more important than originally thought in this regard. In Southeast Asia, so much usable nitrogen is produced in the rice fields by naturally occurring cyanobacteria that rice is often grown for many years on the same land without the addition of fertilizer.