Marine Microalgae 5.5 Photobioreactors for Marine Microalgae 57
Part A | 5.5
ever, the reaction catalyzed by the ethylene forming
enzyme induced metabolic stress, which was detrimental to the host cell.
Microalgal biodiesel production is expected to
be improved though metabolic engineering. Several
transformants have been established for the increased
oil content of the microalgal cell, enhanced biomass
productivity, and improved quality of the lipids. Acetyl
CoA carboxylase (ACCase), which catalyzes the
carboxylation of acetyl-CoA to form malonyl-CoA,
the primary substrate of fatty acids synthesis, has been
overexpressed in diatom cells to elevate the cell lipid
content [5.36]. The vector containing the ACCase gene
and its 5
0 UTR (untranslated region) from the diatom
Cyclotella cryptica was constructed and introduced
into the diatoms C. cryptica and Navicula saprophila.
Stable high ACCase expression transformants were
obtained successfully, yet the predicted increase
of neutral lipid content was not achieved, which
indicates that TAG (triacylglycerol) accumulation
in the microalgal cell is much more complex than
previously assumed. With the expression of the hexose
transporter that transfers the monohexose from the
culture medium into the cell, the transformants of
green algae C. reinhardtii and V. carteri as well as
diatom P. tricornutum have been demonstrated to be
capable to grow in the dark in a medium containing
glucose [5.89–91]. Especially, the glucose transporter
(Glut1) gene transformant of P. tricornutum cultured
in dark conditions showed an almost threefold higher
biomass production than in light conditions [5.91]. On
the other hand, the de-regulation of the light-harvesting
proteins in C. reinhardtii has been demonstrated to be
able to elevate the solar energy conversion efficiencies
in photosynthesis when the light-harvesting chlorophyll
antenna size is minimized [5.92]. This permits a greater
photosynthetic productivity under high cell density
conditions as well as the possibility of culturing cells
under high sunlight conditions. The transformation of
Cinnamomum camphora (C12-TE) and Umbellularia
californica (C14-TE) Acyl-ACP thioesterases genes
into diatom P. tricornutum resulted in an increased
lauric (C12:0) and myristic acid (C14:0) accumulation
mutant [5.93]. Levels of lauric acid of up to 6:2% of total fatty acids and myristic acid of up to 15% by weight
were achieved. Moreover, 7590% of the shorter chain
length fatty acids produced were demonstrated to be
incorporated into triacylglycerols.
5.5 Photobioreactors for Marine Microalgae
Microalgae mass cultivation for the production of useful compounds has been widely discussed since the
1950s. Even though large-scale production of astaxanthin, DHA, and EPA from microalgae have been
achieved, the industrial production of microalgal biofuel is still under development. Lower cost and higher
productivity and efficiency than current bioreactors
are necessary due to the extremely low final price
of biodiesel (1 dollar L
1 ) compared with those high
value-added microalgal products.
Both the biology and the economics of microalgae
mass cultivation are strongly influenced by photobioreactor design. Photosynthetic microalgae can be cultured
in photobioreactors as either an open culture system
or a closed system. Based on their localization, these
photobioreactors can be divided into outdoor culture
systems or indoor culture systems.
Outdoor open culture systems are the simplest
method of algal cultivation due to the low construction
cost and effortless operation. However, the productivity of these systems can be easily affected by several
environmental factors such as contamination of other
microorganisms, changes of weather conditions, and
the disability of transgenic microalgae cultivation. The
need to achieve higher productivity and to maintain
monoculture of algae led to the development of closed
photobioreactors. Despite higher biomass concentration
and better control of culture parameters, CO 2 recycling
efficiency, energy profit ratio, energy payback time, and
cost of production in these enclosed photobioreactors
are not better than those achievable in open systems.
The growth rate and maximum biomass yield of
microalgal strains are affected by culture parameters
(light, temperature, and pH) and nutritional status (CO 2 ,
nitrogen, and phosphate concentration). On the other
hand, increasing the density of cultures decreases photon availability to individual cells. Light penetration
of microalgal cultures is poor, especially at high cell
densities, and such poor photon availability decreases
specific growth rates. Higher biomass yields can be expected if sufficient photons are provided in high density
cultures of microalgae. Two major types of bioreactors
(tubular [5.94, 95] or flat plate [5.96, 97]) are generally
applied for the enclosed system (Fig. 5.1).
