Researchers at the Ruhr-Universität Bochum have determined how green algae can produce hydrogen in the dark; their paper appears in the Journal of Biological Chemistry.
Green algae of the type Chlamydomonas can use light energy for the production of molecular hydrogen (H2). However, Chlamydomonas only forms hydrogen under stress, says Prof. Dr. Thomas Happe, head of the working group Photobiotechnology. The disposal of hydrogen serves as a kind of overflow valve so that excess light energy does not damage the sensitive photosynthetic apparatus.
|“If you want to make green algae produce more hydrogen, it is important to understand all the production pathways.”|
—Prof. Dr. Thomas Happe
Chlamydomonas can also produce hydrogen in the dark. Although this fact has been known for decades, H2 synthesis in the absence of light has barely been studied because much less of the gas is produced in the dark than in the light. Moreover, it is complicated to isolate large quantities of the key enzyme of the dark-reaction: pyruvate:ferredoxin oxidoreductase.
|The unicellular green alga Chlamydomonas can produce H2 in the dark, as well as using solar energy. The RUB researchers uncovered the combination of the proteins responsible. Copyright: Working Group Photobiotechnology, RUB. Click to enlarge.|
Happe’s team reconstructed the core of the dark hydrogen production in vitro, thus demonstrating the underlying mechanism. In order to get to the proteins involved, the researchers had these produced by bacteria.
First they introduced the corresponding genes of the green algae into Escherichia coli—e.g., the gene for the pyruvate:ferredoxin oxidoreductase. E. coli then produced the proteins according to this blueprint. Happe’s team then isolated them from the bacterial cells and analysed how different combinations of proteins interacted with each other under specific environmental conditions.
In so doing, they found out that, under stress in the dark, the algae switch to a metabolic pathway which is normally only found in bacteria or single-celled parasites.
Chlamydmonas has an evolutionarily ancient enzyme. With the help of vitamin B1 and iron atoms, it gains energy from the breakdown of sugars.—Jens Noth from the working group Photobiotechnology
This energy is then used by other green algal enzymes, the hydrogenases, to form hydrogen. The unicellular microalgae switch on this metabolic pathway when they suddenly encounter oxygen-free conditions in the dark because the green algae need oxygen to breathe if they cannot draw their energy from sunlight. The formation of hydrogen in the dark helps the cells to survive these stress condition.
With this knowledge, we have now found another piece of the puzzle to get an accurate picture of H2 production in Chlamydomonas. In future, this could also help to increase the biotechnologically relevant light-dependent H2 formation rate.—Thomas Happ
This work was supported by grants from the Deutsches Zentrum für Luft- und Raumfahrt (ModuLES) and the Volkswagen Foundation (LigH2t) (to T. H.).
J. Noth, D. Krawietz, A. Hemschemeier, T. Happe (2013) Pyruvate:ferredoxin oxidoreductase is coupled to light-independent hydrogen production in Chlamydomonas reinhardtii, Journal of Biological Chemistry, doi: 10.1074/jbc.M112.429985