Modelling and control microalgae for strain performance improvement
Microalgae are photosynthetic microorganisms whose potential has been highlighted in the last decade, especially for feed, food, and renewable energy production. When grown in photobioreactors, they can reach a higher actual photosynthetic yield compared to terrestrial plants, and thus larger biomass productivity. However, optimising this living resource is not straightforward due to the strong light attenuation in the culture that leads to photolimitation in the deepest layer, while the cells in the upper layer are damaged by an excess of light.
We use a simple ODE-based model to show that the average growth rate is determined by the incident light intensity and the optical depth. In the case of vertical cylindrical photobioreactors illuminated from above we prove under general assumptions how the average growth rate is affected by light and optical density. There is an Allee effect when light limits or inhibits the growth rate in outdoor cultures, leading to bi-stability. We propose an original control strategy to stabilize one of the unstable equilibrium and generate a long term photoinhibition stress. Under these conditions, the study of N-competing strains show that the strains less prone to photoinhibition will win the competition. These results are applied in the lab and a Darwinian selection pressure is maintained for 6 months, leading to the emergence of more transparent strains, and finally a doubling of productivity.
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