17-39: Inhibitor tolerance by high local cell density Saccharomyces cerevisiae cultures

Bioethanol production from lignocellulosic raw materials requires a microorganism able to ferment the toxic hydrolyzate created during pretreatment and hydrolysis. A way of increasing the inhibitor tolerance of less tolerant S. cerevisiae strains is to encapsulate the yeast in semi-permeable millimeter-sized capsules. We have investigated the tolerance of S. cerevisiae strains CBS8066 and CCUG53310 towards readily and less convertible inhibitors. Experimental results and numerical simulations of the diffusion - reaction system indicate that the high local cell density in the capsules causes the improved tolerance. This is partly due to consumption of the convertible inhibitors, since the encapsulation did not protect against all inhibitors. With gene expression and proteomic analyses we have shown that the cells are slightly stressed in the capsules already without inhibitors, due to carbon starvation caused by mass transfer limitations into the cell pellet in the core of the capsules. The triggered starvation stress response helps the cells to cope with inhibitory hydrolyzates, and even increases the thermo-tolerance of the cells. The variability in inhibitor tolerance is very large among different strains of S. cerevisiae. Flocculating strains have, in different studies, been shown to have high tolerance. Strongly flocculating yeast strains (eg. CCUG53310) can have flocs similar in size to the encapsulated yeast. We hypothesize that the high local cell density in the flocs gives advantages for the same reasons as for the encapsulated yeast. Thus, mass transfer limitations may actually lead to higher productivity in inhibitory lignocellulosic media.