Bioprocess Engineering of Phototrophic Marine Organisms 9.2 Growth Characteristics of Phototrophic Suspension Cultures 261
Part B | 9.2
traction or chromatography are used to fractionate the
cell extract into the final purified compounds. Topics
related to downstream bioseparation processes such
as biomass extraction and chromatography are beyond
the scope of this chapter because they are generic to
all forms of bioprocess technology, not just marine
bioprocess technology. Excellent references on bioprocess engineering fundamentals and bioseparations are
available [9.17–19].
This chapter focuses on the cultivation of phototrophic marine organisms in enclosed photobioreactor systems. However, it is also possible to cultivate
phototrophic marine organisms outdoors in tanks or
ponds. In open pond or tank cultures, the liquid surface is exposed to ambient air and direct sunlight.
The open pond or tank culture is industrially practiced species of photosynthetic microalgae that can
grow under conditions of salinity or pH where potential airborne contaminants cannot survive. A prominent example is the cultivation of the green microalga
Dunaliella at elevated salinity in raceway ponds for
production of beta-carotene. The two major limitations
on open pond culture are sterility and process control.
The manufacture of bioproducts requires a GMP or
good manufacturing practices environment. Therefore,
bioreactors must be enclosed and sterilizable to control contamination. Furthermore, bioreactor cultivation
must be carried out with pure cultures under precisely
controlled environmental conditions to ensure that the
same product is reliably produced in the cell culture
from batch to batch. Simultaneous control of contamination, temperature, pH, nutrient composition, aeration,
and light is impossible to achieve in open pond or tank
cultures.
9.2 Growth Characteristics of Phototrophic Suspension Cultures
Phototrophic marine organisms in liquid suspension
culture share common growth characteristics. Below,
concepts underlying photosynthetic biomass stoichiometry and specific growth rate are used to estimate
the amounts and rates of photosynthetic biomass
production.
9.2.1 Nutrient Requirements and
Photosynthetic Biomass Stoichiometry
Photosynthetic Biomass Production
Phototrophic liquid suspension cultures utilize dissolved carbon dioxide as the carbon source and light
as the energy source for photosynthesis. A detailed
discussion of the complex biochemistry behind photosynthesis is beyond the scope of this chapter, and so
the reader is encouraged to consult any of the excellent textbooks that cover photosynthesis, particularly
in algal systems [9.20–22]. However, the basic idea of
photosynthesis is to convert carbon dioxide to carbohydrates and ultimately to cellular biomass by an array
of reduction reactions. Within the cell, photosynthesis is localized within organelles called chloroplasts.
Within the chloroplasts are thylakoid membranes that
contain complex molecules called chlorophylls that absorb light in the photosynthetically active radiation
(PAR) range of 400700 nm wavelength. The absorption of light by chlorophyll induces electron flow and
transport that is used to drive reactions within two
photosystems (I and II). Photosystem I carries out
the reduction of the chemical energy carrier NADP
C
to NADPH (nicotinamide adenine dinucleotide phosphate) whereas Photosystem II splits water into O 2 and
hydrogen ions (H
C ). The proton gradient generated
by Photosystem II produces the chemical energy carrier adenosine triphosphate (ATP) through the process
of photophosphorylation. The NADPH and ATP generated by these processes subsequently reduce CO 2 to
carbohydrates by a series of non-light requiring (dark)
reactions collectively called the Calvin cycle. The overall stoichiometric equation for photosynthesis is
6CO 2 C 6H 2 O
hv
! C 6 H 12 O 6 C 6O 2 ;
(9.1)
where hv is the light energy. Note that CO 2 is consumed and O 2 is generated in a 1 W 1 stoichiometric
ratio. However, this stoichiometric equation ignores the
incorporation of other elements such as nitrogen and
phosphorous into cellular material.
