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macromolecules for the biomass growth) the energy-transducing reactions,
namely, for photosynthetic bacteria, the water photolysis and the photophosphorylation mechanisms.
The number of independent kinetic laws that remain to be established is
lower than for an unstructured compartment model. Also, it becomes much
easier to assign the influence of a rate limiting process to a specific metabolic
reaction, which increases the robustness and predictivity of such a model. This
approach, which has become popular for representing aerobic culture processes
[-1], will be extensively used in Sect. 6 for modeling the growth of Spirulina
platensis under different light-limiting conditions.
2.2 Main Components of Cell Material
Detailed composition of the main components of cell material, such as proteins,
carbohydrates, lipids and nucleic acids is required to establish stoichiometries
for growth. This provides a global formula for biomass either by averaging each
compound from its molar fraction experimentally observed in specific operating
conditions, or by taking into account the production rates of each compound
given by a biochemically structured model for growth [1]. The first approach
leads to a fixed stoichiometric equation for biomass growth that is only valid for
limited applications, while the second approach enables allowance for marked
changes in biomass composition when the micro-organisms are cultivated over
a wide range of limiting conditions.
Based on ultrastructure [-2, 3] and biochemical studies [-4, 5], the following
composition may be assumed for the active biomass of the cyanobacterium
Spirulina platensis:
-
the mass protein content is 60-74% with a phycocyanin content of about
25% as the main pigment in photosystem II;
the mass carbohydrate content is 12-16%. Carbohydrates are mainly
located in the cell wall, except for an intracellular sulfated glycogen occurring as
carbon reserve [-6];
-
the mass lipid content is 8-12% located mainly in membranes of cells
except for polyhydroxybutyrate as carbon reserve [7, 8];
-
the mass nucleic acid content is about 4% (78% RNA and 22% DNA) [7];
- the carbon reserves for dark respiration are mainly sulfated glycogen [-6]
and polyhydroxybutyrate [-8] in equimolar proportions.
S. platensis also synthesizes a complex sulfated exopolysaccharide [7, 9]
which together with the active biomass gives the total biomass.
The relative mass fraction of each main component varies with the cultivation conditions, i.e. it depends on the limiting factor governing the biomass
synthesis (light transfer, CO2 transfer, mineral concentration, bicarbonate concentration etc.). The elemental formula for the constitutive macromolecules and
the average composition of active biomass (excluding exopolysaccharide) are
given in Table 1.
J.-F. Cornet et al.
macromolecules for the biomass growth) the energy-transducing reactions,
namely, for photosynthetic bacteria, the water photolysis and the photophosphorylation mechanisms.
The number of independent kinetic laws that remain to be established is
lower than for an unstructured compartment model. Also, it becomes much
easier to assign the influence of a rate limiting process to a specific metabolic
reaction, which increases the robustness and predictivity of such a model. This
approach, which has become popular for representing aerobic culture processes
[-1], will be extensively used in Sect. 6 for modeling the growth of Spirulina
platensis under different light-limiting conditions.
2.2 Main Components of Cell Material
Detailed composition of the main components of cell material, such as proteins,
carbohydrates, lipids and nucleic acids is required to establish stoichiometries
for growth. This provides a global formula for biomass either by averaging each
compound from its molar fraction experimentally observed in specific operating
conditions, or by taking into account the production rates of each compound
given by a biochemically structured model for growth [1]. The first approach
leads to a fixed stoichiometric equation for biomass growth that is only valid for
limited applications, while the second approach enables allowance for marked
changes in biomass composition when the micro-organisms are cultivated over
a wide range of limiting conditions.
Based on ultrastructure [-2, 3] and biochemical studies [-4, 5], the following
composition may be assumed for the active biomass of the cyanobacterium
Spirulina platensis:
-
the mass protein content is 60-74% with a phycocyanin content of about
25% as the main pigment in photosystem II;
the mass carbohydrate content is 12-16%. Carbohydrates are mainly
located in the cell wall, except for an intracellular sulfated glycogen occurring as
carbon reserve [-6];
-
the mass lipid content is 8-12% located mainly in membranes of cells
except for polyhydroxybutyrate as carbon reserve [7, 8];
-
the mass nucleic acid content is about 4% (78% RNA and 22% DNA) [7];
- the carbon reserves for dark respiration are mainly sulfated glycogen [-6]
and polyhydroxybutyrate [-8] in equimolar proportions.
S. platensis also synthesizes a complex sulfated exopolysaccharide [7, 9]
which together with the active biomass gives the total biomass.
The relative mass fraction of each main component varies with the cultivation conditions, i.e. it depends on the limiting factor governing the biomass
synthesis (light transfer, CO2 transfer, mineral concentration, bicarbonate concentration etc.). The elemental formula for the constitutive macromolecules and
the average composition of active biomass (excluding exopolysaccharide) are
given in Table 1.