Cellular adaptations of yeast to freeze-thaw

We study the survival of Saccaromyces cerevisiae when exposed to cycles of freeze (77K) and thaw (298 K), followed by growth. A "wild-type" population of cells has a ~2% survival rate upon exposure to a single cycle. However, under experimental evolution, we found that the population survival fraction increased to ~70% within ~25 cycles. Compared to the initial wild-type cells, we found that these "evolved" cell types displayed increased mass density, lower cellular volumes and lower cytoplasmic fluidity, together with increased basal levels of a glass-forming sugar, trehalose. Our measurements of the cellular growth rates and associated thermal fluxes using calorimetry indicate different nutrient utilization characteristics for the evolved cells. Further, when subjected to a stronger selection i.e. exposure to multiple (3) consecutive cycles of freeze-thaw before growth, we found that the cells evolved to exhibit significantly lower cytoplasmic fluidity compared to the weaker selection regime while achieving similar survival rates. Altogether, these point to an underlying mechanical adaptation of yeast to these extreme environmental perturbations.