Biomedical Engineering Reference
In-Depth Information
there was no change in lipid content of a similarly engineered diatom
Cyclotella
cryptica
(Dunahay et al., 1995; Dunahay et al., 1996). Three possible strategies exist
for enhanced lipid production: biochemical engineering (BE), genetic engineering
(GE), and transcription factor engineering (TFE). BE approaches are currently the
most widely established in microalgal lipid production (Courchesne et al., 2009).
Radakovits et al. (2010) discussed the potential of manipulating the central car-
bon metabolism in eukaryotic microalgae through genetic engineering to enhance
lipid production. They suggested that it should be possible to increase production
of not only carbon storage compounds, such as TAGs and starch, but also designer
hydrocarbons that may be used directly as fuels.
Another possibility is to engineer the light-harvesting antennae in autotrophic
algae. Smaller antennae lead to greater photosynthetic efficiency (Mitra and Melis,
2008); mutating genes that control antennae biogenesis is a possible mechanism
for enhancing photosynthetic efficiency (Scott et al., 2010). Possibilities exist to
improve solar energy conversion efficiency from the present the 1-4% to 8-12% to
realize fully the potential of microalgal co-production systems in
Chlamydomonas
perigranulata, C. reinhardtii, Chlorella vulgaris, Cyclotella
sp.,
Dunaliella salina,
Scenedesmus obliquus,
and
Synechocystis
PCC 6714 (Stephens et al., 2010).
A mechanistic model
developed by Flynn et al. (2010) explores cellular chlorophyll
and photosynthetic efficiency to optimize commercial algal biomass production.
The model predicts that genetically modified strains with a large antenna size, indi-
cated by a low Chl:C ratio, are more suitable for commercial biofuel production
than strains selected from nature. However, for the generation of hydrogen and
hydrocarbons as biofuels, smaller light-collecting antennae seem to be more effi-
cient in
Botryococcus braunii
(Eroglu and Melis, 2010). Three races (Race A, B,
and L) of the strain
Botryococcus braunii
are recognized (Banerjee et al., 2002);
these races are regarded as a potential source of renewable fuel with yields of hydro-
carbons reaching up to 75% of algal dry mass. A
Botyrococcus
Squalene Synthase
(BSS) gene from a Race B variant of
B. braunii
has been sequenced, amplified as
a 1,403-bp fragment, and expressed as a heterologous protein in
E. coli
BL21 cells.
Following Isopropyl-β-D-thiogalactoside (IPTG) induction, recombinant squalene
synthase activity was detected, suggesting that a key hydrocarbon synthesis gene
from a commercial alga can be isolated and cloned into a heterologous expression
system. This opens the door for large-scale hydrocarbon synthesis in more amenable
systems such as
E. coli
and may help reduce the problems associated with the vis-
cous nature of
Botyrococcus
cultures (Banerjee et al., 2002).
13.9 SUMMARY
Innovative ways to optimize maximum microalgal biomass production and techno-
logical advances for transesterification would be necessary to make microalgae more
cost effective for biodiesel production and to sustain an economically viable micro-
algal biotechnological industry (Figure 13.1). Improvements at various intermedi-
ary stages of culturing, selection of strains of algae, harvesting, and extraction of
bio-fuel production and co-products could bring down the production costs. Norsker
et al. (2011) state that by optimizing irradiation conditions, mixing, photosynthetic
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