Apparently, the impact of stress due to the sulfate was more severe on ethanol production by the parent P. stipitis. In addition, it is clear that sulfate does not seem to be involved in physiological metabolism of P. stipitis because no obvious change in sulfate concentration occurred during fermentation. Importantly, the present study demonstrates that the influence of sulfate concentration on P. stipitis was significant and cannot be neglected. At the optimum pH range of 4 to 7 for P. stipitis growth and fermentation , sulfuric acid is mostly present in a dissociated form. Thus, the inhibitory effect of sulfate is expected to be different from that of acetic acid. It is suggested inhibition by sulfate may be due to hyperosmotic and ionic stresses that are imposed as the sulfate concentration increases. When yeast cells encounter these types of stresses, the surviving cells may tend to activate specific stress response proteins in order to minimize the consequences of the stress-associated damage. Therefore, it is believed that the tolerance to sulfate stress in this study has resulted from specific changes to various molecular pathways in the adapted P. stipitis; these changes may have given rise to a faster activation of the appropriate stress responses