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26

Lab Times

5-2016

Analysis

Exploiting food crops for energy production has turned out to be a massive

failure. But researchers are currently working on the second and third gen-

eration of biofuels, using transgenic trees or algae as their resources.

Renewable energy

Fotolia/angelo19

From Biomass to Biofuel

T

here’s no doubt about our need to

move from a doomed fossil-based to

a sustainable bio-based economy. To

be at the forefront of this move to reduce

greenhouse gas emissions and alleviate cli-

mate change, biofuels need to become an

important renewable energy produced and

consumed around the world.

Yet our first attempt at producing bio-

fuels has faced many criticisms. From a

technical point of view, there was nothing

wrong with the approach: extraction of en-

ergy-containing molecules, like sugars and

cellulose, from crop plants followed by fer-

mentation to produce bioethanol. Soon,

however, many researchers were voicing

their concerns about the competition be-

tween the use of land for food and fuel, and

research into these first generation biofuels

has largely fallen out of favour in many Eu-

ropean countries.

Bioethanol under fire

With its sugarcane bioethanol, Bra-

zil has shown that it is possible to meet re-

newable fuel targets but the system is un-

der fire by many, criticising its environ-

mental impact and low efficiency. In fact,

the European Academies Science Adviso-

ry Council (EASAC) has recently declared

that “current first-generation biofuels ap-

pear to provide little or no greenhouse gas

reduction once all impacts of biomass cul-

tivation and fuel production are taken into

account”. The same EASAC report also calls

for a combined effort from the scientific

community to accelerate the generation of

advanced biofuels sourced from alternative

biomasses, as well as improving the perfor-

mance of conversion technologies (EASAC

Sustainable Biofuels, 2012).

After failed first generation biofuels, the

obvious progression was tomove away from

any food sources, and choose either ined-

ible parts of the plant or crops grown on

marginal and poor land specifically to pro-

duce biofuels. Unfortunately for research-

ers, this solved one problem but created an-

other: lignin. This had never been an issue

with first generation biofuels, as the system

relied on easily accessible sweet glucose in

crops like sugarcane and maize. However,

the move towards alternative crops, like

poplar and willow, meant high levels of

lignin had to be removed before cellulose

could be processed.

Trees with less lignin

Engineering plants to produce less

lignin has been on Wout Boerjan’s agenda

for a long time. Curiously, the team, based

at the VIB Institute in Belgium, started ge-

netically engineering poplar trees over 20

years ago for a completely different pur-

pose. “Initially, we wanted to reduce the

amount of lignin in trees because lignin

is a problem for paper making,” says the

researcher but “the very same problem is

there if you want to produce biofuels from

wood.”

Their original work targeted cinnamoyl-

CoA reductase (CCR), the enzyme involved

in one of the first steps in the lignin bio-

synthetic pathway. Re-evaluating their

trees for the purpose of biofuel produc-

tion, they found that extracting glucose was

much easier with low CCR levels (

PNAS,

111:845-50). “Reducing CCR activity re-

duces the production of lignin and we can

make plants which have less lignin, and as

such, cellulose is more easily extractable,”

says Boerjan.

One more problem

At this stage, the results seemed prom-

ising but there was yet another problem.

Lignin is not only vital to provide strength

to the plant (in fibre cells), it’s also essential

to maintain a functional vasculatory sys-

tem (in vessel cells). A collapsed vasculato-

ry system caused by low lignin would mean

poor nutrient transport for the plant and in-

evitably lower yields.

To solve this issue, they came up with

an extremely clever use of genetic tools:

they decided to put back what they took

away! In other words, they re-introduced

the downregulated CCR enzyme but this

time under the control of a vessel-specif-