Obtaining higher yields of butanol involves manipulation of the metabolic networks using
metabolic engineering and
genetic engineering. While significant progress has been made,
fermentation pathways for producing butanol remain inefficient. Titer and yields are low and separation is very expensive. As such, microbial production of butanol is not cost-competitive relative to petroleum-derived butanol. Although unproven commercially, combining electrochemical and microbial production methods may offer a way to produce butanol from
sustainable sources.
Escherichia coli Escherichia coli, or
E. coli, is a
Gram-negative,
rod-shaped bacterium.
E. coli is the microorganism most likely to move on to commercial production of isobutanol.
Clostridia n-Butanol can be produced by
fermentation of biomass by the
A.B.E. process using
Clostridium acetobutylicum,
Clostridium beijerinckii.
C. acetobutylicum was once used for the production of
acetone from
starch. The butanol was a by-product of fermentation (twice as much butanol was produced). The feedstocks for biobutanol are the same as those for ethanol:
energy crops such as
sugar beets,
sugar cane,
corn grain,
wheat and
cassava, prospective non-food energy crops such as
switchgrass and even
guayule in North America, as well as
agricultural byproducts such as
bagasse,
straw and
corn stalks. According to
DuPont, existing bioethanol plants can cost-effectively be retrofitted to biobutanol production. Additionally, butanol production from
biomass and agricultural byproducts could be more efficient (i.e. unit engine motive power delivered per unit solar energy consumed) than
ethanol or
methanol production. A strain of
Clostridium can convert nearly any form of
cellulose into butanol even in the presence of oxygen. A strain of
Clostridium cellulolyticum, a native cellulose-degrading microbe, affords isobutanol directly from cellulose. A combination of
succinate and ethanol can be fermented to produce
butyrate (a precursor to butanol fuel) by utilizing the metabolic pathways present in
Clostridium kluyveri. Succinate is an intermediate of the
TCA cycle, which metabolizes glucose.
Anaerobic bacteria such as
Clostridium acetobutylicum and
Clostridium saccharobutylicum also contain these pathways. Succinate is first activated and then reduced by a two-step reaction to give
4-hydroxybutyrate, which is then metabolized further to
crotonyl-coenzyme A (CoA). Crotonyl-CoA is then converted to butyrate. The genes corresponding to these butanol production pathways from
Clostridium were cloned to
E. coli.
Cyanobacteria Cyanobacteria are a
phylum of
photosynthetic bacteria. They are suited for isobutanol biosynthesis when genetically engineered to produce isobutanol and its corresponding
aldehydes. Isobutanol-producing species of cyanobacteria offer several advantages as biofuel synthesizers: • Cyanobacteria grow faster than plants and also absorb sunlight more efficiently than plants. This means they can be replenished at a faster rate than the plant matter used for other biofuel biosynthesizers. • Cyanobacteria can be grown on non-arable land (land not used for farming). • Cyanobacteria
bioreactors require high energy to operate. Cultures require constant mixing, and the harvesting of biosynthetic products is energy-intensive. This reduces the efficiency of isobutanol production via cyanobacteria.
Bacillus subtilis Bacillus subtilis is a
gram-positive rod-shaped bacteria.
Bacillus subtilis offers many of the same advantages and disadvantages of
E. coli, but it is less prominently used and does not produce isobutanol in quantities as large as
E. coli.
S. cerevisiae is an ideal candidate for isobutanol biofuel production for several reasons: •
S. cerevisiae can be grown at low
pH levels, helping prevent contamination during growth in industrial bioreactors.
S. cerevisiae, however, has proved difficult to work with because of its inherent biology: • As a eukaryote,
S. cerevisiae is genetically more complex than
E. coli or
B. subtilis, and is harder to genetically manipulate as a result. •
Anodes are placed in a mixture of H2O and CO2. • An electric current is run through the anodes, and through an
electrochemical process H2O and CO2 are combined to synthesize
formic acid. • A
culture of
C. necator (composed of a
strain tolerant to electricity) is kept within the H2O and CO2 mixture. • The culture of
C. necator then converts formic acid from the mixture into isobutanol. • The biosynthesized isobutanol is then separated from the mixture, and can be used as a biofuel. ==Feedstocks==