Balancing Osmotic Pressure

The primary challenge of inhabiting a high salinity environment is balancing osmotic pressure. Since these environments contain high salt concentrations, water from the cells of organisms spontaneously diffuses out of the cytoplasm in order to restore osmotic balance. This leaves cells dehydrated and thus, eventually causes cell death. In order to ameliorate this predicament, halophiles use one of two unique strategies that function to increase the osmolarity of the cell, both of which as illustrated in figure 4. One strategy employs the accumulation from the environment or synthesis of organic compounds called compatible solutes in the cytoplasm of cells. These compatible solutes include polyalcohols, sugars, ectoines, betaines and amino acids.
For instance, the cell walls of halobacterium are physiologically optimized to survive in high salinity environments. These cell walls are composed of glycoproteins that are stabilized through interaction with sodium ions (Na+), which bind to the outer surface and ensure cellular integrity. In fact, the cell wall disintegrates and the cell lysis when there is an insufficient concentration of Na+ (Mescher and Strominger 1976). This occurs due to the fact that the high content of negatively charged acidic amino acids aspartate and glutamate in the glycoprotein are no longer interacting with the positively charged sodium ions, which leaves these negatively charged carboxyl group of these amino acids to repel each other, leading to cell lysis. Sugars can also be present in archaeal lipids, like the predominant membrane lipids of many Euryarchaeota, glycerol diether glycolipids, which contain negative charges and are similarly stabilized by interaction with sodium ions (Mescher and Strominger 1976). In halophiles that use the potassium ion as their compatible solute, naturally the cytoplasm is relatively acidic and thus, cytoplasmic proteins typically contain lower levels of hydrophobic amino acids and lysine, a positively charged basic amino acid, than proteins of nonhalophiles. This is reasonable as in a highly ionic cytoplasm, polar proteins would tend to
These organisms contain light-sensitive pigments are present, including red and orange carotenoids, especially the C50 pigments called bacterioruberin, and inducible pigments involved in energy conservation (Schulten 1978). Since this process does not involve chlorophyll it is not considered photosynthesis. In low aeration environments, certain haloarchaea synthesize a protein called bacteriorhodopsin, which is then inserted into their cell membranes. Retinal is conjugated to bacteriorhodopsin. Retinal is a carotenoid-like molecule that can absorb light energy and pumps a proton across the cytoplasmic membrane. Therefore, transfer of these cells from growth under high-aeration to oxygen-limiting growth conditions triggers bacteriorhodopsin synthesis and its insertion into their cytoplasmic membranes (Schulten 1978). Through the activity of bacteriorhodopsin, as protons accumulate on the outer surface of the membrane, a proton motive force is generated that is coupled to ATP synthesis through the activity of a proton-translocating ATPase, which enables growth in anoxic conditions. Moreover, the light-stimulated proton pump functions to drive the uptake of nutrients, including the K+ needed for osmotic balance and to pump sodium ions out of the cell by activity of a Na+–H+ antiport system. Other metabolic processes are indirectly driven by light as well, because amino acids