Page 101 - Lignocellulosic Biomass to Liquid Biofuels

Bioconversion of lignocellulosic biomass to bioethanol and biobutanol 77

(FA) [74], to stress [75] as well as to increase multitolerance to high tem-
perature, acidity and high ethanol production [76].

3.2.1.3 Direct microbial conversion
A method to convert cellulosic biomass to ethanol is the direct microbial
conversion (DMC), in which both ethanol and all required enzymes are
produced by a single microorganism [23,77,78]. Several strains of Fusarium
oxysporum can convert D-xylose and cellulose to ethanol in a one-step
process, but there is no robust organism available that can produce cellu-
lases or other cell wall degrading enzymes in conjunction with ethanol
with a high yield [22].
The development of simultaneous saccharification and fermentation
(SSF) of biomass to ethanol by native or genetically modified microbial
strains has been studied intensively [79 83]. This approach combined the
cellulase enzymes and fermenting microbes in one vessel.
A single-step process was proposed for converting lignocellulose to
ethanol using mesophilically native isolated strains. The strain Bacillus sp.
THLA0409 was identified as a dominant cellulose-degrading bacterium,
while the strain K. oxytoca THLC0409 was determined as a dominant
sugars-utilizing bacterium. The coculture of these two strains remarkably
enhanced the utilization efficiency of hydrolyzates from acid-pretreated
raw bamboo, Napiergrass, rice straw, and ethanol production [84].
During the last decade, several wild-type and genetically engineered
bacteria, fungi, and yeasts have been proposed for application in a con-
solidated bioprocessing (CBP) system, where all the processes, including
enzyme production, enzymatic saccharification, and fermentation of
the resulting sugars to bioethanol or other valuable products, proceed
simultaneously [25,72]. The potential microorganisms and their
suitable characteristics for CBP alcohol production were comprehen-
sively reviewed and discussed [85]. These microorganisms have com-
bined saccharolytic and ethanologenic capabilities: they naturally
degrade cellulose and ferment the resulting sugars into ethanol, lactate,
acetate, carbon dioxide, and hydrogen. In particular the following strain
groups were investigated:
• Bacteria (Clostridium thermocellum, Clostridium phytofermentans,
Thermoanaerobacterium sp.)
• Fungi (Mucor circinelloides, F. oxysporum, Fusarium verticillioides,
Acremonium zeae, A. oryzae, Paecilomyces variotii)