From Science & Research Microalgae to PHB How cyanobacteria could transform our industry PHB (polyhydroxybutyrate) offers a promising substitute to comparable fossil-based plastics, which shows similar material properties as polypropylene, while at the same time being biobased and biodegradable. Currently, PHB is mostly produced in heterotrophic bacteria, which require sugar as a carbon source for growth. Those sugars are often produced in large monocultures, such as cornfields, which themselves have negative consequences on the environment. Furthermore, the usage of crops like corn, which could also be used as a food source, raises ethical questions. The research group of Karl Forchhammer in Tübingen has now developed a sustainable alternative, which can convert atmospheric CO 2 to high-quality PHB. The secret lies in so-called cyanobacteria, which are often referred to as microalgae (Figure 1, 2). Just like algae, cyanobacteria are capable of using photosynthesis. This is a process, where sunlight is used as an energy source, to fix atmospheric CO 2 for the cells. The CO 2 can then be further converted by the cyanobacteria into valuable products, such as PHB (cf. bM 03/14, bM 01-05-06/17, 04-05/20) Besides CO 2 and sunlight, cyanobacteria require only a low-cost salt medium. Alternatively, sewage water can be used, allowing cyanobacteria to grow, while at the same time cleaning the water. Additionally, their ability to sequester CO 2 from the atmosphere enables them to clean CO 2 -rich exhausts, for example from coal power plants. This makes cyanobacteria an ideal production system for sustainable products. Unfortunately, cyanobacteria naturally produce only small amounts of PHB, making the production economically unfeasible. Instead, cyanobacteria are currently produced as food supplements (for example the superfood Spirulina) or used for the production of high-value fine chemicals, such as pigments. However, those products are only produced in small quantities, hence the positive impact on the environment is limited. To unleash the full potential of cyanobacteria, they have to be produced in large amounts of bulk products, with bioplastics, such as PHB, for example. The working group at the University of Tübingen (Germany) focuses on the analysis of cyanobacterial metabolism. Moritz Koch, who did his PhD in this group, has specifically focused on metabolic engineering strategies, enabling him to rationally reprogram the cells for the production of PHB (Figure 3). This allowed him to unleash the natural potential of the small microbes, resulting in unprecedented amounts of accumulated PHB. Under optimized conditions, the amounts per cell-dry-weight were increased from the naturally produced 10 % to more than 80 %. These are not only the highest amounts ever achieved in any photoautotrophic organism but are also in a comparable range with heterotrophic bacteria, which are currently used for the production of PHB. Understanding the science In order to improve the cyanobacterial PHB production, the researchers first had to deepen their understanding of the intracellular metabolism. This is essentially the mechanism of how CO 2 , once it’s taken up, travels through the cells and gets converted into the different molecules a cell contains. After years of intensively studying proteins and molecular regulators, that are involved in the PHB biosynthesis, the research group of Forchhammer came to a breakthrough: they discovered a new molecular regulator, which serves as a metabolic switch, channelling large fractions of the intracellular carbon towards PHB. Based on this discovery Moritz Koch created a cyanobacterial chassis for further development. Additionally, he overexpressed the biosynthesis genes required for the PHB production, which further boosted the PHB accumulation. Finally, after systematically testing and optimizing the cultivation conditions, Koch discovered ideal conditions which favour cyanobacterial growth and carbon flux going towards PHB. Although many researchers worldwide have already tried to improve cyanobacteria for the production of bioplastics, most of them had only limited success. Based on the recent results, the group from Tübingen was able to demonstrate for the first time, which metabolic potential cyanobacteria have, and that they can compete with currently used, heterotrophic bacteria, which still rely on sugars as a carbon source. This brings cyanobacteria, for the first time, in the range of an economically feasible PHB production (Figure 4). 20 bioplastics MAGAZINE [03/21] Vol. 16
What’s next So far, most of the work on cyanobacteria relies on laboratory-scale experiments. In the next stage, the research group plans to collaborate with companies, which will test the technology in pilot plants. This will provide further insights into how their sustainable production system can be upscaled. However, until cyanobacteria are established for the mass production of products like bioplastics, it will take years, maybe even decades. Still, the long-term benefits are clear and it is expected to see many more products which are based on cyanobacteria. In the future, where more strict carbon taxes are expected, carbon-neutral production systems, like microalgae, become more economically feasible. The company Photanol (Amsterdam, the Netherlands), recently introduced in bioplastic MAGAZINE, is a great example of how cyanobacteria can be produced on a larger scale for the production of industrially relevant products. It is expected that more companies like them will emerge in the future and provide with their products a substantial contribution to the way how we produce our everyday products. Nevertheless, until this is the case, the pollution of our environment continues, may it be in form of plastic trash in the seas, or as CO 2 in the atmosphere. “Recent meta-studies have shown very clearly that it requires our society to come up with a more holistic solution to prevent the worst outcomes of our most pressing ecological crises”, says Koch. “It will not be sufficient, nor quick enough, to rely completely on technological innovations”, he continues. “Instead, societal as well as political changes, are required”, he says. “We should hence advocate for a more sustainable society, which can live within our planetary boundaries”. AT Figure 1: Cyanobacterial culture Figure 3: Moritz Koch, the leading researcher From Science & Research https://uni-tuebingen.de/en/ Figure 4: Cyanobacteria with PHB granules, before and after optimization Figure 2: Microscopic picture of the individual cells bioplastics MAGAZINE [03/21] Vol. 16 21
