A research group from the University of Turku, Finland, has developed a new protocol to deliver sustained hydrogen photoproduction in green algae under a train of strong white light pulses interrupted by longer dark phases. As reported in a paper in the RSC journal Energy & Environmental Science, under the new protocol, hydrogen photoproduction proceeds for up to 3 days with the maximum rate occurring in the first 6 hours.

Photobiological water splitting to molecular hydrogen (H₂) and oxygen (O₂), also known as direct water biophotolysis, has been considered as one of the most promising and environmentally friendly approaches for generating bulk quantities of clean H₂ biofuel. Many species of cyanobacteria and eukaryotic green algae, including the model organism Chlamydomonas reinhardtii, are capable of catalyzing this reaction.

… Efficient H₂ photoproduction in green algae occurs in the light after a period of dark anaerobic incubation. The reaction is transient due to a rapid, within seconds, inhibition of H₂ase by O₂, which is co-produced in the water-splitting reaction. One of the approaches to achieve sustained H₂ photoproduction in C. reinhardtii cultures is to deprive them of sulfur. Sulfur deprivation prevents the efficient repair of the light-damaged D1 reaction center protein of PSII, thus leading to a gradual loss of the water-splitting activity in algal cells over time. As a consequence, the actively respiring algae establish an anaerobic environment in the sealed photobioreactor, induce the H₂ase enzyme(s) and continuously produce H₂ gas for several days.

Although the loss of active PSII centers sustains H₂ photoproduction in algae, it also downregulates the direct, water oxidation-dependent flow of electrons to the H₂ase, resulting in low overall efficiency of the process. Sulfur-deprivation requires extensive and time-consuming centrifugations, which make this protocol difficult for application even in laboratory scale projects (yet a few alternatives have been suggested).

In the current work, we demonstrate that efficient H₂ photoproduction can be sustained in growing C. reinhardtii cultures for at least three days by switching the algal suspensions from continuous light to a train of short strong light pulses superimposed on either darkness or permanent low light illumination. The protocol is very simple, non-damaging to algae and reproducible even under strict autotrophic conditions.

—Kosourov et al.

For decades, researchers believed that the main obstacle to the longer-term hydrogen production in algae in light was the destruction of the hydrogenase enzyme, a key element in this process, which is caused by oxygen.

Since algae constantly release oxygen during their photosynthesis that occurs simultaneously with the production of hydrogen, maintenance of anaerobic conditions in illuminated cultures has been particularly troublesome, said Senior Researcher Sergey Kosourov, a member of the research group and lead author of the paper. This led to techniques such as sulfur-deprivation to extend hydrogen production.

The researchers at the University of Turku showed that the production of hydrogen could be significantly extended by simply exposing the anaerobic algal cultures to a train of strong yet short light pulses, which are interrupted by longer dark periods.

Under these conditions, algal cultures exposed to sunlight do not accumulate oxygen in the medium. In addition, algae steer the electrons resulting from the decomposition of water and charged by sunlight into hydrogen production instead of biomass accumulation. The process lasts for, at least, several days and the maximum rate of the production of hydrogen occurs during the first eight hours, Kosourov said.

The research indicated clearly that a major obstacle to efficient hydrogen production is not oxygen but a strong competition between two metabolic pathways: carbon dioxide fixation leading to the biomass accumulation and the hydrogenase enzyme catalyzing photoproduction of hydrogen.

The study opens up new possibilities for the construction of efficient living cell factories for the production of biofuels and different chemicals directly from sunlight, carbon dioxide and water. The research provides important information on how to avoid wasting solar-driven energy in biomass production and how to apply this energy directly for the production of useful bio-products, said Yagut Allahverdiyeva-Rinne, leader of the research group and Associate Professor of Molecular Plant Biology.

The new method developed by the researchers is valuable both for the basic research of the photosynthesis of algae and for the research and development work of the industrial sector when producing new technologies for the large-scale production of carbon-neutral biofuels.

The research is funded by Kone Foundation and the NordAqua Nordic Centre of Excellence of NordForsk Bioeconomy programme. The project is part of the Center of Excellence in Molecular Biology of Primary Producers funded by the Academy of Finland and led by Academician of Science Eva-Mari Aro from the University of Turku.

Resources

• Sergey Kosourov, Marina Jokel, Eva-Mari Aro & Yagut Allahverdiyeva (2018) “A new approach for sustained and efficient H₂ photoproduction by Chlamydomonas reinhardtii.” Energy Environ. Sci. doi: 10.1039/C8EE00054A