Active glycerol uptake is a mechanism underlying halotolerance in yeasts: a study of 42 species.

A comparison of 42 yeast species with respect to growth in the presence of high NaCl concentration and characteristics of glycerol uptake is presented. The yeast species were classified into four classes on the basis of their ability to grow in the presence of 1, 2, 3 or 4 M NaCl. Considering that two different types of active-transport systems for glycerol uptake have been described, Na+/glycerol and H+/glycerol symports, glycerol transport was investigated by testing for proton uptake upon glycerol addition in cells incubated in the absence and in the presence of NaCl. Only strains belonging to the two higher classes of salt tolerance showed constitutive active glycerol uptake, and could accumulate glycerol internally against a concentration gradient. Five of these strains exhibited a H+/glycerol symport. All the other strains showed evidence of the activity of a salt-dependent glycerol uptake similar to that described in the literature for Debraryomyces hansenii. The strains within the two lower classes of salt tolerance showed, to varying degrees, glycerol active uptake only when glycerol was used as the carbon and energy source, suggesting that this uptake system is involved in glycerol catabolism. The results within this work suggest that active glycerol uptake provides a basis for high halotolerance, helping to maintain a favourable intracellular concentration of glycerol. The relation between the constitutive expression of such carriers and a higher level of salt-stress resistance suggests that this may be an evolutionary advantage for growth under such conditions.