issue 03/2021

Figure 2: Microscopic picture of the individual cells
Science and 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 highvalue
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, 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).
What’s next
So far, most of the work on cyanobacteria rely 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
28 bioplastics MAGAZINE [03/21] Vol. 16