Introduction
Climate change is a threat to the several life forms of our planet. This phenomenon is partially due to the preferential use of fossil fuels to sustain human activities such as heating, agriculture or transportation. The use of biofuels is one of the most promising alternatives, as it would help reduce the effects of climate change, while avoiding the use of non-renewable resources such as petroleum or gas as sources of energy (Hill, Nelson, Tilman, Polasky, & Douglas, 2006). Among the most common biofuels are alcohols that can be obtained from microbial fermentation using different kinds of carbon sources. Ethanol is the best known and most studied alcohol that can be obtained by microbial fermentation using yeast or bacteria, while butanol is a less common but increasingly attractive biofuel due to several characteristics, since it is capable of producing more energy if properly harnessed, and has a lower vapor pressure and is less hygroscopic than ethanol (Dürre, 2007). Many different processes have been developed to obtain alcohols from sugars or starch, but the use of these substrates to produce biofuels would compete with food supplies. To avoid this problem, biofuels should be obtained from non-food substrates, such as lignocellulosic biomass. Several different approaches have been employed to use this abundant substrate, most of which start with the hydrolysis of the biomass to obtain sugars that can be subsequently fermented to the desired compounds. This hydrolysis is normally achieved through energy intensive processes, involving high temperatures and aggressive chemical conditions. Consolidated bioprocessing is an alternative that integrates enzyme production, saccharification and fermentation (Yamada, Hasunuma, & Kondo, 2013) that can be achieved in mild conditions. Consolidated bioprocesses that use anaerobic thermophilic organisms capable of degrading lignocellulosic biomass are expected to meet sustainability standards, as they would enable production of biofuels from renewable resources by means of low energy demanding procedures (Lynd, Weimer, Zyl, & Pretorius, 2002).
The use of cellulolytic thermophilic bacteria, such as C. thermocellum , has been extensively studied for the production of ethanol from cellulose (Lamed & Zeikus, 1980; Tian et al., 2016). However, most of these organisms are unable to produce butanol, and/or to ferment pentoses derived from hemicellulose degradation, thereby limiting the efficient use of lignocellulosic biomass (Demain, Newcomb, & Wu, 2005). In recent years, several species of the thermophilic genusThermoanaerobacterium have received increased attention due to their capability to use different biomass substrates to produce solvents. The genus Thermoanaerobacterium comprises 8 validly described species: T . aciditolerans, T. aotearoense, T. saccharolyticum, T. thermosaccharolyticum, T. thermostercoris (formerlyT. thermostercus ), T. thermosulfurigenes, T. xylanolyticumand T. butyriciformans (Onyenwoke & Wiegel, 2015). T. saccharolyticum and T. thermostercoris can produce ethanol (Romano et al., 2010; Shaw et al., 2008) and several T. thermosaccharolyticum isolates have been reported to produce hydrogen (Cao et al., 2009) and/or butanol (Li, Zhang, Yang, & He, 2018) from different biomass sources. These results point out the potential of these microorganisms for the synthesis of diverse bioproducts, and particularly biofuels, from untreated or minimally treated biomass, and spur further research in order to analyze their potential application in consolidated bioprocesses.
We have isolated and characterized a new strain, T. thermosaccharolyticum GSU5, an anaerobic thermophilic bacterium that is capable of producing ethanol and butanol from a variety of substrates. In this work we present the genomic sequence of GSU5 and analyze its phenotypic traits, especially those pertaining to solvent production, in comparison to the type strain of the species. Additionally, genes related to solventogenesis of all sequenced Thermoanaerobacteriumare compared to gain new insights into their unique metabolic properties.