Journal of Plankton Research Vol.17 no.l pp.59-69. 1995 Osmotic adjustment in marine yeast Norma Y.Hernandez-Saavedra, Jose Luis Ochoa and Rafael Vazquez-Dulhalt 1 Center for Biological Research, PO Box 128, La Paz, Baja California Sur, 23000 Mexico and 'Department of Microbiology, University of Alberta, Edmonton, Alberta, T6G 2E9, Canada Abstract. 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., Cryptococciis albidus var. albidiis, 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 1~' NaCI; however, it showed the highest specific cell growth in the range of 0 to 1.54 mol I" 1 NaCI. Aureobasidium pullulans, on the other hand, showed the lowest specific growth rate, but the highest halotolerance range from 0 to 5.13 mol I"' NaCI. Debaryomyces hansenii, in contrast, showed higher specific growth at this salinity range Cryptococcus albidus var. albidus and Rhodolorula rubra showed similar specific cell growth rate values and halotolerance between 0 and 2.45 mol I" 1 NaCI. 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. Introduction Fungi are a heterogeneous group of organisms with many kinds of vegetative, as well as sexual, life cycles. Less than 1% of the described fungal species are obtained from marine environments and only 0.36% are marine-occurring yeasts (Kohlmeyer and Kohlmeyer, 1979). The biology of marine fungi has been reviewed (Moss, 1986), including the physiology of their osmotolerance (Blomberg and Adler, 1992), based on a few cases: Debaryomyces hansenii (Gustafsson and Norkrans, 1976; Adler et al., 1985; Larsson et al., 1990), Zygosaccharomyces rouxii (Van Zyl and Prior, 1990), Hansenlua anomala (Van Eck et al., 1989), Sporobolomyces salmonicolor (Gervais, 1991) and Candida tropicalis (Saubenova et al., 1989). Thus, it is necessary to increase our knowledge about the mechanisms of haloadaptation, halophilism or halo- tolerance of a larger number of examples since there may be different strategies that may be ecologically important. The terminology currently employed in studies concerned with the osmotic responses of the cell has been reviewed (Reed, 1984). The physiological adaptation involving an increase in the concentration of compatible solutes in the cell is called osmotic adjustment and the compatible solutes are called osmotica. © Oxford University Press 59 at University of Arizona on February 29, 2012 http://plankt.oxfordjournals.org/ Downloaded from