Medium Components
In phototrophic liquid suspension cultures, carbon dioxide is supplied to the culture by placing the liquid
medium into contact with an aeration gas containing
CO 2 . The CO 2 dissolves into the liquid phase from
the gas phase. In addition to dissolved CO 2 and light,
phototrophic liquid suspension cultures require both
Part B | 9.2
traction or chromatography are used to fractionate the
cell extract into the final purified compounds. Topics
related to downstream bioseparation processes such
as biomass extraction and chromatography are beyond
the scope of this chapter because they are generic to
all forms of bioprocess technology, not just marine
bioprocess technology. Excellent references on bioprocess engineering fundamentals and bioseparations are
available [9.17–19].
This chapter focuses on the cultivation of phototrophic marine organisms in enclosed photobioreactor systems. However, it is also possible to cultivate
phototrophic marine organisms outdoors in tanks or
ponds. In open pond or tank cultures, the liquid surface is exposed to ambient air and direct sunlight.
The open pond or tank culture is industrially practiced species of photosynthetic microalgae that can
grow under conditions of salinity or pH where potential airborne contaminants cannot survive. A prominent example is the cultivation of the green microalga
Dunaliella at elevated salinity in raceway ponds for
production of beta-carotene. The two major limitations
on open pond culture are sterility and process control.
The manufacture of bioproducts requires a GMP or
good manufacturing practices environment. Therefore,
bioreactors must be enclosed and sterilizable to control contamination. Furthermore, bioreactor cultivation
must be carried out with pure cultures under precisely
controlled environmental conditions to ensure that the
same product is reliably produced in the cell culture
from batch to batch. Simultaneous control of contamination, temperature, pH, nutrient composition, aeration,
and light is impossible to achieve in open pond or tank
cultures.
9.2 Growth Characteristics of Phototrophic Suspension Cultures
Phototrophic marine organisms in liquid suspension
culture share common growth characteristics. Below,
concepts underlying photosynthetic biomass stoichiometry and specific growth rate are used to estimate
the amounts and rates of photosynthetic biomass
production.
9.2.1 Nutrient Requirements and
Photosynthetic Biomass Stoichiometry
Photosynthetic Biomass Production
Phototrophic liquid suspension cultures utilize dissolved carbon dioxide as the carbon source and light
as the energy source for photosynthesis. A detailed
discussion of the complex biochemistry behind photosynthesis is beyond the scope of this chapter, and so
the reader is encouraged to consult any of the excellent textbooks that cover photosynthesis, particularly
in algal systems [9.20–22]. However, the basic idea of
photosynthesis is to convert carbon dioxide to carbohydrates and ultimately to cellular biomass by an array
of reduction reactions. Within the cell, photosynthesis is localized within organelles called chloroplasts.
Within the chloroplasts are thylakoid membranes that
contain complex molecules called chlorophylls that absorb light in the photosynthetically active radiation
(PAR) range of 400700 nm wavelength. The absorption of light by chlorophyll induces electron flow and
transport that is used to drive reactions within two
photosystems (I and II). Photosystem I carries out
the reduction of the chemical energy carrier NADP
C
to NADPH (nicotinamide adenine dinucleotide phosphate) whereas Photosystem II splits water into O 2 and
hydrogen ions (H
C ). The proton gradient generated
by Photosystem II produces the chemical energy carrier adenosine triphosphate (ATP) through the process
of photophosphorylation. The NADPH and ATP generated by these processes subsequently reduce CO 2 to
carbohydrates by a series of non-light requiring (dark)
reactions collectively called the Calvin cycle. The overall stoichiometric equation for photosynthesis is
6CO 2 C 6H 2 O
hv
! C 6 H 12 O 6 C 6O 2 ;
(9.1)
where hv is the light energy. Note that CO 2 is consumed and O 2 is generated in a 1 W 1 stoichiometric
ratio. However, this stoichiometric equation ignores the
incorporation of other elements such as nitrogen and
phosphorous into cellular material.
Medium Components
In phototrophic liquid suspension cultures, carbon dioxide is supplied to the culture by placing the liquid
medium into contact with an aeration gas containing
CO 2 . The CO 2 dissolves into the liquid phase from
the gas phase. In addition to dissolved CO 2 and light,
phototrophic liquid suspension cultures require both