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There are two species in the genus Halobacterium. Most biologists now place this genus with methanogenic (methane-producing) bacteria in the Archaebacteria, a separate kingdom of organisms. Halobacteria thrive in very salty environments, such as the Dead Sea and the Great Salt Lake. In general, halobacteria prefer environments with NaCl concentration of about 5 Molar, and cannot tolerate environments with NaCl concentration below about 3 Molar.

Halobacteria are unique in that they perform photosynthesis without chlorophyll. Instead, their photosynthetic pigments are bacteriorhodopsin and halorhodopsin. These pigments are similar to sensory rhodopsin, the pigment which humans and other animals use for vision. Bacteriorhodopsin and halorhodopsin are embedded in the cell membranes of halobacteria and each pigment consists of retinal, a vitamin-A derivative, bound to a protein. Irradiation of these pigments causes a structural change in their retinal, referred to as photoisomerization. Retinal photoisomerization leads to the synthesis of ATP, the same high-energy compound synthesized during the light reactions of higher plants. Interestingly, halobacteria also have two additional rhodopsins, sensory rhodopsin-I and sensory rhodopsin-II which regulate phototaxis, the directional movement in response to light. Bacteriorhodopsin and halorhodopsin seem to have an indirect role in phototaxis as well.

See also Plant pigment.



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Demmig-Adams, B., and W. W. Adams III. "Photosynthesis: Harvesting Sunlight Safely." Nature 403 (January 2000): 371-374.

Li, X. P., O. Bjorkman, C. Shih, et al. " A Pigment-binding Protein Essential for Regulation of Photosynthetic Light Harvesting." Nature 403 ; (January 2000): 391-395.

Peter A. Ensminger


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Calvin cycle

—Dark reactions of photosynthesis which use the ATP and NADPH made by the light reactions to synthesize carbohydrates.


—Green organelle in higher plants and algae in which photosynthesis occurs.

Cyanobacteria (singular, cyanobacterium)

—Photosynthetic bacteria, commonly known as blue-green alga.


—Biological molecule, usually a protein, which promotes a biochemical reaction but is not consumed by the reaction.


—A cell whose genetic material is carried on chromosomes inside a nucleus encased in a membrane. Eukaryotic cells also have organelles that perform specific metabolic tasks and are supported by a cytoskeleton which runs through the cytoplasm, giving the cell form and shape.


—Membrane enclosed structure within a eukaryotic cell which is specialized for specific cellular functions.


—Cell without a nucleus, considered more primitive than a eukaryote.


—Pores in plant leaves which function in exchange of carbon dioxide, oxygen, and water during photosynthesis.


—The material that bathes the interior of chloroplasts in plant cells.


—A membranous structure that bisects the interior of a chloroplast.

Additional topics

Science EncyclopediaScience & Philosophy: Philosophy of Mind - Early Ideas to Planck lengthPhotosynthesis - History Of Research, Location Of Light Reactions, Cam Photosynthesis, Photorespiration, Cyanobacteria, Anaerobic Photosynthetic Bacteria - Light reactions, Dark reactions, Photosynthesis in lower organisms, Chloroxybacteria