Synechocystis gathered momentum in modelling studies and biotechnological applications owing to multiple factors like fast growth, ability to fix carbon dioxide into valuable products, and the relative ease of genetic manipulation. Synechocystis physiology and metabolism, and consequently, the productivity of Synechocystis-based photobioreactors (PBRs), are heavily light modulated. Here, we set up a turbidostat-controlled lab-scale cultivation system in order to study the influence of varying orange-red light intensities on Synechocystis growth characteristics and photosynthetic activity. Synechocystis growth and photosynthetic activity were found to raise as supplied light intensity increased up to 500 μmol photons m-2 s-1 and to enter the photoinhibition state only at 800 μmol photons m-2 s-1. Interestingly, reverting the light to a non-photo-inhibiting intensity unveiled Synechocystis to be able to promptly recover. Furthermore, our characterization displayed a clear correlation between variations in growth rate and cell size, extending a phenomenon previously observed in other cyanobacteria. Further, we applied a modelling approach to simulate the effects produced by varying the incident light intensity on its local distribution within the PBR vessel. Our model simulations suggested that the photosynthetic activity of Synechocystis could be enhanced by finely regulating the intensity of the light incident on the PBR in order to prevent cells from experiencing light-induced stress and induce their exploitation of areas of different local light intensity formed in the vessel. In the latter case, the heterogeneous distribution of the local light intensity would allow Synechocystis for an optimized usage of light.

Analysis of the light intensity dependence of the growth of Synechocystis and of the light distribution in a photobioreactor energized by 635 nm light.

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1-butanol production from carbon dioxide by Synechococcus elongtus in Turkey

The oxygenic photoautotroph Synechocystis sp. PCC6803 is a present-day cyanobacterium, a lineage with billions of years of history. In this thesis, we explore ways to optimize the use of Synechocystis for the realization of a sustainable society, in which the carbon cycle is closed by using sunlight to produce fuel and chemical feedstock out of CO2. A deeper understanding of metabolism in Synechocystis is required to be able to reach this goal in the future. State-of-the-art photobioreactors were used to precisely control and measure parameters of growth and develop a new medium, as described in chapter 2. This chapter explores the limits of growth of Synechocystis and exposes nutrient limitations. In chapters 3 and 4, the photosynthetic apparatus itself is closely investigated with special emphasis on the role of the light-harvesting complexes typically found in cyanobacteria, revealing a remarkable flexibility in coping with varying light intensities. Because of the desire to use sunlight to fuel the assimilation of CO2, it is of paramount importance to be able to deal with the steady rhythm of day and night, as well as varying light intensities during the day. Chapters 5 to 7 cover the circadian clock of Synechocystis, an essential mechanism to time gene expression and metabolism. The clock of Synechocystis was shown to be robust and in control of large number of cellular processes, crucial for coping with the night and allowing for additional regulation to enhance production of compounds of interest. Chapter 8 discusses all findings in a broader context.

P. van Alphen

Papers

Physiological studies to optimize growth of the prototype biosolar cell factory Synechocystis sp. PCC6803