Lignocellulosic substrates derived from waste biomass have become an
attractive feedstock for the production of inexpensive, more
environmentally-friendly biofuels.  For example, spent sulfite liquor
(SSL), a carbohydrate-rich effluent produced in sulfite pulping, can be
used to add value to the pulp and paper industry by using the sugars it
contains to produce ethanol.  However, using SSL in such a capacity
requires a robust, ethanologenic microorganism that can withstand the
substrate toxicity that is due to the presence of inhibitory compounds
like furfural, 5-hydroxymethylfurfural (HMF) and acetic acid.
Saccharomyces cerevisiae is currently used for the production of
ethanol from SSL.  This industrially well-established yeast, though a
robust starting organism for SSL fermentation, will still succumb to
toxicity and inhibition, especially in the most inhibitor rich forms of
SSL such as hardwood SSL (HWSSL).  To establish a S. cerevisiae
strain that can overcome such a complex and incompletely understood form
of inhibitory pressure, a genome shuffling method was developed to create
a better SSL fermenter.  This method aims to improve polygenic traits by
generating a pool of mutants with improved phenotypes, followed by
iterative recombination between their genomes.  Through five rounds of
shuffling and screening, three strains were obtained that are able to not
only survive in HWSSL, but grow to a limited extent.  Our results show
that the tolerance of these strains to SSL translates into an increased
capacity to produce ethanol over time using this substrate, due to
continued viability of the yeast population.