Dissertation defence (Molecular Plant Biology): FM Lauri Kakko

FM Lauri Kakko defends the dissertation in Molecular Plant Biology titled “Enhancing Carbon Flux Control in Synechocystis sp. PCC 6803 Through Translational Tuning and Electron Sink Engineering” at the University of Turku on 23 October 2026 at 12.00 (University of Turku, Dentalia, Arje Scheinin lecture hall, Lemminkäisenkatu 2,Turku).

Opponent: Associate Professor Pia Lindberg (Uppsala University, Sweden)

Custos: Associate Professor Pauli Kallio (University of Turku)

Summary of the Doctoral Dissertation:

Cyanobacteria are microscopic organisms that, like plants, use sunlight to turn carbon dioxide and water into sugars. Researchers are trying to harness them as living factories that make useful chemicals directly from CO₂, without fossil raw materials. The difficulty is that most of the carbon they fix is used for growth and storage, not for the product we want.

This doctoral research tested two ways to steer that carbon in the model cyanobacterium Synechocystis. The first was to fine-tune a synthetic pathway that makes 3-hydroxybutyrate, a small molecule used as a building block for polyhydroxybutyrate, a biodegradable plastic. Unlike polyhydroxybutyrate, which stays inside the cell, 3-hydroxybutyrate is released into the growth medium and can be collected without breaking the cells. The second was to remove a protein, Flv3, that normally acts as a safety valve in photosynthesis by passing spare electrons to oxygen.

The best strain from the first approach produced 11.7 grams of 3-hydroxybutyrate per litre under bright light and extra carbon dioxide. That is several times more than earlier cyanobacterial studies. Measuring the proteins in the cells showed why: high production depended on a balanced amount of the three pathway enzymes, not on making as much as possible of any one of them. The same strains produced much less under ordinary light and carbon dioxide, so growing conditions mattered as much as the genetic changes.

Removing Flv3 had a broader effect. The cells reduced parts of their carbon-fixation machinery, increased the pathway that feeds central metabolism, began consuming sugar they had previously exported, and stored more glycogen and polyhydroxybutyrate, especially in bright light. Closing the electron safety valve can therefore redirect resources toward storage compounds, but the outcome depends on the conditions and on what other outlets for carbon the cell still has.

Together, the results give a practical lesson for designing photosynthetic production systems. Both the balance of the enzymes in an engineered pathway and the cell’s own use of light energy have to be considered, and both only work under the right growing conditions. The work shows that cyanobacteria can approach the product levels of conventional industrial microbes while using carbon dioxide as their raw material, although cost and large-scale production remain open questions.