There is an increasing global need for developing new sustainable alternatives and technological solutions for replacing oil‐based products currently in use. In response to this demand, photosynthetic cyanobacteria have been extensively studied as nextgeneration biotechnological hosts for the large‐scale production of different carbonbased chemicals of interest directly from CO2. The strategy is based on the intrinsic capacity of cyanobacterial cells to utilize solar energy for fixing CO2 into complex organic molecules, together with our ability to engineer the cells for increased product range and improved performance. This Thesis is an expansion to the long‐term basic research carried out at the Molecular Plant Biology (Department of Biochemistry, University of Turku, Finland), which aims to understand the molecular mechanisms of photosynthesis and associated regulatory networks. The key focus of the Thesis is on i) the toxicity of a range of end‐products and related biosynthetic intermediates, and ii) enzymatic factors associated with the native regulation of intracellular cofactor redox homeostasis in cyanobacteria. 

The first part of the Thesis concentrated on evaluating the growth‐inhibitory effects of several chemicals and metabolites associated with hydrocarbon biosynthesis, towards a range of cyanobacterial species. The objective was to gain information which could be used for rational selection of the most appropriate target chemicals, engineering strategies and suitable hosts for biotechnological production trials and further photosynthetic cell factory. The comparison revealed significant differences between the apparent toxicity effects that decreased in the series from aldehydes to alcohols to alkanes, while ethanol and propane appeared as the most prominent candidates for continuous production. The length of the carbon chain was shown to have a clear impact in regards to the physiological tolerance, which expectedly reflected the solubility and capacity to penetrate through the cell membrane. The work also underlined the strain‐specific features between different cyanobacteria, and complications in direct quantitative comparison between alternative hosts. 

The second part of the Thesis concentrated on elucidating the biological role of the enzyme pyridine nucleotide transhydrogenase PntAB as part of the regulation of NAD(H)/NADP(H) homeostasis in cyanobacteria. Generation and characterization of Synechocystis sp. PCC 6803 deletion mutant ΔpntA devoid of the native transhydrogenase activity revealed that the enzyme plays a central role in the maintenance of sufficient NADPH supply under mixotrophic growth conditions when the photosynthetic activity of the cell is limited. The activity of PntAB was also linked with ATP metabolism and the photosynthetic repair mechanisms, which together are important considerations for the design of any engineered cyanobacterial systems.