Energy metabolism of Candida (Yarrowia) lipolytica yeast under nonstress and salinity stress conditions

The effect of a high (9%) NaCl concentration in the nutrient medium on the growth and energy metabolism of the halotolerant yeast Candida (Yarrowia) lipolytica (Wick.) Van der Walt and Arx. was investigated. The yeast cells readily utilized hexoses, organic acids (succinate, acetate, and citrate), ethanol, and glycerol as sole sources of carbon and energy. Oxygen consumption by intact cells was mediated by two terminal oxidases, i.e., the cytochrome oxidase and the alternative oxidase, albeit to varying extents at different growth phases, with the greater participation of the alternative pathway upon reaching the stationary growth phase. The capacity of cells to grow on succinate (which is utilized exclusively through the main respiratory chain), as well as the observation that micromolar concentrations of ethidium bromide (an inhibitor of mitochondrial transcription) almost totally inhibited cell growth on both fermentable and nonfermentable substrates, suggests that the oxidative phosphorylation system plays an important, if not exclusive, role in the cell energy budget of this yeast strain. The oxidative phosphorylation system was not accessible to glucose repression. Mitochondria isolated from the yeast cells grown under normal conditions displayed respiratory and phosphorylative activities, indicating a respiratory chain with three points of energy conservation. Salinity stress (9% NaCl) resulted in diminished (by two to three times) growth rates (when cells utilized sucrose, ethanol, or glycerol), a complete loss of capacity to utilize succinate, and enhanced respiration resistant to cyanide. As can be inferred from the analysis of oxidative and phosphorylating activities of mitochondria, salinity stress was accompanied by pronounced changes in the organization of the respiratory chain, including a delayed expression of the alternative oxidase, lower rates of the oxidation of NAD-linked substrates, and dramatically increased compensatory activity of succinate oxidase. The mechanisms underlying the halotolerance of yeast cells are discussed.