The extreme halophiles or halobacteria, class Halobacteria, are another major group of archea, with 15 genera in one family, the halobacteraceae. They are aerobic chemoheterotrophs with respiratory metabolism and require complex nutrients, usually proteins and amino acids, for growth. Species are non motile or motile by lophotrichous flagella.
The most obvious distinguishing character of this family is its absolute dependence on a high concentration of NaCl. These prokaryotes require at least 1.5M NaCl (80%, wt/vol), and usually have a growth optimum at about 3 to 4 M NaCl (17 to 23%). They will grow at a salt concentration approaching saturation (about 36%).
Halobacterium�s cell wall is so dependent on the presence of NaCl that it disintegrates when NaCl concentration drops to about 1.5M. Thus halobacteria grow only in high salinity habitats such as marine saltenrs and salt lakes such as the Dead Sea between Israel and Jordan, and the Great Salt Lake in Utah. They also can grow in the food products such as salted fish and cause spoilage.
Halobacteria often have red to yellow pigmentation from carotenoids that are probably used as protection against strong sunlight. They can reach such high population levels that salt lakes; salterns and salted fish actually turn red.
Halobacterium and other extremely halophilic bacteria have significantly modified the structure of their proteins and membranes rather than simply increasing the intracellular concentration of solutes, the approach used by most osmotolearnt microorganisms. These extreme halophiles accumulate enormous quantities of potassium in order to remain hypertonic to their environment; the internal potassium concentration may reach 4 to 7 M. The enzymes, ribosomes, and transport proteins of these bacteria require high levels of potassium for stability and activity. In addition, the plasma membrane and cell wall of Halobacterium are stabilized by high concentration of sodium ion. If the sodium concentration decreases too much, the wall and plasma membrane literally disintegrates. Extreme halophilic bacteria have successfully adapted to environmental conditions that would destroy most organisms. In the process they have become so specialized that they have lost ecological flexibility and can prosper only in few extreme habitats.
The best-studied member of the family is Halobacterium salinarium. This prokaryote is unusual because this can trap light energy photosynthetically without the presence of chlorophyll. When exposed to low oxygen concentration , some strains of Halobacterium synthesize a modified cell membrane called the purple membrane, which contains the protein bacteriorhdopsin. ATP is produced by unique type of photosynthesis without the participation of bacteriophyll or chlorophyll.
Halobacterium actually has four rhodopsins, each with different functions. Bacteriorhodopsin drives outward proton transport for purposes of ATP synthesis. Halorhodopsin uses light energy to transport chloride ions into the cell and maintain a 4 to 5 M intracellular KCl concentrations. Finally there are two rhodopsins that act as photorecptors, one for red light and one for blue. They control flagellar activity to position the organism optimally in the water column. Halobacterium moves to a location of high light intensity, but one in which ultraviolet light is not sufficiently intense to be lethal.
