General Microbiology - Schlegel, H. 1987

Phototrophic Bacteria and Photosynthesis
Utilization of Light Energy by Halobacteria

Species belonging to the genus Halobacterium (H. halobium, H. cutirubrum) form a physiologically highly specialized group of Bacteria. Their natural habitats are highly concentrated or saturated salt solutions, such as the waters of the Great Salt Lake in the USA or salt ponds where salt is produced by solar evaporation of seawater. The adaptation of these bacteria to such extreme environments is due to the fact that the intracellular salt concentration is as high as that of the surrounding medium. Optimal growth of these bacteria occurs in 3.5–5.0 M NaCl solutions. Furthermore, normal functioning of Enzymes isolated from their Cells requires salt concentrations exceeding two molar. Halobacteria belong to the archaebacteria (Sec. 3.13).

The rod-shaped, motile cells of H. halobium are colored red, orange, or yellow by the carotenoids they contain. Their Cell/33.html">Plasma Membrane features dark red patches about 0.5 µm in diameter, which cover roughly half of The Cell surface (Fig. 12.18). These patches are formed by the so-called purple membrane. Its color is due to the presence of Bacteriorhodopsin, a pigment similar to the rhodopsin found in animal visual cells. Thanks to this pigment, exposure to light generates a proton gradient across the membrane. The purple membrane thus Functions as a light-driven proton pump, leading to the generation of an electrochemical Membrane Potential. Charge equilibration can be coupled with ATP synthesis, allowing the purple membrane to mediate a unique type of phosphorylation. This light-converted energy complements the energy derived from aerobic substrate oxidation¹.

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Fig. 12.18. Schematic representation of Halobacterium halobium. Shown are The Plasma Membrane (pink with patches of purple membrane) and the functioning of the purple membrane as a light-driven proton pump.

¹ Halobacteria include not only the genus Halobacterium, but also other genera (Halococcus, Haloarcula, Natrococcus, Natrobacterium). Most of these organisms are capable of synthesizing bacteriorhodopsin under O₂ deficiency. Some halobacteria can grow by deriving energy solely from Photosynthesis mediated by bacteriorhodopsin. — Note by the Editor.