Biology Reference
In-Depth Information
FUTURE PROSPECTS
This chapter has discussed methods to improve yields of biofuel production pathways.
There are many aspects of industrial biofuel production that we lack the space to discuss
further. As discussed in the introduction, constraints on economic margins require that the
yields of fuel from biomass must be extremely close to the theoretical yield. While
impressive yields quite close to theoretical are beginning to appear in the literature, results
obtained in laboratory shake flasks, or even small fermentation vessels, rarely scale
predictably to large-scale fermentation processes. Researchers often tailor their engineered
microbes for laboratory conditions rather than realistic industrial situations. For instance,
production on industrial scales will likely occur in anaerobic environments, due to the
difficulty in aerating thousand-liter or larger vessels. Also, production results are often
(though fortunately not always) reported using rich media, which are far too expensive to
use at industrial scales. A biofuel pathway or a host organism optimized to produce
biofuel in rich, oxygenated medium may not achieve high yields in minimal medium, in
an anaerobic environment. While proof-of-concept results are certainly useful to
demonstrate that production in a new host is indeed possible, redesigning a pathway to
work at industrial scales often requires efforts that match or exceed those required to
achieve the demonstration level. Furthermore, other advantages of laboratory-scale
production that are taken for granted, such as lack of contaminants and absence of phage,
will probably not apply to industrial scales. These are issues that can by addressed
through synthetic biology.
Other complications of biofuel production need to be considered. The sugar stream that the
biofuel production host will be converting into biofuels will most likely be the hydrolysate
of a high-yielding lignocellulosic feedstock such as switchgrass. Most biofuel produced at
the laboratory scale is done with glucose or glycerol; in contrast, plants are composed of
five- and six-carbon sugars. Cells growing on purified glucose may have different metabolite
pools than cells growing on such complex carbon mixtures, because these sugars enter
central carbon metabolism through different routes. It may be necessary to refine the
biofuel synthesis pathway to accommodate these metabolic perturbations. In addition, it
will probably be necessary to modify the host organism so all sugars are consumed
simultaneously rather than sequentially. This is necessary because it leads to more efficient
biofuel production and more complete sugar conversion. The host organism can also be
engineered to use actual plant biomass, rather than biomass hydrolysate, in order to further
reduce processing costs.
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Given these large obstacles, production of economically viable
biofuel may seem incredibly daunting. We anticipate that with applications of both existing
and as-yet undiscovered synthetic biology tools, advanced biofuels will become an integral
part of the energy economy.
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