Page 103 - Lignocellulosic Biomass to Liquid Biofuels

Bioconversion of lignocellulosic biomass to bioethanol and biobutanol 79

[92,94,107 117]. Moreover, some strains of C. beijerinckii and Clostridium
aurantibutyricum can synthesize isopropanol instead of acetone and ethanol,
respectively, whereas Clostridium tetanomorphum synthesizes butanol and
ethanol and does not synthesize any other solvents [118], as well as
Clostridium sporogenes [119]. Some researchers investigated cocultures of C.
thermocellum and C. saccharoperbutylacetonicum with the addition of cellulase
[120], and of C. acetobutylicum, Clostridium butylicum, and C. beijerinckii
with microorganisms having enzymes capable of simultaneously hydrolyz-
ing cellulose and hemicellulose [121 125]. Another solution is the devel-
opment of genetically engineered strains with activated endogenous
cellulose enzyme [126].
Efficient genetic tools are crucial for the metabolic engineering of
Clostridia, in order to enhance solvent production, to improve butanol tol-
erance, to increase the ratio of butanol to the solvent, and to allow the
strain to grow on complex cellulosic substrates [127].
Since butanol is toxic for bacteria, it is essential to find bacterial strains
tolerating higher butanol concentrations, such as the use of an “enrichment
culture,” with the medium containing the compound of interest. A signifi-
cant number of studies has been directed toward microbial production of
butanol by different metabolically engineered mesophilic and thermophilic
organisms [128,129]. With respect to mesophilic microorganisms, butanol
was produced by engineered strains of cyanobacterium, Synechococcus elonga-
tus, [130] and bacteria, Clostridium tyrobutyricum [131,132], C. beijerinckii,
and C. acetobutylicum, in order to enhance butanol tolerance and yield
[95,118,133 144]. A number of thermophilic organisms have been engi-
neered for butanol production: Thermoanaerobacterium thermosaccharolyticum
[145], T. saccharolyticum [146], G. thermoglucosidasius [147],and Pyrococcus fur-
iosus [148]. Moreover, other butanol-producing microorganisms have been
engineered, with the aim to increase tolerance to ferulic acid and “acid
crash” (C. beijerinckii) [149,150], to increase tolerance to lignocellulose-
derived inhibitors (Clostridium sp. strain BOH 3 ) [151], and to facilitate
xylose transport and metabolism (C. tyrobutyricum) [152].
In any case, selection of bacterial strain in ABE process depends on the
raw material. For example, C. acetobutylicum is more suitable for starch-
based medium, whereas C. saccharobuylicum is for molasses-based medium
[92,126].
The classical ABE fermentation by Clostridium strains takes place under
anaerobic conditions. Nevertheless, butanol can be synthesized under aer-
obic conditions, by introducing the corresponding genes in Clostridia