Macromolecular synthesis by yeasts under frozen conditions

Although viable fungi have been recovered from a wide variety of icy environments, their metabolic capabilities under frozen conditions are still largely unknown. We investigated basidiomycetous yeasts isolated from an Antarctic ice core and showed that after freezing at a relatively slow rate (0.8°C min⁻¹), the cells are excluded into veins of liquid at the triple junctions of ice crystals. These strains were capable of reproductive growth at −5°C under liquid conditions. Under frozen conditions, metabolic activity was assessed by measuring rates of [³H]leucine incorporation into the acid-insoluble macromolecular fraction, which decreased exponentially at temperatures between 15°C and −15°C and was inhibited by the protein synthesis inhibitor cycloheximide. Experiments at −5°C under frozen and liquid conditions revealed 2–3 orders of magnitude lower rates of endogenous metabolism in ice, likely due to the high salinity in the liquid fraction of the ice (equivalent of ≈ 1.4 mol l⁻¹ of NaCl at −5°C). The mesophile Saccharomyces cerevisae also incorporated [³H]leucine at −5°C and −15°C, indicating that this activity is not exclusive to cold-adapted microorganisms. The ability of yeast cells to incorporate amino acid substrates into macromolecules and remain metabolically active under these conditions has implications for understanding the survival of Eukarya in icy environments.