Question

Explain why many extreme halophiles use KCL (instead of NaCl) as their compatible solute. [SHORT ANSWER]

Answer 1

• Microorganisms adapt to high salt environments by synthesizing variety of organic and inorganic solutes to counter external osmotic pressure. The cells adapt to changes in external media such as increasing NaCl or salinity by accumulating small molecules in the cytoplasm that counteract the external osmotic pressure. Inorganic cations such as K⁺ play a role in osmotic balance and the osmotic response. The high concentration of organic anions in many halophiles requires counter ions such as K⁺. Bacteria or microbes that live in high salt concentrations (halophiles) exclusively use inorganic ions for osmotic balance. Halophilic aerobic archaea bacteria have shown high concentration of K⁺ and Cl^-. There are also bacteria with exceedingly high intracellular K⁺. Species like Salinibacter ruber, an obligatory aerobic halophilic bacterium, K⁺ is the major intracellular component for osmotic balance of the cell. Extremely halophilic microorganisms that accumulate KCl for osmotic balance possess excess of acidic amino acids in their proteins. Excess of acidic amino acids is due to presence of high content of glutamine (Gln) and asparagine (Asn), which yield glutamic acid (Glu) and aspartic acid (Asp) upon acid hydrolysis. Other species such as Halorhodospira halophila use different strategy to cope with high salt. This includes presence of an acidic proteome. The species accumulates high KCl concentrations at high salt concentrations.