Living organisms need electrons in many places, as they use them to form chemical compounds. Algae and other organisms which carry out photosynthesis release electrons from water with the help of sunlight and then distribute them in the cell. The ferrous protein PETF is responsible for this—it transports the electrons to ferredoxin-NADP+ oxidoreductase (FNR), so that NADPH is formed and carbohydrates are finally synthesized from carbon dioxide.

The production of hydrogen through hydrogenases is among the many other processes for which PETF provides the necessary electrons. Hydrogenases are very efficient enzymes that contain a unique active center comprising six iron atoms, where the electrons supplied by PETF are bound to protons. Molecular hydrogen is produced in this way.

Dihydrogen (H₂) has the highest mass energy density of all known fuel types and as it can be generated from and converted back into water, it is one of the most attractive energy carriers to appease both the world’s climate and energy crisis. The solar-driven bio-H₂ production by microalgae like Chlamydomonas (C.) reinhardtii complements chemical technologies for solar fuel generation. Upon sulfur or nitrogen depletion C. reinhardtii switches to anaerobic growth conditions. As a consequence of anaerobiosis, the [FeFe]-hydrogenase HYDA1 is expressed. It receives electrons from photosystem I (PSI) via the plant-type photosynthetic electron transport ferredoxin (PETF) for catalyzing the reversible reduction of protons to H₂. Under normal growth conditions PETF provides photosynthetic electrons for a variety of different metabolic pathways such as the assimilation of nitrate, sulfate and ammonia, as well as the reductive regeneration of glutathione. Most of its electrons are, however, used for CO₂ fixation mediated by the PETF-dependent ferredoxin-NADP+-oxidoreductase (FNR).

While [FeFe]-hydrogenases can achieve very high turnover rates of up to 104 molecules H₂ per second in vitro, H₂ evolution is strongly limited in vivo by the O₂ sensitivity of the hydrogenase and the availability of reduced PETF. The latter issue has been addressed in several studies demonstrating that through down-regulation of competing processes the electron flow of photosynthetic electrons can be redirected towards the hydrogenase HYDA1 inducing enhanced H₂ photoproduction. … for stable photosynthetic growth a certain level of FNR activity has to be conserved and independent PETF is mandatory to dissipate at least a minor fraction of electrons to other essential redox pathways. In the long run a more subtle approach will be favored to develop a healthy growing algal strain with a strong but not self-destructive solitary focus on H₂ production.

—Rumpel et al.

In this new study, the researchers followed a new approach at the molecular level aiming to reduce the PETF affinity for FNR without affecting its interaction with HYDA1.

With the help of nuclear magnetic resonance spectroscopy, on which magnetic resonance imaging in medicine is also based, the scientists working with Sigrun Rumpel, a post doc at the Max Planck Institute for Chemical Energy Conversion in Mülheim, investigated the amino acids of PETF that interact with the hydrogenase and those that interact with FNR.

They found that only two amino acids of PETF are important for binding FNR. When the researchers modified these two amino acids and the enzyme FNR, PETF was no longer able to bind FNR as efficiently. Thus, the enzyme transferred less electrons to FNR and more to the hydrogenase. In this way, the scientists increased the hydrogen production by a factor of five.

For a technically feasible hydrogen production with the help of algae, its efficiency must be increased by a factor of 10 to 100 compared to the natural process. Through the targeted modification of PETF and FNR we have taken a step towards achieving this objective. These results represent a path to the economically-viable regenerative production of fuels with the help of microalgae.

—Sigrun Rumpel

Modifying electron transfer pathways could further improve hydrogen production in the future; the researchers are exploring the combination of different modifications.

Resources

• Sigrun Rumpel, Judith F. Siebel, Christophe Farès, Jifu Duan, Edward Reijerse, Thomas Happe, Wolfgang Lubitz, Martin Winkler (2014) “Enhancing hydrogen production of microalgae by redirecting electrons from photosystem I to hydrogenase,” Energy & Environmental Science, doi: 10.1039/C4EE01444H