Inhibitor tolerance and flocculation: Characterization of a yeast strain suitable for 2nd generation bioethanol production

Conference poster, 2011

Robust second generation bioethanol processes require microorganisms able to obtain high yields and production rates while fermenting inhibiting hydrolysates. However, tolerance towards inhibitors like, carboxylic acids, furan aldehydes and phenolic compounds, is still an issue and the factors contributing to improved tolerance are not well known. In this study, the constitutively flocculating Saccharomyces cerevisiae strain CCUG 53310, with good ability to ferment toxic hydrolysates, was compared with S. cerevisiae CBS 8066 in order to characterize the mechanisms of flocculation and the fermentative performance in different inhibitory media. The flocculation of CCUG 53310 depended on cell wall proteins and was partly inhibited by mannose. The flocculating cells also exhibited a significantly higher hydrophobicity than the cells of the non-flocculating strain CBS 8066, which might contribute to the flocculation. The flocculating strain was more tolerant to carboxylic acids and furan aldehydes, but more sensitive to phenolic compounds. Surprisingly, the expression increase of YAP1, ATR1 and FLR1, known to confer resistance against lignocellulose-derived inhibitors, upon addition of various inhibitors to the fermentation medium, was less in CCUG 53310 than in CBS 8066 in most cases. This indicates that the flocculating strain experienced the cultivation conditions as less stressful. The flocculation in itself is a likely cause of this by creating subinhibitory local levels of inhibitors for most cells, allowing the cells in flocs to experience a lower collective stress level.