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It has been shown that the intracellular concentrations of Na+, K+, and Cl- ions in Desulfonatronum thiodismutans depend on the extracellular concentration of Na' ions. An increase in the extracellular concentration of Na+ results in the accumulation of K+ ions in cells, which points to the possibility that these ions perform an osmoprotective function. When the concentration of the NaCI added to the medium was increased to 4%, the concentration gradient of Cl- ions changed insignificantly. It was found that D. thiodismutans contains two forms of hydrogenase--periplasmic and cytoplasmic. Both enzymes are capable of functioning in solutions with high ionic force; however they exhibit different sensitivities to Na+, K+, and Li+ salts and pH. The enzymes were found to be resistant to high concentrations of Na+ and K+ chlorides and Na+ bicarbonate. The cytoplasmic hydrogenase differed significantly from the periplasmic one in having much higher salt tolerance and lower pH optimum. The activity of these enzymes depended on the nature of both the cationic and anionic components of the salts. For instance, the inhibitory effect of NaCl was less pronounced than that of LiCl, whereas Na+ and Li+ sulfates inhibited the activity of both hydrogenase types to an equal degree. The highest activity of these enzymes was observed at low Na+ concentrations, close to those typical of cells growing at optimal salt concentrations.