Photosynthesizing Bacteria

(also photosynthetic bacteria), microorganisms that use light (radiant energy) as the energy for their life processes. During photosynthesis such organisms assimilate carbon dioxide and other inorganic substances, as well as organic compounds. Photosynthesizing bacteria include purple and green bacteria, as well as blue-green algae, whose cell structure closely resembles that of purple and green bacteria.

Purple and green bacteria, which are also known as sulfur bacteria, contain chlorophylls that differ in composition—bacteriochlorophylls a, b, c, d, and e—and carotenoids. They are strict or facultative anaerobes. In contrast to higher plants and various algae, they do not emit oxygen during photosynthesis, inasmuch as they use hydrogen sulfide, thiosulfate, sulfur, molecular hydrogen, or organic compounds rather than water as the hydrogen (or electron) donor in the reduction of CO₂. Some purple bacteria, in oxidizing hydrogen sulfide and thiosulfate, accumulate sulfur in their cells, which they may further oxidize to sulfates. In addition to CO₂, these microorganisms are also capable of assimilating organic compounds, such as acetic acid (acetate) and pyruvic acid (pyruvate). Some species grow principally on the basis of the photoassimilation of carbon dioxide—that is, they are photoautotrophs. Others require the presence of organic substances—that is, they are photoheterotrophs. Some species can use the energy formed during respiration or fermentation in addition to radiant energy and thus grow in the dark. Many species fix molecular nitrogen.

Blue-green algae contain chlorophyll a, carotenoids, and pigments that belong to the phycobilin group. Like plants, they emit oxygen during photosynthesis because they use water as the hydrogen donor. The majority of species grow only in the presence of light—that is, they are strict phototrophs. Some species are able to assimilate organic compounds to an insignificant degree. A substantial number fix molecular nitrogen.

Bacteria of the genus Halobacterium, which contain no chlorophyll, effect a special form of photosynthesis. These bacteria are halophilic microorganisms, that is, they grow on media with high concentrations of sodium chloride. They are heterotrophs. They contain the carotenoid retinol, which is bound with protein in a complex called bacteriorhodopsin and which participates in the use of light energy for the synthesis of adenosine triphosphate (ATP).