Abstract
Formic acid is one of the major inhibitory compounds present in hydrolysates derived from lignocellulosic materials, the presence of which can significantly hamper the efficiency of converting available sugars into bioethanol. This study investigated the potential for screening formic acid tolerance in non-Saccharomyces cerevisiae yeast strains, which could be used for the development of advanced generation bioethanol processes. Spot plate and phenotypic microarray methods were used to screen the formic acid tolerance of 7 non-Saccharomyces cerevisiae yeasts. S. kudriavzeii IFO1802 and S. arboricolus 2.3319 displayed a higher formic acid tolerance when compared to other strains in the study. Strain S. arboricolus 2.3319 was selected for further investigation due to its genetic variability among the Saccharomyces species as related to Saccharomyces cerevisiae and availability of two sibling strains: S. arboricolus 2.3317 and 2.3318 in the lab. The tolerance of S. arboricolus strains (2.3317, 2.3318 and 2.3319) to formic acid was further investigated by lab-scale fermentation analysis, and compared with S. cerevisiae NCYC2592. S. arboricolus 2.3319 demonstrated improved formic acid tolerance and a similar bioethanol synthesis capacity to S. cerevisiae NCYC2592, while S. arboricolus 2.3317 and 2.3318 exhibited an overall inferior performance. Metabolite analysis indicated that S. arboricolus strain 2.3319 accumulated comparatively high concentrations of glycerol and glycogen, which may have contributed to its ability to tolerate high levels of formic acid.
Highlights
The importance of identifying alternative energy sources has become necessary due to the continuous depletion of limited fossil fuel stock and for the creation of a safe and sustainable environment
The presence of 35 mM formic acid prevented cell growth on most strains (Fig 1A), growth was observed for strains S. paradoxus DBVPG6466, S. kudriavzeii IFO1802, S. arboricolus 2.3319 and S. cerevisiae NCYC2592
Strains S. paradoxus DBVPG6466, S. kudriavzeii IFO1802, S. arboricolus 2.3319 and S. cerevisiae NCYC2592 were only tolerant to 20 mM formic acid on Yeast Nitrogen Base (YNB) medium plates (Fig 1B, Table 2), suggesting that YPD as an enriched medium may have a higher buffering capacity than YNB, a minimal medium
Summary
The importance of identifying alternative energy sources has become necessary due to the continuous depletion of limited fossil fuel stock and for the creation of a safe and sustainable environment. Attention has focused on renewable or alternative sources of energy, as a PLOS ONE | DOI:10.1371/journal.pone.0135626. Screening of Yeasts for Tolerance to Formic Acid respectively, but did not have any additional role in the study design, data collection and analysis, decision to publish, or preparation of the manuscript. KAS and TGP moved to SABMiller and PepsiCo, during this project, after which they were not involved in any further research. The specific roles of these authors are articulated in the ‘author contributions’ section
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