Modelling of growth and product formation of Porphyridium purpureum.

In this contribution experimental data and simulations of growth and product formation of the unicellular microalgae Porphyridium purpureum are presented. A mathematical model has been developed for a better understanding of growth and product formation in production plants. The model has been refined with the results of several cultivations in a new photobioreactor designed especially for the study of microalgal kinetics under highly defined illumination conditions. In this photobioreactor light is generated by an external light source and then distributed by means of optical fibres into an internal draft tube which also serves as irradiation element. All cultivations were performed in turbidostate mode. The influence of different light intensity changes, including stepwise change and light-dark cycles in the range from millisecond to second, has been investigated and the results were integrated into the mathematical model. The structured mathematical model consists of three levels: metabolic flux, control of macromolecules and the reactor level. A new linear optimization approach has been realized, enabling the model to describe even very different cultivation conditions. Output variables are among others the commercially interesting macromolecules of the microalgae, e.g. polysaccharides, pigments and polyunsaturated fatty acids. Thus, reliable predictions of the specific production rates of these products are possible for the production in a larger scale.

Scale-down of microalgae cultivations in tubular photo-bioreactors--a conceptual approach.

Rational design of large-scale bioreactors is still suffering from inadequate scale-up of technical parameters from lab to large scale and from missing kinetic information concerning the physiological reactions of the specific strain under cultivation. Therefore, simulations of processes expected in large-scale have to be carried out as far as possible and experiments have to be performed in small-scale reactors mimicking the situation in large scale. This procedure is referred to as scale-down. In this paper a concept to accomplish this task is proposed. Firstly, interactions between light transfer, fluid dynamics, and microbial metabolism are described. Secondly, a procedure is given to decompose the interactions by simulation on the one hand and by finding physiological parameters in model reactors on the other. Light transfer can be calculated by Monte Carlo methods, while fluid dynamics is handled by CFD. Ideally illuminated model photo-bioreactors and pilot reactors with enforced flow field are proposed to measure physiological parameters especially induced by light/dark cycles generated by interaction of turbulences and light attenuation.