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.