Improved Photosynthetic Microalgae Production of H2

Improved Photosynthetic Microalgae Production of H2

Author: Roswitha Harrer

The idea that microalgae can be exploited to produce hydrogen goes back to the middle of the 20th century. Today, scientists and engineers are more and more abandoning traditional suspension-culture-based reactor technologies in favor of systems based on immobilized biofilms. Now, a joint study by the University of Turku and VTT Technical Research Centre of Finland has shown that also the material in which the thin films are immobilized should be switched towards more suitable nanofibrous systems [1].



Photosynthetic Biohydrogen Production

Although it has been known since the 1940s that the unicellular green algae Chlamydomonas reinhardtii is capable of producing hydrogen, larger scale exploitation did not start before the end of the 1990s when scientists discovered that hydrogen photoproduction soared when limiting the sulfur supply in the growth medium. Several reactor types were tried in the following. Suspension-culture bioreactors have been used in many designs, but they still suffer from high maintenance costs and a low light utilization efficiency. Panels of flat biofilms could be a solution. Immobilized microalgae films are currently regarded as the most promising photoreactor design in the biofuel production technology.

The biofilm-based reactors allow for more efficient light utilization and a significant decrease of water usage in the system, says Yagut Allahverdiyeva-Rinne, a professor of molecular plant biology at the University of Turku, who, together with Tekla Tammelin of VTT, led the study. Immobilized in the biofilm, the cells consume less energy for growth and can act as true biocatalysts, converting solar energy into chemicals, she adds.



Porous Network of Nanocellulose

Immobilization matrices have been mainly alginate, an anionic polysaccharide harvested from brown algae. But this material bears problems: “An alginate polymer has relatively poor mechanical durability and low porosity,” says Allahverdiyeva-Rinne.

Alternatively, both research teams have developed and tested cellulose nanofibrils, CNFs. Apart from providing mechanical strength, the CNFs’ fibrillary structure offered porosity, in combination with transparency, a large surface area, and hygroscopicity, the authors argued. Porosity is especially important for biofilms because vital nutrients and gases can diffuse to and from the cells, allowing high cell concentrations and production activities.

The researchers also oxidized the fibrils with TEMPO, the 2,2,6,6-tetramethylpiperidine-1-oxyl radical. This treatment enhanced fibrillation and yielded perfectly transparent nanocellulose films, the researchers reported.



Cellulose Nanofibrils Versus Alginate

Comparing the TEMPO–CNF behavior with that of alginate, the scientists observed that the nanofibrils were as biocompatible as the established matrix but yielded more hydrogen gas. In addition, the film-containing TEMPO–CNF hydrogels could be dried and rewetted if cyanobacteria were employed as the photosynthetically active microorganisms, with almost full recovery of their catalytic activity.

The researchers also reported that Ca2+ cross-linking improved the mechanical properties of the hydrogel, and cationization with a polyelectrolyte accelerated the attachment of the cells. Both modifications hint on future developments. Besides general optimizations, the scientists plan to focus on controlling the pore structure so that also larger biochemicals than hydrogen can be produced. “TEMPO–CNF is definitely worthy of further research efforts,“ says Allahverdiyeva-Rinne.


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