Cyanobacteria are excellent model organisms for studying the regulation of photosynthesis. Furthermore, they serve as biocatalysts in light-driven whole-cell biotransformation applications, utilising solar energy for the production of chemicals. However, introducing additional electron sinks in the form of heterologous enzymes into a tightly regulated network of native pathways is challenging and requires a thorough understanding of photosynthesis and its regulation.

In this thesis, I focus on the Mehler-like reaction mediated by flavodiiron proteins, their electron donors, and activity modulation. I show that ferredoxin serves as their primary electron donor while their activity is dynamically controlled by changes in cytosolic pH. Furthermore, I demonstrate that a strong heterologous electron sink with high NAD(P)H consumption can outcompete flavodiiron proteins and other auxiliary electron transport pathways for electrons, thereby maintaining the photosynthetic electron transport chain in an oxidised state. Identifying and addressing the bioenergetic bottleneck is a crucial prerequisite for improving and deploying this technology on an industrial scale. If the bottleneck is cofactor availability, adding glucose at the start of the biotransformation reaction increases NAD(P)H production and enhances biotransformation. Similarly, a CO2-rich atmosphere or a modified light spectrum can improve the biotransformation of enzymes where the bottleneck is their protein accumulation.

The results of my thesis expand our understanding of the regulation of flavodiiron proteins and the impact of biotransformation on the photosynthetic apparatus of Synechocystis sp. PCC 6803. Furthermore, they stress that the characterisation of each enzyme/strain combination is an essential step in engineering cyanobacteria as a future solar-to-chemical platform.