GENERAL MICROBIOLOGY - T.P. Pyrog - 2004

18. PHOTOTROPHIC BACTERIA AND PHOTOSYNTHESIS

18.4. UTILIZATION OF LIGHT ENERGY BY HALOBACTERIA

Halobacteria, which belong to archaebacteria, inhabit highly concentrated or saturated salt solutions. Optimal growth of halobacteria occurs in a 3.0–3.5 M sodium chloride solution. Their ability to survive in such extreme environments is due to the fact that the intracellular salt concentration is just as high as that of the surrounding medium.

The rod-shaped, motile Cells of Halobacterium halobium are colored red, orange, and yellow due to the presence of carotenoids. Halobacterial Cells also contain Bacteriorhodopsin—a pigment that forms specialized structures known as purple membranes, which appear as dark red patches 0.5 µm in diameter within Cell/30.html">The Plasma Membrane. These purple membranes cover approximately half of The Cell surface. Upon exposure to light, bacteriorhodopsin generates a proton gradient across the membrane. Thus, the purple membrane acts as a light-driven proton pump, which establishes an electrochemical Membrane Potential. The dissipation of this potential (charge equilibration) can be coupled with ATP synthesis, allowing the purple membrane to facilitate a unique type of phosphorylation. In addition to light-derived energy, halobacteria can also generate energy through aerobic substrate oxidation.