Cyanobacteria are photosynthetic bacteria formerly known as blue-green algae. Most are found in the soil and in freshwater and saltwater environments. The majority of species are unicellular, but some may remain linked and form filaments.

Cyanobacteria, which are autotrophic, serve as important fixers of nitrogen in food chains. In addition, cyanobacteria, a key component of the plankton found in the oceans and seas, produce a major share of the oxygen present in the atmosphere, while also serving as food for fish. Some species of cyanobacteria coexist with fungi to form lichens.

Cyanobacteria have played an important role in the development of Earth. Scientists believe that they were the first photosynthetic organisms to occur on Earth’s surface. Beginning about 2.7 billion years ago, the oxygen produced by cyanobacteria enriched Earth’s atmosphere and converted it to its modern form. This conversion made possible all life forms that use oxygen for cellular respiration.

In the human intestine, bacteria play an important role in our digestive health. Some species synthesize several vitamins not widely obtained in food, especially vitamin K. Bacteria also often break down certain foods that otherwise escape digestion in the body. Signals from gut microbes spur capillary development in the small intestine, increasing absorption of nutrients into the bloodstream.

Unfortunately, many bacteria are pathogenic; that is, they cause human disease. Such diseases as cholera, tuberculosis, gonorrhea, syphilis, scarlet fever, food poisoning, Lyme disease, plague, tetanus, typhoid fever, and most pneumonias are due to bacteria. In many cases, the bacteria produce powerful toxins that interfere with normal body functions and bring about disease. Botulism (food poisoning) and tetanus toxins are examples. In other cases, bacteria grow aggressively in the tissues (for example, tuberculosis and typhoid fever), destroying them and thereby causing disease.