Photosynthesis

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.