Cyanobacteria – which, like plants, photosynthesise and convert carbon dioxide (CO₂) into sugar and oxygen – have to cope with widely varying CO₂ concentrations in their surroundings. Now, researchers have mapped the mechanism behind this. The discovery could eventually change how bacteria are used – from developing medicines to producing climate-friendly chemicals.
Cyanobacteria have played an enormous role in shaping the current world.
Photosynthesis originated in cyanobacteria more than 2.4 billion years ago, and then the Earth’s atmosphere went from being rich in CO2 to also containing oxygen.
Over time, the increasing levels of oxygen in the atmosphere laid the foundation for the development of animals on Earth – and thus humans.
Throughout Earth’s history, cyanobacteria have also had to adapt to a wide range of CO2 concentrations: not only in the atmosphere but also in microenvironments such as the ocean.
Now, for the first time, researchers have identified how cyanobacteria adapt to these fluctuating levels of CO2.
The discovery, with potential real-world applications, has been published in New Phytologist.
“Cyanobacteria can potentially produce industrially useful substances, and in this context, knowing how to create optimal growth conditions for these bacteria is important. With this discovery, we understand better how cyanobacteria adapt to CO2 and how CO2 can help regulate the growth of these bacteria,” explains a researcher behind the discovery, Taina Tyystjärvi, Associate Professor, University of Turku, Finland.
A single missing protein leaves bacteria defenceless against CO₂
The discovery is based on Taina Tyystjärvi’s many years of work on the machinery cyanobacteria use to read genes.
RNA polymerase is the cell’s copying machine – an enzyme complex that converts DNA recipes into the proteins that form the basis of cells and thus of life.
In their experiments, the team removed a small, non-essential piece of the RNA polymerase copying machine – and were surprised to find a mutant cyanobacterium that thrived at normal CO₂ but died at high CO2, which boosts growth of normal cyanobacteria.
This mutant became the focus of detailed study, enabling the team to pinpoint the signalling pathways involved in how cyanobacteria adapt to CO₂.
Genetic tweaks expose bacteria’s hidden CO₂ sensor
The scientists used bioinformatics, biochemistry and 3D modelling of proteins to improve understanding of the mutant cyanobacteria.
First and foremost, they found that the cyanobacterium lacked a subunit of RNA polymerase and that this subunit plays a role in CO2 adaptation.
They then identified how mutations in other genes could counteract the mutation in the subunit of RNA polymerase, enabling the cyanobacterium to grow even with high concentrations of CO₂ in the environment.
The research showed that changes in the ssr1600 gene can trigger an interaction between three proteins (Ssr1600, SigC and Slr1861). Together, they form a biochemical signalling pathway – a kind of domino chain – that enables cyanobacteria to sense CO₂ concentrations and adjust their growth accordingly.
When CO2 itself becomes the growth signal
The point is simple: when this signal cascade is disrupted, as in the mutant studied by the researchers, a special enzyme complex accumulates. It acts as a brake, inhibiting the genes that normally build cell walls, drive photosynthesis and absorb nutrients.
The special signalling system acts as the cell’s internal control tower, keeping track of everything from growth to photosynthesis. In addition, the signalling cascade is part of a regulatory mechanism whose function is not yet fully understood, but it appears to control not only photosynthetic activity but also how fixed carbon is distributed between growth and storage.
“The discovery of a special CO₂-controlled signalling chain shows how environmental CO₂ concentrations can flick genes on and off – giving us a clearer picture of just how resilient and adaptable microorganisms are,” says Taina Tyystjärvi.
Findings may pave the way for green production
Taina Tyystjärvi says that the research results may be of interest to others besides cyanobacteria researchers.
In many parts of the world, industry sees opportunities in using cyanobacteria to produce a wide range of useful substances, including medicines and other valuable chemicals.
However, high production yields require controlling the distribution of fixed carbon between growth and chemical production.
By tweaking this signalling chain – and using synthetic biology to redirect the bacteria’s output – Taina Tyystjärvi thinks highly efficient, CO₂-neutral production could eventually be achieved.
She also thinks that reusing nutrients from wastewater would be optimal, since cyanobacteria effectively collect both phosphate and nitrate or ammonium from wastewater.
“When we first identified this mutant, we expected it to thrive in high CO₂ – after all, that is its food source. Instead, it died. That indicates an adaptation we might be able to harness, helping cyanobacteria flourish even at very high CO₂ concentrations and creating conditions for optimal growth,” concludes Taina Tyystjärvi.
