Genetic and physiological alterations occurring in a yeast population continuously propagated at increasing temperatures with cell recycling

This work investigated the effects of increasing temperature from 30°C to 47°C on the physiological and genetic characteristics of Saccharomyces cerevisiae strain 63M after continuous fermentation with cell recycling in a system of five reactors in series. Steady state was attained at 30°C, and then the temperature of the system was raised so it ranged from 35°C in the last reactor to 43°C in the first reactor or feeding reactor with a 2°C difference between reactors. After 15 days at steady state, the temperature was raised from 37°C to 45°C for 25 days at steady state, then from 39°C to 47°C for 20 days at steady state. Starter strain 63M was a hybrid strain constructed to have a MAT a/α, LYS/lys, URA/ura genotype. This hybrid yeast showed vigorous growth on plates at 40°C, weak growth at 41°C, positive assimilation of melibiose, positive fermentation of galactose, raffinose and sucrose. Of 156 isolates obtained from this system at the end of the fermentation process, only 17.3% showed the same characteristics as starter strain 63M. Alterations in mating type reaction and in utilization of raffinose, melibiose, and sucrose were identified. Only 1.9% of the isolates lost the ability to grow at 40°C. Isolates showing requirements for lysine and uracil were also obtained. In addition, cell survival was observed at 39–47°C, but no isolates showing growth above 41°C were obtained.