Biofuels: Ethanol Producers

Abstract

Two groups of microbes, saccharolytic and ethanologenic, are important in ethanol production. The former makes sugar utilisation possible and the latter converts the sugar to ethanol. Many promising or potential ethanologenic and saccharolytic microbes fall within the respective phyla Ascomycota and Neocallimastigomycota for fungi, and Proteobacteria and Fibrobacteres for bacteria. Saccharomyces sp. (Ascomycota) and Zymomonas mobilis (Proteobacteria) are the only microbes naturally capable of producing ethanol close to theoretical maximum, with Saccharomyces cerevisiae predominant for current ethanol production based on starch and sugar feedstocks. To enable cellulosic ethanol technologies, microbial capability and efficiency must be enhanced by appropriately designed mixed‐culture systems and/or genetically modified microbes. As cellulosic ethanol nears commercialisation, these two approaches are key to sustainable and economical production.

Key Concepts:

  • Ethanol producers are known as ethanologens.

  • Currently, commercial production of bioethanol depends on yeast, especially species of Saccharomyces cerevisiae, for fermentation of hexose (6‐carbon) sugars.

  • Zymomonas mobilis is the only bacteria naturally capable of producing ethanol as the sole product and the only ethanologen that uses a nonglycolytic metabolic pathway.

  • Saccharolytic or hydrolytic microbes are an important complement to ethanologens in ethanol production.

  • Production of ethanol from carbohydrate oligomers (2–10 monosaccharide units) or polymers (>10 monosaccharide units) usually requires hydrolysis to simple hexose or pentose sugars.

  • Of the ten major phylogenetic phyla of fungi, only two are currently identified as relevant to ethanol production.

  • Thermophilic microorganisms and species of the genus clostridium are expected to play greater roles in future ethanol production.

  • Microbial consortia, comprising different saccharolytic and diverse sugar fermenting microbes, provide one promising strategy for cellulosic ethanol production.

  • Other cellulosic ethanol strategies are genetically modifying microbes capable of producing hydrolytic enzymes as well as co‐fermenting 5‐carbon and 6‐carbon sugars.

Keywords: bioethanol; fermentation; ethanologens; yeast; saccharolytic microbe; recombinant microbes; metabolic pathways; microbial consortium

Figure 1.

Ethanol production from fermentation and synthetic routes. CBP, consolidated bioprocessing; SSCF, simultaneous saccharification and co‐fermentation; SSF, simultaneous saccharification and fermentation and SHF, separate hydrolysis and fermentation. Highlighted boxes are processes where microbial catalysis is applicable.

Figure 2.

Metabolic pathways used in ethanol production, including one variant of the Wood–Ljungdahl pathway.

Figure 3.

Phylogenetic classification showing microbes of current ethanologenic relevance. Purple=ethanol as main product. Brown=more recognised for its saccharolytic activities. Others: refers to other groups in the phylum, subphylum, class or clade and are not of relevance to ethanol production.

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Darku, Irene D, and Richard, Thomas L(Dec 2011) Biofuels: Ethanol Producers. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0020373]