Direct ethanol production from cellulosic materials by consolidated biological processing using the wood rot fungus Schizophyllum commune

Highlights

• Bioethanol production was observed using a fermentable basidiomycete S. commune.

• S. commune converted multiple sugars including xylose to ethanol.

• Simultaneous saccharification and fermentation using S. commune was carried out.

• Ethanol production from wood chips was moderated by co-culture of S. commune with T. cucumeris.

Abstract

In the present study, ethanol production from polysaccharides or wood chips was conducted in a single reactor under anaerobic conditions using the white rot fungus Schizophyllum commune NBRC 4928, which produces enzymes that degrade lignin, cellulose and hemicellulose. The ethanol yields produced from glucose and xylose were 80.5%, and 52.5%, respectively. The absolute yields of ethanol per microcrystalline cellulose (MCC), xylan and arabinogalactan were 0.26 g/g_-MCC, 0.0419 g/g_-xylan and 0.0508 g/g_{-arabinogalactan}, respectively. By comparing the actual ethanol yields from polysaccharides with monosaccharide fermentation, it was shown that the rate of saccharification was slower than that in fermentation. S. commune NBRC 4928 is concluded to be suitable for CBP because it can produce ethanol from various types of sugar. From the autoclaved cedar chip, only little ethanol was produced by S. commune NBRC 4928 alone but ethanol production was enhanced by combined use of ethanol fermenting and lignin degrading fungi.

Introduction

Second-generation bioethanol is a renewable biofuel made from lignocellulosic biomass that does not compete with food and forage use (Wang et al., 2009). Lignocellulosic biomass includes crop residues and un-utilised forest resources and is mainly composed of cellulose, hemicellulose and lignin. Bioethanol production from cellulosic biomass requires pretreatment such as delignification and saccharification. Physicochemical pretreatment such as sulphite pretreatment (Zhu et al., 2009, Luo et al., 2010, Wang et al., 2009), diluted acid pretreatment (Mosier et al., 2005, Yan et al., 2009), and steam explosion (Horn and Eijsink, 2010, Sassner et al., 2005) can solubilise lignin and produce fermentable sugars from polysaccharides but can also form furfural, which is a strong fermentation inhibitor (Nilsson et al., 2005, Ewanick et al., 2007, Sassner et al., 2005). Enzymatic pretreatment is preferable for fermentation because inhibitors are not generated. There have been several reports on microbial pretreatment using lignin-degrading fungi (Bak et al., 2010, Holmgren and Sellstedt, 2008). Delignification enables access to cellulose and hemicellulose by saccharification enzymes (Ryu et al., 2013).

Similar to the case for saccharification, simultaneous saccharification and fermentation (SSF) processes, in which a two-step process is conducted in a single vessel, are being actively studied (Narra et al., 2015, Olofsson et al., 2008, Kádár et al., 2004). SSF is considered to be effective in reducing costs but a lower efficiency in raw material utilisation will increase costs during bioethanol production from lignocellulose. Saccharomyces cerevisiae is one of the most commonly used microbes in the brewing industry and for ethanol production but it cannot ferment xylose, which is the most abundant pentose in hemicellulose (Ho et al., 1998, Sedlak and Ho, 2004). Many genetically modified S. cerevisiae strains and ethanol fermenting bacteria have been produced to ferment xylose (Ho et al., 1998, Bertilsson et al., 2009). Moreover, the capability to produce fermentable sugars has been added to these fermenting microbes (Lynd et al., 2005, Wingren et al., 2003). Such multifunctional microbes can play multiple roles, such as saccharification of polysaccharides and fermentation of both glucose and xylose, so they are used in consolidated bio-processing (CBP) (de Almeida et al., 2013, Anasontzis and Christakopoulos, 2014, Huang et al., 2014).

Several wood rot basidiomycete, which include lignin degrading groups, have been reported to produce ethanol directly (Okamoto et al., 2010, Okamoto et al., 2014, Mizuno et al., 2009). In a previous report, we selected wood rot fungi that produced ethanol with high efficiency (Horisawa and Nishida, 2014). Schizophyllum commune NBRC4928 gave the highest yields amongst the strains we tested. White rot basidiomycetes, which can degrade lignin and polysaccharides, may possess multiple functionalities, such as delignification, saccharification, and ethanol fermentation. S. commune is a very common white-rot fungus that is distributed all over the world. In the present study, ethanol conversion efficiencies from various sugars during fermentation using S. commune NBRC 4928 were estimated. Direct fermentation tests on S. commune NBRC4928 were conducted using cellulose, hemicellulose and wood chips as raw materials.