Heterocystous cyanobacteria, Dps proteins and H2 production

Abstract: To mitigate climate change, CO2-emitting technologies have to be exchanged by renewable alternatives such as H2. H2 can be produced by cyanobacteria, but major efforts to enhance H2 production yields by genetic modifications or optimised cultivation conditions resulted in energetic photo-to-H2 conversion efficiencies of 4.0 %, far from its theoretical maximum. New concepts to enhance photobiological H2 production are described in two separate thesis chapters.In photobioreactors, photosynthesis can lead to high O2 concentrations promoting oxidative stress that decreases the photosynthetic efficiency. Genetic modifications could potentially increase the cellular robustness facing oxidative stress e.g. by the introduction of Dps proteins. This protein class is known to mitigate oxidative stress, but cyanobacterial Dps proteins are fairly unexplored. In the 1st thesis chapter, I searched to identify the function of five Dps proteins from the filamentous and heterocystous cyanobacterium Nostoc punctiforme. Since the physiologically active Dps proteins are twelve subunit complexes, various methods were utilised to verify their multimeric state and stability. All five NpDps formed high multimeric complexes that allowed for further enzymatic characterisations. In spectroscopic analyses NpDps1-3 were found to utilise H2O2 for Fe2+ oxidation, whereas NpDps4 only used O2. NpDps4 crystal structures revealed an uncommon ferroxidase center (FOC) with a His character. This His-type FOC was found across the cyanobacterial phylum. Based on their O2 and H2O2 consumption, all four NpDps display interesting candidates to enhance the cellular robustness in photobioreactors.To enhance H2 production yields, a reallocation of photosynthetic energy from cell growth to H2 production is required. In the 2nd thesis chapter, Nostoc PCC 7120 ΔhupW was set under iron starvation to evaluate this cultivation strategy for the purpose of H2 production. The Fe-limited culture comprised a ~ 5.3 fold lower chlorophyll a and a ~ 4.5 fold higher specific carbohydrate concentration as compared to the control. The Fe-limited cells retained long filaments with high heterocyst frequency of ~ 6 %. The microoxic environment inside heterocysts enables efficient H2-production from O2-senstive photo fermentative pathways. Therefore, iron-starvation could display the basis of enhanced H2 production on the cost of growth. For this purpose a biofilm-containing photobioreactor was designed.