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Mitigating global climate change and CO 2 emissions has pushed for the continuous search of right types of biomass for the production of cellulosic fuels and chemicals (Albers et al., 2016; Lynd, 2017). This biomass selection has used crop wastes (e.g., corn stover and rice straw) and wood residues to avoid competition with foodstuffs. For an effective CO 2 removal with minimum land use change, energy-dedicated crops such as switchgrass and miscanthus are considered (Schmer et al., 2008; Searchinger et al., 2018). Meanwhile, the chemical composition of hydrolysates (i.e., partially degraded biomass) greatly diverges depending on the type of biomass (Mahdieh

Lactic acid is mainly used to produce bio-based, bio-degradable polylactic acid. For industrial production of lactic acid, engineered Saccharomyces cerevisiae can be used. To avoid cellular toxicity caused by lactic acid accumulation, pH-neutralizing agents are used, leading to increased production costs. In this study, lactic acid-producing S. cerevisiae BK01 was developed with improved lactic acid tolerance through adaptive laboratory evolution (ALE) on 8% lactic acid. The genetic basis of BK01 could not be determined, suggesting complex mechanisms associated with lactic acid tolerance. However, BK01 had distinctive metabolomic traits clearly separated from the parental strain, and lactic acid production was improved by 17% (from 102 g/L to 119 g/L). To the best of our knowledge, this is the highest lactic acid titer produced by engineered S. cerevisiae without the use of pH neutralizers. Moreover, cellulosic lactic acid production by BK01 was demonstrated using acetate-rich buckwheat husk hydrolysates. Particularly, BK01 revealed improved tolerance against acetic acid of the hydrolysates, a major fermentation inhibitor of lignocellulosic biomass. In short, ALE with a high concentration of lactic acid improved lactic acid production as well as acetic acid tolerance of BK01, suggesting a potential for economically viable cellulosic lactic acid production.

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