The Halophilic Archaea — Evolutionary Relationships and Adaptation to Life at High Salt Concentrations

Abstract

Red halophilic Archaea (Halobacteriaceae) are found worldwide in environments in which salt concentrations exceed 2.5-3 M, such as salt lakes and solar saltern crystallizer ponds. They also occur on products such as salted fish and salted hides. In salt lakes they are often found in numbers sufficiently high to impart a reddish color, due to their content of carotenoid and possibly also retinal pigments. KCl is accumulated intracellularly in molar concentrations to provide osmotic balance with the salinity of the outside medium. The presence of high intracellular salt concentrations requires special adaptations of the cells’ enzymes and other macromolecules to be stable and active under physiological conditions. One of these adaptations is the presence of a large excess of acidic amino acids in the proteins, and a paucity of hydrophobic residues. In the absence of salt these proteins are unstable and unfold, thus the constant presence of high salt concentrations is required for the cells’ integrity and viability. Within the limitations dictated by the requirement for a minimal and high salt concentration, the physiological diversity among the members of the Halobacteriaceae is substantial. In addition to respiratory metabolism on a variety of carbon and energy sources, species may live by anaerobic respiration using different electron acceptors, some are able to ferment arginine, and certain species may also use light energy via the light driven primary ion pumps bacteriorhodopsin and halorhodopsin. Some species are adapted to life in the extremely high magnesium and calcium concentrations of the Dead Sea. Analysis of 16S rRNA nucleotide sequences, combined with renewed attempts in recent years to isolate additional types, has yielded a complex picture of phylogenetic relationships within the group. In spite of the progress made, those species most abundant in such environments as saltern crystallizer ponds still defy our attempts to isolate them, as shown by the fact that direct amplification of halophilic archaeal 16S rRNA sequences from the brines yields phylotypes different from the cultured species of halophilic Archaea.