Regulation of the Potassium to Sodium Ratio and of the Osmotic Potential in Relation to Salt Tolerance in Yeasts

Abstract

By using the isotope pairs 22Na-24Na and 42K-86Rb, the uptake and retention of Na and K was studied in the salt-tolerant Debaryomyces hansenii and in the less tolerant Saccharomyces cerevisiae at NaCl levels of 4 mm and 0.68, 1.35, and 2.7 M in the medium. The ratio of K to Na is much higher in the cells than in the media, and higher in D. hansenii than in S. cerevisiae under comparable conditions. The difference between the two species is due to a better Na extrusion and a better uptake of K in D. hansenii. The kinetics of ion transport show that at about the time when extrusion of Na could be demonstrated in D. hansenii, K-Rb previously lost to an easily washable compartment of the cells was reabsorbed in both organisms. More H+ was given off from S. cerevisiae than from D. hansenii in the course of these events. The findings fit the working hypothesis tested, which regards salt tolerance as partly dependent on the ability to mobilize energy to extrude Na from the cells and to take up K. The volume changes in S. cerevisiae are greater and are more slowly overcome than those in D. hansenii. The total salt level of the cells is not sufficient to counteract the osmotic potential of the medium, so that additional osmoregulatory mechanisms must be involved in determining halotoler'ance.

Osmoregulatory active sodium‐glycerol co‐transport in the halotolerant yeast Debaryomyces hansenii

“…Maintaining the sodium gradient requires in turn an active mechanism that transports sodium out of the cell against its gradient. As was shown earlier (Norkrans, 1968;Norkrans and Kylin, 1969) in D. hunsenii in contrast to Saccharomyces cerevisiae, the intracellular sodium concentration is maintained low and does not increase significantly when the extracellular sodium chloride concentration is increased. Our observations of proton movements associated with glycerol transport when extracellular sodium chloride was present (but not in its absence) and of the collapse of the glycerol gradient after addition of the protonophore CCCP lend support to the proposed existence (Blomberg, 1988) in D. hansenii of a sodium-proton exchange mechanism that pumps sodium out of the cells using the proton gradient maintained by the plasma membrane proton pump a t the expense of metabolic energy.…”

Intracellular Na+ and K+ distribution in Debaryomyces hansenii. Cloning and expression in Saccharomyces cerevisiae of DhNHX1

“…Debaryomyces hansenii tolerates high salinity levels (Lages et al, 1999;Ramos, 1999Ramos, , 2006, and its halotolerant behaviour has been known for decades (Norkrans, 1968;Norkrans & Kylin, 1969) although the basis for this characteristic is not well understood. While the main strategy to avoid salt stress in the model yeast Saccharomyces cerevisiae is to extrude sodium out of the cell in order to keep low levels of the cation and high intracellular ratios of K 1 /Na 1 (Camacho et al, 1981;Haro et al, 1991), several groups have reported that D. hansenii accumulates large amounts of sodium without apparent physiological problems.…”

“…In addition, a correlation between the intracellular glycerol and arabinitol concentrations and the salinity of the medium has been reported (Larsson et al, 1990). Moreover, exhaustive biochemical work has suggested that K 1 and Na 1 transporters should play important roles in salt tolerance (Norkrans & Kylin, 1969;Prista et al, 1997;Thomé-Oritz et al, 1998). Nevertheless, the molecular information supporting previous biochemical work on D. hansenii is scarce.…”

Glycolytic sequence and respiration of Debaryomyces hansenii as compared to Saccharomyces cerevisiae

“…Debaryomyces hansenii has become increasingly important during the last few years as a model of osmotolerance of eukaryotic microorganisms, which is partly due to the diversion of metabolism towards the production of glycerol (Adler et al, 1985;Adler and Gustafsson, 1980;Blomberg and Adler, 1992). An important characteristic of this yeast is its ability to accumulate and tolerate significant amounts of Na + when incubated or grown in the presence of high Na + concentrations (Norkrans and Kylin, 1969;Prista et al, 1997;Thomé et al, 1998). Its osmotolerance mechanism does not seem to result from either the resistance of its glycolytic enzymes to the salt (Neves et al, 1997) or the particular properties of its monovalent cation transport systems (González-Hernández et al, 2004).…”