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 Rn NaCl. Considering that two different types of active-transport systems for glycerol uptake have be en 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 belongin g to the two higher classes of salt tolerance showed constitutive active gl ycerol uptake, and could accumulate glycerol internally against a concentra tion gradient, five of these strains exhibited a H'/glycerol symport. All t he other strains showed evidence of the activity of a salt-dependent glycer ol uptake similar to that described in the literature for Debraryomyces han senii. The strains within the two lower classes of salt tolerance showed, t o varying degrees, glycerol active uptake only when glycerol was used as th e carbon and energy source, suggesting that this uptake system is involved in glycerol catabolism. The results within this work suggest that active gl ycerol 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-str ess resistance suggests that this may be an evolutionary advantage for grow th under such conditions.