Biomedical Engineering Reference
In-Depth Information
content, desirable environmental stress tolerance and a product profile optimized
for biofuel conversion. However, hunting in nature for such microalgae strains that
are simultaneously endowed with such talents has proved to be an uphill struggle.
2 Progress and Challenges in Algal Feedstock Development
Nearly two decades ago, the ''Aquatic Species Program'' in the USA amassed a
microalgal collection of over 3,000 strains which eventually was winnowed down
to around 300 species with potential as biofuel producers. Some of the strains were
put into open ponds for outdoor cultivation testing; however, none of them
demonstrated a combination of the superior traits demanded [
6
]. For example,
although the growth rate could reach 50 g/m
2
/day in mass culture, the yearly
average was only 10 g/m
2
/day, a level far below the industrial requirements. Most
importantly, some of the key traits were mutually exclusive for a given microalgal
isolate, such as high growth rate and high oil content [
6
]. These roadblocks
were also encountered by Japanese researchers in the ''Biological Fixation and
Utilization of CO
2
'' project, where some strains were isolated that could tolerate
and grow rapidly under high concentrations of CO
2
. These pioneering endeavors
demonstrated that only a small fraction of microalgal species and strains in nature
harbors the potential for large-scale biodiesel production, and few of them are
simultaneously endowed with high biomass productivity and high lipid content,
strong environmental tolerance and a product profile optimized for biofuels [
4
].
Genetic engineering of microalgae has also been attempted to improve lipid
productivity. In 1995, one of the key enzymes for lipid biosynthesis, encoded by
the acetyl-CoA carboxylase (ACCase) gene, was identified. Targetting the gene,
transgenic Cyclotella cryptica and Navicula saprophila were developed, but neither
showed increased lipid content [
7
-
11
]. More recently, in a Chlamydomonas
reinhardtii mutant with inactivated ADP-glucose pyrophosphorylase, an enzyme
involved in starch synthesis, the triacylglycerol content exhibited a 10-fold increase,
from 2 to 20.5%, under nitrogen starvation [
11
]. However, its growth slowed down,
resulting in lower overall lipid yield than that of the wild type. So far, there have been
few success stories in improving oil productivity by engineering individual genes or
pathways in oleaginous microalgae. As a result, transgenic microalgae at product
costs competitive with petroleum-derived fuels have been rare [
12
].
Such humbling outcomes resulted from a number of realities, some of which were
recognized only recently. First, the key traits of oleaginous microalgae are inherently
correlated and interact with each other, thus selection and engineering strategies that
treat each trait independently would usually be futile. Many such phenotypes were
observed, such as the link between photosynthesis and cellular carbon flux, the
apparent conflicts between algal growth rate and oil content, the interactions between
biosynthesis pathways of neutral lipids (e.g. triacylglycerol) and polar lipids
(phospholipids etc.), the relationship between neutral lipid production and pigment
synthesis, and the links between the various oil-induction programs induced by light
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