206
J.-F. Cornet et al.
5 Limitation
by the Carbon
Source
5.1 Different Cases in C02 Transfer and HCO~ Consumption
Rates
At high pH values (8 < pH < 11), the total dissolved carbon (CO2 + HCO~+ CO~-) is essentially composed of bicarbonate HCO~ and carbonate CO~ions. For example, at pH 9.5, the concentration of carbonate and bicarbonate
ions is about 2000 times higher than the CO2 concentration. This is an important specificity compared with the classical approach used for gas transfer rate
analysis, because this creates a large buffer reserve of total dissolved carbon in
the culture media. This implies that the assumption of non-accumulation of
dissolved gases in the medium is no longer valid and that the mass balance
equations should not be written at steady state and should account for an
accumulation term in the liquid medium. This entails solving non-stationary
differential equations.
Cyanobacteria are known to concentrate intracellular bicarbonate if the
extracellular concentration is low [11-14, 90, 91]. This concentration proceeds
against the HCO~ concentration gradient but maintains the activity of the
Rubisco at a high level. However, this active transport consumes part of the
produced ATP and the energy yield of photosynthesis thus decreases, resulting
in lower growth rates.
The study of the limitation by the carbon source and its modeling are
therefore complex problems since information has to be obtained at three levels:
- the study of the CO2 gas-liquid transfer and any limitation by the COz
transfer rate;
- the study of chemical equilibria in complex media with high ionic strength
to calculate the CO2 HCO~-CO z--concentrations in the culture medium as
function of pH;
- the study of HCO~ limiting concentrations on the physiology and metabolism of S. platensis to postulate a kinetic law.
Moreover, the general modeling of these coupled phenomena requires mass
balances on O2 and CO2 in the gas phase to be performed to supplement the
above information.
Two principal cases may be examined:
1 - limiting concentration of bicarbonate: in this case, the Rubisco controls the
bioconversion rates (growth rate, glycogen and exopolysaccharide synthesis
rates) which depend on the bicarbonate concentration. This bicarbonate concentration is related to the gas-liquid mass balance and to the dissociation
equilibria. Either at steady state or in transient state, the bicarbonate concentration is obtained by solving the mass balance equations (Eqs. 9 15) enabling one
to link bicarbonate concentration to process parameters, i.e. gas liquid volumetric transfer coefficient kLa, gas flow rate, entrance molar fraction of CO2,
total pressure;
J.-F. Cornet et al.
5 Limitation
by the Carbon
Source
5.1 Different Cases in C02 Transfer and HCO~ Consumption
Rates
At high pH values (8 < pH < 11), the total dissolved carbon (CO2 + HCO~+ CO~-) is essentially composed of bicarbonate HCO~ and carbonate CO~ions. For example, at pH 9.5, the concentration of carbonate and bicarbonate
ions is about 2000 times higher than the CO2 concentration. This is an important specificity compared with the classical approach used for gas transfer rate
analysis, because this creates a large buffer reserve of total dissolved carbon in
the culture media. This implies that the assumption of non-accumulation of
dissolved gases in the medium is no longer valid and that the mass balance
equations should not be written at steady state and should account for an
accumulation term in the liquid medium. This entails solving non-stationary
differential equations.
Cyanobacteria are known to concentrate intracellular bicarbonate if the
extracellular concentration is low [11-14, 90, 91]. This concentration proceeds
against the HCO~ concentration gradient but maintains the activity of the
Rubisco at a high level. However, this active transport consumes part of the
produced ATP and the energy yield of photosynthesis thus decreases, resulting
in lower growth rates.
The study of the limitation by the carbon source and its modeling are
therefore complex problems since information has to be obtained at three levels:
- the study of the CO2 gas-liquid transfer and any limitation by the COz
transfer rate;
- the study of chemical equilibria in complex media with high ionic strength
to calculate the CO2 HCO~-CO z--concentrations in the culture medium as
function of pH;
- the study of HCO~ limiting concentrations on the physiology and metabolism of S. platensis to postulate a kinetic law.
Moreover, the general modeling of these coupled phenomena requires mass
balances on O2 and CO2 in the gas phase to be performed to supplement the
above information.
Two principal cases may be examined:
1 - limiting concentration of bicarbonate: in this case, the Rubisco controls the
bioconversion rates (growth rate, glycogen and exopolysaccharide synthesis
rates) which depend on the bicarbonate concentration. This bicarbonate concentration is related to the gas-liquid mass balance and to the dissociation
equilibria. Either at steady state or in transient state, the bicarbonate concentration is obtained by solving the mass balance equations (Eqs. 9 15) enabling one
to link bicarbonate concentration to process parameters, i.e. gas liquid volumetric transfer coefficient kLa, gas flow rate, entrance molar fraction of CO2,
total pressure;