OSMOTIC ADJUSTMENT IN MARINE YEAST

The effect of environmental salinity on cell growth, and on the composition and accumulation of compatible solutes, or osmotica, of five yeast strains (Aureobasidium pullulans, Candida sp., Cryptococcus albidus var. albidus, Debaryomyces hansenii and Rhodotorula rubra) was compared. All these yeast were isolated from marine environments, but were able to grow in the absence of salt and should therefore be considered as halotolerant strains. According to their specific cell growth rates at different salt concentrations, these strains vary in their capacity to osmotically adjust to modifications in external salinity. Candida sp. appears to be the most sensitive since the maximum salt concentration at which it can grow is 1.54 mol l(-1) NaCl; however, it showed the highest specific cell growth in the range of 0 to 1.54 mol l(-1) NaCl. Aureobasidium pullulans, on the other band, showed the lowest specific growth rate, but the highest halotolerance range from 0 to 5.13 mol l(-1) NaCl. Debaryomyces hansenii, in contrast, showed higher specific growth at this salinity range. Cryptococcus albidus var. albidus and Rhodotorula rubra showed similar specific cell growth rate values and halotolerance between 0 and 2.45 mol l(-1) NaCl. The protein and carbohydrate content of the biomass of the different yeast cells, as a result of external salinity variation, remained practically constant. The most important effects of the increase in salt concentration in the culture medium were the reduction of cell volume and the accumulation of low-molecular-weight metabolites (LMWM), which appear to act as osmoregulators. Glycerol was found as the major compatible solute in the different marine yeasts studied herein with a total contribution of 64-96% of the internal cell osmolarity. Other LMWM, like carbohydrates and amino acids, contributed to a lesser extent to compensate for the rise in osmotic pressure promoted by the salinity of the external environment.