Osmoadaptive strategies in halophilic fungi

Abstract

Osmoregulatory mechanisms are essential for fungi to survive in hypersaline environments. The osmoregulatory mechanisms, namely 'organic osmolyte' and 'salt-in-cytoplasm' mechanisms were studied in obligate and facultative halophilic fungi isolated from various athalassohaline, thalassohaline and polyhaline econiches, as a response when grown at solar salt concentrations optimal for their growth, as well as one concentration below and one above the optimal. All the halophilic fungi accumulated sugars sucrose and trehalose, and polyols erythritol, ribitol, arabitol, xylitol, inositol, mannitol, and dulcitol at each of these salt concentrations. An increase in the solar salt concentrations in the growth medium resulted in an increase in the total concentration of osmolytes accumulated by the mycelia; a positive correlation was also seen with individual osmolytes. These fungi showed low intracellular accumulation of sodium and potassium cations, which may therefore not contribute significantly in osmoregulation; however, a high concentration of Na sup(+) was detected on the fungal cell wall, indicating this to be a protective mechanism for the fungal cell. This indicates that the halophilic fungi examined are essentially osmo-conformers, operating on a mechanism of organic osmolyte accumulation for adaptation to hypersaline environments.