Together we are strong! Inhibitor tolerance conferred by good neighbors?
Conference poster, 2010
Bioethanol from lignocellulosic materials is one of the desired alternatives to meet the increased demand of renewable fuels. However, there are challenges in several steps of lignocellulose processing, including pretreatment, hydrolysis and fermentation. Using flocculating strains in the fermentation process gives a number of advantages. For example, the cells can be accumulated in the bioreactors leading to high cell concentration and rapid fermentation. They are easily separated using sedimentation and can thus be recycled to the bioreactors. Some of these strains are also better than non-flocculating strains at tolerating the possible inhibitors in the cultivation media. These inhibitors make it hard for the yeast to ferment the hydrolyzate and detoxification is often necessary.
A flocculating yeast strain was isolated from a Swedish ethanol plant (Domsjö Fabriker AB) fermenting sulphite liquor, and registered at Culture Collection University of Gothenburg as CCUG 53310. It has been shown that this strain can successfully ferment lignocellulosic hydrolyzates, where the freely suspended reference strain, CBS 8066, failed to assimilate any sugar. However, upon encapsulation in Ca-alginate capsules, the strain CBS 8066 was able to successfully withstand the effect of the inhibitors and ferment lignocellulosic hydrolyzate.1
There are similarities between yeast cells living in large flocs and yeast living inside a capsule, such as a high local cell concentration. It is hypothesized that this greatly enhanced local biomass concentration strongly contributes to this increased tolerance, since more cells will be able to survive. That high biomass leads to a greater number of living cells have also been known for a long time.2 Further it is thought that the cells on the outside of the floc, as well as in the outer layer inside the capsule, will convert most of the inhibitors and partly die, protecting the inner lying cells.
The effect of different inhibitor classes, furan aldehydes, organic acids and phenolic compounds, present in the hydrolyzate on the flocculating strain as well as the freely suspended strain was also investigated, finding that the flocculating strain was indeed a lot better at withstanding the effect of furan aldehydes and carboxylic acids. Interestingly though, the CBS 8066 strain could tolerate the presence of phenolic compounds in the growth medium significantly better. This shows that higher biomass cannot help against all inhibitors, but there are also other underlying reasons, i.e. differences in the strains, yet to be investigated.