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Title of the thesis: Advancing hydrogen-oxidizing ‘Knallgas’ bacteria as production hosts for biotechnology

Thesis defender: Tytti Jämsä 
Opponent: Associate Professor Sandy Schmidt, University of Groningen, Netherlands
Custos: Professor Paula Jouhten, Aalto University School of Chemical Engineering

Plants and microbes convert carbon dioxide effectively into valuable products. Plants fix carbon dioxide from the atmosphere into food and materials using sunlight as their energy source. Hydrogen-oxidizing bacteria, in turn, use hydrogen for carbon fixation, which is more energy-efficient than photosynthesis. When hydrogen is produced sustainably, these bacteria enable the integration of carbon-neutral bioproduction with the hydrogen economy and open new possibilities for sustainable production.

Hydrogen-oxidizing bacteria are already used industrially for single-cell protein production, and they also have significant potential for producing bioplastics, polyhydroxyalkanoates (PHA). However, their widespread use is limited by the high cost of raw materials, the diversion of cellular resources to undesired products, and a limited product range. This thesis aimed to address these challenges.

The results showed that carbon dioxide-rich reject gas from biogas upgrading can be used as a carbon source despite its impurities. It supported the growth and PHA production by Cupriavidus necator at levels comparable to those achieved with pure carbon dioxide. This approach can lower production costs and reduce emissions from biogas production.

It was also shown that blocking PHA production redirects cellular resources more efficiently toward protein production. Engineered Xanthobacter strains produced higher amounts of single-cell protein in continuous cultivation, which simplifies cultivation control at an industrial scale.

The research further expanded the use of these bacteria beyond carbon dioxide-based products to new production processes. Nearly 30 g/L of xylose was reduced to xylitol in C. necator by exploiting its NAD⁺-reducing hydrogenase for atom-efficient cofactor recycling.

These results advance hydrogen-oxidizing bacteria as versatile and efficient production hosts.

Key words: Biotechnology, single-cell protein, PHA, bioplastic, xylitol, hydrogen economy, carbon dioxide

Thesis available for public display 7 days prior to the defence at .

Contact information: tytti.jamsa@vtt.fi