The metabolic response of Saccharomyces cerevisiae to continuous heat stress Femke I.C. Mensonides 1 , J. Merijn Schuurmans 1 , M. Joost Teixeira de Mattos 1 , Klaas J. Hellingwerf 1 , and Stanley Brul 1,2 1 Laboratory for Microbiology, Swammerdam Institute for Life Sciences, University of Amsterdam, Nieuwe Achtergracht 166, 1018 WV Amsterdam, The Netherlands. 2 Food Processing & New Technologies, Unilever Research, Olivier van Noortlaan 120, 3133 AT Vlaardingen, The Netherlands. Corresponding author: Femke I.C. Mensonides, Laboratory for Microbiology, Swammerdam Institute for Life Sciences, University of Amsterdam, Nieuwe Achtergracht 166, 1018 WV Amsterdam, tel: +31 20 525 7025/7068, fax: +31 20 525 7056, email: mensonides@science.uva.nl Summary A study has been initiated to integrate molecular and physiological responses of Saccharomyces cerevisiae to heat stress conditions. We focus our research on a quantification of the energetics of the stress response. A series of continuous heat stresses was applied to exponentially growing cells of the strain X2180-1A at 28°C, by increasing the growth temperature to 37, 39, 40, 41, 42, or 43°C. Here, the results on cell growth and viability, as well as on anabolic and catabolic rates are presented. We observed a surprisingly ‘thin line’ for the cells between growing, surviving, and dying, with regard to growth temperature. The heat stress showed a dual effect on catabolism: immediately after the temperature increase a strong peak was seen, after which a new, steady level was reached. In addition, the yield on glucose decreased with increasing temperature. Our results indicate that life at elevated temperatures is energetically unfavourable and a non-lethal heat stress invokes a redistribution of catabolic and anabolic fluxes. Key words: biomass yield, cell growth, cell viability, glucose flux, Saccharomyces cerevisiae, temperature stress Introduction In nature, microorganisms encounter a constantly changing environment. In order to survive, they have to be able to sense and properly respond to (potentially) threatening conditions like high temperature and nutrient depletion. These responses to stresses tend to be complex (see for instance Estruch, 2000; and Andreishcheva & Zvyagilskaya, 1999). We set out to quantify the response to heat stress on the molecular and physiological level. The data will be included in a mathematical model describing and predicting microbial adaptation and growth. Here, we will present the effects of a series of continuous heat stresses on cell growth, cell viability, and several metabolic fluxes. Methods Strain, media, and growth conditions The Saccharomyces cerevisiae strain X2180-1A (MATa SUC2 mal mel gal2 CUP1) was grown in fermentors at 28°C with a stirring rate of 700 rpm and an aeration rate of a fermentor volume of air per minute. The growth medium consisted of 0.67% YNB w/o amino acids (Difco) and 1% glucose in 100 mM potassium phthalate at pH 5.0.