Researchers demonstrate new method to increase catalytic activity of cyanobacteria

Although cyanobacteria are known to stain water green through their unique pigments, they are colloquially referred to as “blue-green algae.”

Cyanobacteria are environmentally friendly and readily available biocatalysts for the production of new chemicals and, thanks to researchers at TU Graz, could soon be used in large-scale technological applications. Image Credit: Lunghammer - Graz University of Technology.

These bacteria can change light energy into chemical energy because of their highly active photosynthetic cells. This feature makes them suitable for biotechnological applications, where they could be utilized as readily available and environmentally friendly biocatalysts for producing new chemicals through specifically introduced enzymes.

Limited light availability

What sounds good in theory continues to face difficulties in the practical large-scale technological implementation. At present, a conclusive limiting factor is the availability of light.

When cyanobacteria are densely grown, i.e. in high concentrations, only the cells located on the outside receive enough light. Inside it’s pretty dark. This means that the amount of catalyst cannot be increased at will. After a cell density of a few grams per litre, the photosynthetic activity and thus the productivity of the cells decreases sharply. This is of course a considerable disadvantage for large-scale biotechnological production.”

Robert Kourist, Institute of Molecular Biotechnology, Graz University of Technology

By contrast, already established biocatalysts like yeasts can be utilized with cell densities of 50 g/L and more. The major drawback in the established production organisms is that they rely on agricultural products as a basis for growth and, hence, consume several resources.

Kourist added, “Algae-based catalysts can be grown from water and CO₂, so they are ‘green’ in a two-fold sense. For this reason, intensive efforts are underway to increase the catalytic performance of cyanobacteria.”

Making better use of available light

Along with Ruhr University Bochum and the Finnish University of Turku, the algae working for THE team from Graz University of Technology has currently succeeded in accurately increasing this catalytic performance by mainly redirecting the photosynthetic electron flow to the required catalytic function.

For the first time, we were able to measure the supply of photosynthetic energy directly in the cells in a time-resolved manner so that we were able to identify bottlenecks in the metabolism.”

Marc Nowaczyk, Chair of Plant Biochemistry, Ruhr University Bochum

Hanna Büchsenschütz, the study’s first author and doctoral student from Graz University of Technology, stated, “We have switched off a system in the genome of the cyanobacterium that is supposed to protect the cell from fluctuating light. This system is not necessary under controlled cultivation conditions, but consumes photosynthetic energy. Energy that we prefer to put into the target reaction.”

Thus, it is possible to address the issue of low productivity of cyanobacteria caused by high cell densities.

To put it another way, we can only use a certain amount of cells. That’s why we have to make the cells go faster. We have developed a method using so-called metabolic engineering that makes cyanobacteria a great deal more mature for biotechnological application.”

Robert Kourist, Institute of Molecular Biotechnology, Graz University of Technology

Apart from enhancing the productivity of the cell itself by targeted interventions at the genetic level, the Graz team is also looking at novel ideas for the process of algal cultivation. One method is the direct introduction of light sources into the cell suspension, for instance, through small LEDs.

Experiments are being conducted even with new geometries. In this way, cyanobacteria in the form of encapsulated small spheres, known as “beads”, can absorb higher levels of light in general.

“It is very important to develop all measures on the way to large-scale industrial application of algae-based biocatalysts in an integrated way. This is only possible with interdisciplinary research that looks at the function of an enzyme in the same way as we look at engineering in the photosynthetic cell,” Robert Kourist concluded.