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| Cover of Science Issue 335. |
It has been a while since we have done
a journal club, and so today I thought I would write about an article
I recently read. Adam J. Wargacki, who works in Yasuo Yoshikuni's
group, recently published the above titled paper.
In light of problems associated with
the use of fossil energy sources (cost, scarcity, environmental
impact, and geopolitical considerations), the search for new energy
sources is starting to become more important. The paper in discussion
today proposes that a coupled system consisting of brown macroalgae
and engineered bacteria could be used.
Of course there are more traditional
biofuel feedstock sources such as corn and sugarcane. There a couple
of hurdles such as the debate about “food versus fuel” and
technical hurdles such as the degradation of lignocellulosic matter
that need to be solved. We have previously discussed how microalgaecould get around the difficulties of corn and sugarcane in the production of bioefuels. At the
current time, genetically manipulating these algae is technically
difficult. So Bioarchitecture Labs is proposing the use of brown macroalgae (ak seaweed) as an industrial feedstock.
Why?
The group claims that macroalgae have a
couple of advantages. For one, these macroalgae are grown in sea
water which does not compete with rare sweet water resources. This
also means that macroalgae are not cultivated in agricultural lands
used for food production leaving the “food vs fuel” debate out.
Lastly, brown macroalgae do not contain lignin which means that
sugars required for the production of biofuels can easily be obtained
by crushing the harvested biomass.
What did Bio Architecture Labs
actually do?
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| Logo of Bio Architecture Labs |
Of course there are challenges trying
to use brown algae as a biofuel feedstock. The challenges consists of
the fact that brown algae are rich in other sugars (mainly alginate,
mannitol and glucan). The degradation of manitol and glucan by
bacteria is associated with metabolic imbalances generating excess
reducing equivalents. The group realized that the challenges
associated with manitol break-down could be counteracted by also
breaking down alginate which consumes two reducing equivalents. The
problem: Most bacteria including E. coli which is a well-known model
organism strain that has traditionally been used for other
biotechnological applications do not have pathways to degrade
alginate. And thus, most of the paper describes how Bio Architecture
Labs was able to still use seaweed as a feedstrock.
The group first showed that E. coli
transformed with plasmid containing an engineered alginate lyase was
able to secrete and degrade alginate effectively. Next, the group
successfully identified and incorporated a functional heterologous
alginate transport and metabolic system into E. coli. The paper
describes this step as the most challenging step because the only
alginol transport system known until then consisted out of a
complicated outer-membrane protein complex and inner-membrane ABC
transporter system which was never successfully shown to be
incorporated into E. coli. Instead, the group ended up trying to find
a way around the challenge of incorporating that system by relying on
homology searches of simpler transporter and symporter systems that
have previously been successfully incorporated into E. coli. The
group found a hypothetical match in a 30 kb fragment in the genome of
Vibro spledidus. Bio Architecture Labs then created a fosmid
(similar to cosmids but more stable) library and screened for
fragments that would allow E. coli cells to grow on medium
containing alginate oligomers. The research group identified three
genes this way that were previously also found using the homology
search strategy. To prove the necessity and functionality of these
genes in alginate import and metabolism, each gene was individually
knocked out and tested. Lastly, the group looked for further genes
that could help in the import and metabolism of alginate by looking
for other genes surrounding the initial 30 kb fragment and identified
a couple of more candidates which the group further characterized.
The result: After several rounds of
optimization on synthetic medium, E. coli containing the full
alginate metabolism pathway were able to produce ethanol directly
from Saccharina japonica at a final titer of ~ 4.7% which represents
an approximate yield of 80% of maximum theoretical yield. This
represents terrific efficiencies in the pathway.
So what's the significance?
Erik Stokstad noted in a commentary
appearing in the same issue of Science that E. coli could be modified
to produce other fuels and other valuable chemicals from the same
feedstock source. As to major hurdles and sustainability, the
commentary points out that to just replace 1 % of U.S. Gasoline supply
would require growing seaweed in an area of over 11, 000 square
kilometers. To put this into perspective, the US does not currently
do any significant amount of seaweed farming, and due to the large
area requirement the technology would obviously only be feasible in
coastal regions. Still, in the search for alternative energy sources, there will be no silver bullet. Seaweed-derived biofuels can be one of many important sources. Or cautionary note, according to the commentary, a full-life cycle
analysis is currently being performed right now so there is no final answer yet when it comes to sustainability aspects. My guess is that seaweed-derived biofuels can be sustainable if a number of stringent criteria are fulfilled. Of course, for the reader of
this blog, the question of coupling several processes such as
bioremediation with production of renewable chemicals is always an
interesting aspect to further explore.
Source:
2. Erik Stoktad “Engineered Superbugs
Boost Hopes Of Turning Seaweed Into Fuel”. Science 335, 273 (2012).
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