PhD Defence by Lars Sebastian Stegemüller

Principal supervisor
Professor Irini Angelidaki
DTU Chemical and Biochemical Engineering

Examiners
Associate Professor Seyed S. Mansouri
DTU Chemical Engineering

Dr Ioannis Fotidis
Aristotle University of Thessaloniki
Greece

Niels-Henrik Norsker
Algiecel A/S

Chairperson at defence
Tenure Track researcher Antonio Grimalt Alemany
DTU Chemical and Biochemical Engineering

The defence will also be held virtually. If you wish to follow the defense virtually you can sign up by sending an e-mail to Dimitar Karakashev dbka@kt.dtu.dk no later than 4 December at 9:00. You will then receive an invitation to join the virtual defence.

Popular summary

Microalgae are versatile organisms with a wide range of potential applications, including as food for humans, feed for aquaculture, and sources of nutraceuticals and pharmaceuticals. The cultivation of microalgae offers a sustainable approach due to their ability to grow with sunlight, fixate CO2 and uptake inorganic nitrogen and phosphorus sources from waste sources. Depending on the strain, microalgae are rich in protein, carbohydrates, lipids and offer highly valuable components such as astaxanthin or β‐carotene. Currently, microalgae biomass is mainly produced for high‐value applications related to human consumption, including nutraceuticals and pharmaceuticals.

High production costs of 5 to 50€ per kg make a wider application spectrum unfeasible. This Ph.D. thesis focused on the implementation of novel strategies and digital tools to improve current processes and enable a wider application of microalgae in the field of biotechnology. The thesis focused on Haematococcus lacustris which is a microalgae able to produce the high value product astaxanthin. In general, microalgae grow photoautotrophically, which means they utilize (sun)light and carbon dioxide to produce biomass. However, in some cases microalgae can also grow heterotrophically, where instead of light carbon sources such as glucose or acetate, are used as the energy source. The combination of both light energy and energy from organic carbon is then called mixotrophic growth.

Synergistic effects were observed, elevating the growth rates of mixotrophically grown microalgae. Mixotrophic growth on low-cost carbon sources was studied in detail. Moreover, the relation between light intensity and mixotrophic growth was compared to photoautotrophic growth. Simulations are an important part of modern biotechnology, since they offer an easy alternative to excessive experiments for process adaptations and optimizations. However, the applicability and precision of these models rely heavily on an accurate representation of the biological system. A mathematical model description for the growth and product formation was proposed and calibrated accounting for the available light intensity, nitrogen uptake and internal nitrogen storage. Artificial intelligence is omnipresent these days and mainly known for large language models such as Chat-GPT.

However, the field of artificial intelligence also offers other algorithms, which can be used to classify objects such as microscope images. Two of these algorithms were studied to classify single cell images obtained from a scanning microscope to determine morphological trades related to either growth or product formation, which is an important information for control and decision making during the cultivation of microalgae. The work resulted in an accurate online monitoring system of 5 cell stages, unveiling important culture dynamics related to the accumulation of products. In conclusion, this thesis studied different strategies to enhance growth rates and improve process control in microalgae cultivation. Although focused on a specific microalga producing high-value compounds, the proposed mixotrophic strategy, mathematical modelling framework, and AI-based monitoring system offer broad applicability and can be adapted for use with other strains, paving the way for wider biotechnological applications.