Quantitative physiology of non-energy-limited retentostat cultures ofSaccharomyces cerevisiaeat near-zero specific growth rates

So far, the physiology ofSaccharomyces cerevisiaeat near-zero growth rates has been studied in retentostat cultures with a growth-limiting supply of the carbon and energy source. Despite its relevance in nature and industry, the near-zero growth physiology ofS. cerevisiaeunder conditions where growth is limited by the supply of non-energy substrates remains largely unexplored. This study analyses the physiology ofS. cerevisiaein aerobic chemostat and retentostat cultures grown under either ammonium or phosphate limitation. To compensate for loss of extracellular nitrogen- or phosphorus-containing compounds, establishing near-zero growth rates (μ < 0.002 h-1) in these retentostats required addition of low concentrations of ammonium or phosphate to reservoir media. In chemostats as well as in retentostats, strongly reduced cellular contents of the growth-limiting element (nitrogen or phosphorus) and high accumulation levels of storage carbohydrates were observed. Even at near-zero growth rates, culture viability in non-energy-limited retentostats remained above 80 % and ATP synthesis was still sufficient to maintain an adequate energy status and keep cells in a metabolic active state. Compared to similar glucose-limited retentostat cultures, the nitrogen- and phosphate-limited cultures showed a partial uncoupling of catabolism and anabolism and aerobic fermentation. The possibility to achieve stable, near-zero growth cultures ofS. cerevisiaeunder nitrogen- or phosphorus-limitation offers interesting prospects for high-yield production of bio-based chemicals.ImportanceThe yeastSaccharomyces cerevisiaeis a commonly used microbial host for production of various bio-chemical compounds. From a physiological perspective, biosynthesis of these compounds competes with biomass formation in terms of carbon and/or energy equivalents. Fermentation processes functioning at extremely low or near-zero growth rates would prevent loss of feedstock to biomass production. EstablishingS. cerevisiaecultures in which growth is restricted by the limited supply of a non-energy substrate could therefore have a wide range of industrial applications, but remains largely unexplored. In this work we accomplished near-zero growth ofS. cerevisiaethrough limited supply of a non-energy nutrient, namely the nitrogen or phosphorus source and carried out a quantitative physiology study of the cells under these conditions. The possibility to achieve near-zero-growthS. cerevisiaecultures through limited supply of a non-energy nutrient may offer interesting prospects to develop novel fermentation processes for high-yield production of bio-based chemicals.