Compost as an untapped niche for thermotolerant yeasts capable of high‐temperature ethanol production

Abstract

Efficient bioethanol production from lignocellulosic biomass requires thermotolerant yeasts capable of utilizing multiple sugars, tolerating inhibitors and fermenting at high temperatures. In this study, 98 thermotolerant yeasts were isolated from nine compost samples. We selected 37 yeasts that belonged to 11 species; 31 grew at 45°C; 6 strains grew at 47°C, while 9 yeasts could utilize multiple sugars. Many yeast isolates showed high ethanol production in the range of 12–24 g l−1, with fermentation efficiencies of 47–94% at 40°C using 5% glucose. Kluyveromyces marxianus CSV3.1 and CSC4.1 (47°C), Pichia kudriavzevii CSUA9.3 (45°C) produced 21, 22 and 23 g l−1 of ethanol with efficiencies of 83, 87 and 90%, respectively, using 5% glucose. Among these yeasts, K. marxianus CSC4.1 and P. kudriavzevii CSUA9.3 exhibited high tolerance against furfural, 5‐HMF, acetic acid and ethanol. These two strains produced high amounts of ethanol from alkali‐treated RS, with 84 and 87% efficiency via separate hydrolysis and fermentation; 76 and 74% via simultaneous saccharification and fermentation at 47 and 45°C, respectively. Therefore, this study demonstrates compost as a potential anthropogenic niche for multiple sugar‐utilizing, inhibitor‐tolerant ethanologenic yeasts suitable for high‐temperature ethanol production via SHF of rice straw.

Dissecting an ancient stress resistance trait syndrome in the compost yeastKluyveromyces marxianus

“…And though K. marxianus can be isolated from rotting fruit 40,41 , it may not have specialized narrowly to such substrates: a neighboring clade that includes Saccharomyces cerevisiae produces more ethanol than do any of the Kluyveromyces species 6 , suggesting a weaker drive in the latter toward this putative mechanism to kill off microbial competitors in high-sugar environments. Perhaps most compelling among microbial ecology findings is the prevalence of K. marxianus in decomposing plant material beside fruit, including leaf litter and pine needles 42 , cow dung 19 and industrially processed cocoa beans 43 , agave 44 , and sugar cane 16,45 . If the ancestral niche for K. marxianus included plant substrates of this kind, the yeast could have specialized in rapid metabolism of free sugars, leaving the breakdown of lignocellulosic plant cell wall components to other fungal saprotrophs.…”

“…The ecological driving forces for unique traits in K. marxianus are unknown, though sampling studies have found this species at high prevalence in high-temperature agricultural and domestic compost [18][19][20] . We reasoned that some phenotypes unique to K. marxianus could have evolved as a product of adaptation to high-temperature or other niches, and that at relevant loci, we could detect evidence for positive selection from sequence data.…”