Part A | 5.5
ever, the reaction catalyzed by the ethylene forming
enzyme induced metabolic stress, which was detrimental to the host cell.
Microalgal biodiesel production is expected to
be improved though metabolic engineering. Several
transformants have been established for the increased
oil content of the microalgal cell, enhanced biomass
productivity, and improved quality of the lipids. Acetyl
CoA carboxylase (ACCase), which catalyzes the
carboxylation of acetyl-CoA to form malonyl-CoA,
the primary substrate of fatty acids synthesis, has been
overexpressed in diatom cells to elevate the cell lipid
content [5.36]. The vector containing the ACCase gene
and its 5
0 UTR (untranslated region) from the diatom
Cyclotella cryptica was constructed and introduced
into the diatoms C. cryptica and Navicula saprophila.
Stable high ACCase expression transformants were
obtained successfully, yet the predicted increase
of neutral lipid content was not achieved, which
indicates that TAG (triacylglycerol) accumulation
in the microalgal cell is much more complex than
previously assumed. With the expression of the hexose
transporter that transfers the monohexose from the
culture medium into the cell, the transformants of
green algae C. reinhardtii and V. carteri as well as
diatom P. tricornutum have been demonstrated to be
capable to grow in the dark in a medium containing
glucose [5.89–91]. Especially, the glucose transporter
(Glut1) gene transformant of P. tricornutum cultured
in dark conditions showed an almost threefold higher
biomass production than in light conditions [5.91]. On
the other hand, the de-regulation of the light-harvesting
proteins in C. reinhardtii has been demonstrated to be
able to elevate the solar energy conversion efficiencies
in photosynthesis when the light-harvesting chlorophyll
antenna size is minimized [5.92]. This permits a greater
photosynthetic productivity under high cell density
conditions as well as the possibility of culturing cells
under high sunlight conditions. The transformation of
Cinnamomum camphora (C12-TE) and Umbellularia
californica (C14-TE) Acyl-ACP thioesterases genes
into diatom P. tricornutum resulted in an increased
lauric (C12:0) and myristic acid (C14:0) accumulation
mutant [5.93]. Levels of lauric acid of up to 6:2% of total fatty acids and myristic acid of up to 15% by weight
were achieved. Moreover, 7590% of the shorter chain
length fatty acids produced were demonstrated to be
incorporated into triacylglycerols.
5.5 Photobioreactors for Marine Microalgae
Microalgae mass cultivation for the production of useful compounds has been widely discussed since the
1950s. Even though large-scale production of astaxanthin, DHA, and EPA from microalgae have been
achieved, the industrial production of microalgal biofuel is still under development. Lower cost and higher
productivity and efficiency than current bioreactors
are necessary due to the extremely low final price
of biodiesel (1 dollar L
1 ) compared with those high
value-added microalgal products.
Both the biology and the economics of microalgae
mass cultivation are strongly influenced by photobioreactor design. Photosynthetic microalgae can be cultured
in photobioreactors as either an open culture system
or a closed system. Based on their localization, these
photobioreactors can be divided into outdoor culture
systems or indoor culture systems.
Outdoor open culture systems are the simplest
method of algal cultivation due to the low construction
cost and effortless operation. However, the productivity of these systems can be easily affected by several
environmental factors such as contamination of other
microorganisms, changes of weather conditions, and
the disability of transgenic microalgae cultivation. The
need to achieve higher productivity and to maintain
monoculture of algae led to the development of closed
photobioreactors. Despite higher biomass concentration
and better control of culture parameters, CO 2 recycling
efficiency, energy profit ratio, energy payback time, and
cost of production in these enclosed photobioreactors
are not better than those achievable in open systems.
The growth rate and maximum biomass yield of
microalgal strains are affected by culture parameters
(light, temperature, and pH) and nutritional status (CO 2 ,
nitrogen, and phosphate concentration). On the other
hand, increasing the density of cultures decreases photon availability to individual cells. Light penetration
of microalgal cultures is poor, especially at high cell
densities, and such poor photon availability decreases
specific growth rates. Higher biomass yields can be expected if sufficient photons are provided in high density
cultures of microalgae. Two major types of bioreactors
(tubular [5.94, 95] or flat plate [5.96, 97]) are generally
applied for the enclosed system (Fig. 5.1).