Kinetics and Energetics of Photosynthetic Micro-Organisms in Photobioreactors
1. Introduction
159
The use of activities of photosynthetic micro-organisms, either in a closed vessel
under light irradiation or in an open outdoor pond, calls for a quantitative
understanding of their behaviour and of the rate controlling processes. As for
other bioprocesses, where different classes of models provide adequate framework for such endeavours, the cultivation of photosynthetic micro-organisms
needs specific problems to be treated, leading to particular models. These
models can be applied to process development, process optimization or process
control and can also be useful guidelines for the design of new culture vessels
and for the formulation of the composition of nutrient feed.
The problems to be overcome in modeling the behaviour of a photosynthetic
microorganism growing in an illuminated reactor derive mainly from the complexity of the physiological regulation of metabolism and the physical transfer of
light energy inside the medium. Considerable reduction of these two levels of
complexity is required to obtain mathematical models that are tractable for the
comprehensive study of photosynthetic cultures. The danger of such a simplification is that it can result in loss of realism and so lead to erroneous conclusions.
Hence the real difficulty lies in determining the appropriate degree of detail for
the description at a physiological level or at the physical level for the radiative
transfer characterization.
This paper presents a basis for simulating batch and continuous cultures of
photosynthetic micro-organisms under varying conditions of incident light
energy and nutrient limitations. The cyanobacterium Spirulina platensis is taken
as an example of such micro-organism because it presents classical photosynthesis with two photosystems, and it is one of the more studied and documented
in literature. The state of the art in modeling this kind of photosynthetic culture
includes the widely made assumption that growth rate is closely related to light
energy availability inside the medium, thereby paying special attention to the
physical problem of light energy transfer inside a dense culture that absorbs and
scatters light. It is thus important to assess the effects of light energy consumption and light scattering caused by the presence of the micro-organisms. This
produces an uneven light intensity distribution within the reactor and consequently a biochemical conversion rate inhomogeneity inside the culture
medium. This creates a high degree of complexity for the modeling of photosynthetic micro-organism cultures.
Also, a variety of situations occur in which the compositional variables of the
organisms are subject to changes that significantly affect behaviour. In such
cases, representation of the culture variables requires appropriate knowledge of
the biochemistry of the conversion processes and their regulation in the cell
environment.
The characterization of Spirulina platensis growth kinetics thus includes
both physiological and physical aspects. To handle these problems, the general
approach for modeling submerged aerobic or anaerobic cultures is used. It
1. Introduction
159
The use of activities of photosynthetic micro-organisms, either in a closed vessel
under light irradiation or in an open outdoor pond, calls for a quantitative
understanding of their behaviour and of the rate controlling processes. As for
other bioprocesses, where different classes of models provide adequate framework for such endeavours, the cultivation of photosynthetic micro-organisms
needs specific problems to be treated, leading to particular models. These
models can be applied to process development, process optimization or process
control and can also be useful guidelines for the design of new culture vessels
and for the formulation of the composition of nutrient feed.
The problems to be overcome in modeling the behaviour of a photosynthetic
microorganism growing in an illuminated reactor derive mainly from the complexity of the physiological regulation of metabolism and the physical transfer of
light energy inside the medium. Considerable reduction of these two levels of
complexity is required to obtain mathematical models that are tractable for the
comprehensive study of photosynthetic cultures. The danger of such a simplification is that it can result in loss of realism and so lead to erroneous conclusions.
Hence the real difficulty lies in determining the appropriate degree of detail for
the description at a physiological level or at the physical level for the radiative
transfer characterization.
This paper presents a basis for simulating batch and continuous cultures of
photosynthetic micro-organisms under varying conditions of incident light
energy and nutrient limitations. The cyanobacterium Spirulina platensis is taken
as an example of such micro-organism because it presents classical photosynthesis with two photosystems, and it is one of the more studied and documented
in literature. The state of the art in modeling this kind of photosynthetic culture
includes the widely made assumption that growth rate is closely related to light
energy availability inside the medium, thereby paying special attention to the
physical problem of light energy transfer inside a dense culture that absorbs and
scatters light. It is thus important to assess the effects of light energy consumption and light scattering caused by the presence of the micro-organisms. This
produces an uneven light intensity distribution within the reactor and consequently a biochemical conversion rate inhomogeneity inside the culture
medium. This creates a high degree of complexity for the modeling of photosynthetic micro-organism cultures.
Also, a variety of situations occur in which the compositional variables of the
organisms are subject to changes that significantly affect behaviour. In such
cases, representation of the culture variables requires appropriate knowledge of
the biochemistry of the conversion processes and their regulation in the cell
environment.
The characterization of Spirulina platensis growth kinetics thus includes
both physiological and physical aspects. To handle these problems, the general
approach for modeling submerged aerobic or anaerobic cultures is used. It