8-11: High local cell density for efficient 2nd generation bioethanol production by Saccharomyces cerevisiae

Successful implementation of second generation bioethanol production requires a microorganism able to ferment both the hexoses and pentoses in the toxic hydrolysate created during pretreatment and hydrolysis of lignocellulosic materials.  We have shown that the tolerance of Saccharomyces cerevisiae to compounds present in lignocellulosic hydrolysates, inhibiting its metabolism, can be increased by cell encapsulation. Numerical simulations have also shown that the xylose utilization is increased upon encapsulation of a xylose utilizing strain. Both of these benefits are due to the high local cell density of the yeast, where a triggered stress response due to starvation helps the cells to cope with further stress, as seen by gene expression analysis and proteome analysis. Xylose uptake would be promoted in parts of the capsule where the glucose concentration is low.

We have further characterized a constitutively flocculating strain, which was shown to tolerate inhibitors in lignocellulosic hydrolysates too toxic for the reference strain. Since flocculation is nature's way of keeping the yeast cells close together, similar to encapsulation, it is tempting to hypothesize that this gives the cell community similar benefits, a notion that we are currently investigating. If our hypothesis proves correct, high local cell density yeast fermentations can be a solution to an efficient 2^nd generation bioethanol production process, overcoming problems with inhibitor tolerance as well as xylose fermentation.