Bacteria belong to the kingdom Monera. All bacteria are prokaryotic. Bacteria are the oldest, smallest, and most abundant organisms. They can be found in the fossil record tracing back 3.2 billion years; and for the major portion of the time prior to the appearance of the first more complex organisms, they were the sole occupants of the world. For 2.8 billion years, life was microscopic, prokaryotic, anaerobic, and did not liberate oxygen in photosynthesis. Three kinds of bacteria dominated the scene: green sulfur bacteria, purple sulfur bacteria, and purple nonsulfur bacteria (which may be purple, red, or brown). The green sulfur bacteria have a form of chlorophyll called chlorobium chlorophyll; the other two forms of bacteria have bacteriochlorophyll (which is blue-gray in color). The first bacteria to produce oxygen in photosynthesis were the cyanobacteria (also called blue-green algae). Their respiration was anaerobic. In time, the liberation of oxygen as the waste product of photosynthesis necessitated the retreat of anaerobic forms, and aerobic respiration became more common.
The green sulfur bacteria, purple sulfur bacteria, and purple nonsulfur bacteria have survived over billions of years. Because of their own requirements, however, they were forced underground, where they can hide from the oxygen-enriched atmosphere. Although these photosynthetic bacteria require "light," bacteriochlorophyll absorbs light in the infrared zone, which is not visible to the human eye. The energy provided by infrared light is less than that provided by shorter wavelengths and is partly wasted. Additional energy sources are required. These bacteria, then, may tap sulfur compounds such as hydrogen sulfide or thiosulfate, aliphatic acids, or alcohols underground. In a laboratory setting, they sometimes directly exploit molecular hydrogen.
Purple sulfur bacteria grow in decaying organic soils, where sulfur compounds are available. By way of their metabolism, these bacteria create free elemental sulfur, which can be seen inside the bacterial cells.
Some bacteria are chemoautotrophic rather than photosynthetic. They do not use light but instead bring about synthetic reactions by obtaining energy from the oxidation of inorganic molecules such as nitrogen, sulfur, and iron compounds. They may also oxidize gaseous hydrogen.
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