Hydrogen Metabolism

The central carbohydrate metabolism in cyanobacteria is tightly intertwined with its hydrogen metabolism. The cyanobacterium Synechocystis sp. PCC 6803 possesses a hydrogenase, which catalyzes the simplest reaction. It reduces protons (H⁺) to hydrogen (H₂) or else oxidizes in opposite direction hydrogen back to protons. The enzyme requires energy rich electrons for hydrogen production. Under certain conditions these electrons can originate from photosynthesis. However, this so called photoH₂ is immediately oxidized and consumed by the cells.

When cells encounter anaerobic conditions in darkness, they oxidize their carbohydrates and produce so called fermentative hydrogen. When these cells are brought into light, the light reaction of photosynthesis starts immediately. However, the CBB cycle of CO₂ fixation, which accepts photosynthetic electrons, is activated with delay. In this situation electrons dam up in the photosynthetic electron transport chain. The hydrogenase dissipates this congestion by transferring electrons onto protons and by producing photoH₂. As soon as the CBB cycle is active, electrons flow into CO₂ fixation. When photosynthesis is running at full speed, photosystem II produces so much oxygen, that the hydrogenase, which is an oxygen sensitive enzyme, gets inhibited. In order to utilize photosynthetic hydrogen biotechnologically, oxygen has to be removed from the cultures. In addition, the hydrogenase competes with the CBB cycle and other cellular processes for photosynthetic electrons.

In order to maximize photosynthetic hydrogen production, we fused the hydrogenase genetically to photosystem I of photosynthesis (Appel et al., 2020).The respective mutants produce successfully photosynthetic hydrogen and as desired, they do no longer oxidize and consume the photoH₂. We currently remove the oxygen from our cultures enzymatically. However, in this process glucose enters the cells, is oxidized and delivers electrons via anoxygenic photosynthesis to the hydrogenase. The hydrogen production in our mutants thus currently relies on both oxygenic and anoxygenic photosynthesis. Our aim is to base the process exclusively on oxygenic photosynthesis in the long run. In addition, we wish to optimize the electron transfer between photosystem I and the hydrogenase, as this determines the efficiency of the process substantially.

The vision of this project, that we share with many scientists around the world, is to store sun energy via photosynthesis in form of hydrogen. The electrons should originate exclusively from water splitting at photosystem II. When this photosynthetic hydrogen is fed into fuel cells, electrons are transferred in the knallgas reaction back to oxygen. The stored sun energy is released and as a side product, pure water is produced. The circle is closed. This form of energy conservation and utilization is the most sustainable and eco-friendly form one can think of. We are currently still conducting basic research and we cannot be sure that the process can be developed further to reach final product maturity. However, the potential of the process is enormous, and we are determined to give it a try with combined efforts. For this, detailed knowledge concerning the carbohydrate and hydrogen metabolism in cyanobacteria is required.

References

Appel, J., Hueren, V., Boehm, M. and Gutekunst, K. 2020. Cyanobacterial in vivo solar hydrogen production using a photosystem I–hydrogenase (PsaD-HoxYH) fusion complex. Nature Energy.