by Dr. Julia Weiler

Hydrogen production in the dark: The unicellular green alga Chlamydomonas cannot only produce H2 using solar energy, but also in the dark. The RUB researchers uncovered the combination of the proteins responsible. ©AG Photobiotechnologie, RUB

Hydrogen production in the dark: The unicellular green alga Chlamydomonas cannot only produce H2 using solar energy, but also in the dark. The RUB researchers uncovered the combination of the proteins responsible. ©AG Photobiotechnologie, RUB

Biologists at the Ruhr-Universität Bochum have identified a pathway whereby the green algae Chlamydomonas can produce hydrogen in the dark. Their findings were published recently in the “Journal of Biological Chemistry”.

More typically, researchers are interested in light-driven hydrogen synthesis, so this study offers an unusual approach that may have important implications. “Hydrogen could help us out of the energy crisis”, says Prof. Dr. Thomas Happe, head of the working group Photobiotechnology. “If you want to make green algae produce more hydrogen, it is important to understand all of the production pathways.”

Chlamydomonas can use light energy for the production of molecular hydrogen H2. “However, Chlamydomonas only forms hydrogen under stress”, says Dr. Happe. “The disposal of the energy-rich gas serves as a kind of overflow valve so that excess light energy does not damage the sensitive photosynthetic apparatus.”

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, the so-called pyruvate: ferredoxin oxidoreductase. The RUB researchers nevertheless tackled the project.

Dr. Happe’s team reconstructed the core of the dark hydrogen production in vitro, 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 the gut bacterium Escherichia coli, for example, the gene for the pyruvate: ferredoxin oxidoreductase. E. coli then produced the proteins according to this blueprint. Dr. Happe’s team isolated them from the bacterial cells and examined them like a construction kit. In the test tube, the biologists analyzed 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 normally only found in bacteria or single-celled parasites. “Chlamydmonas has an evolutionarily ancient enzyme,” said Jens Noth, from the Photobiotechnology working group. “With the help of vitamin B1 and iron atoms, it gains energy from the breakdown of sugars.”

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, like humans, 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 conditions.

“With this knowledge, we have now found another piece of the puzzle to get an accurate picture of H2 production in Chlamydomonas,” says Dr. Happe. “In the future, this could also help to increase the biotechnologically relevant light-dependent H2 formation rate.”

Reference: 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