CO₂ bioconversion – from greenhouse gas to valuable material

As a waste product from the combustion of fossil fuels, CO₂ has a demonstrable impact on the climate and is considered one of the main causes of global warming. To make the effects of climate change more or less manageable, climate researchers assume that global warming must be limited to an annual average of +1.5°C. Biotechnology can help to achieve climate goals, e.g. by capturing CO₂ and using it sustainably. That makes biotechnology an effective instrument on the way to a sustainable economy, the bio-economy.

What to do with excess CO₂? First and foremost, it is important for all stakeholders to reduce emissions right from the start! However, without CO₂ emissions there would be no traffic, no industry, no heating of private households, in short, no life as we know and love it. It is therefore necessary to "capture" and retain the CO₂ that is currently still unavoidable – so that it is not released into the atmosphere and causes further damage. A crazy idea? Not at all – and there are now many approaches to use gaseous CO₂ as a raw material and to incorporate it into a newly produced solid or liquid substance.

A few pertinent examples: Evonik and Siemens use electricity from renewable sources and specialized bacteria to convert CO₂ into valuable specialty chemicals. The polymer manufacturer Covestro uses carbon dioxide as a raw material to produce polyols as components for soft polyurethane foam, the basis for mattresses, for example. The Swiss start-up Climeworks is working on capturing CO₂ from the atmosphere in order to use it for the cultivation of plants or the production of carbon-neutral fuels.

Microorganisms become production specialists

What role does biotechnology play in this context? What can it contribute to the "material" use of the gaseous compound CO₂ or the pure element carbon (C) from industrial waste streams – e.g. from flue gases? Biotechnology is based on the use of living organisms such as microorganisms, algae or plant cells and uses their ability to metabolize one substance into another.

One example is the pure CO₂ produced during the production of bioethanol: researchers from BRAIN AG, in cooperation with Südzucker AG, have been looking for microorganisms that use CO₂ as the only source of carbon in their metabolism in a broad range of sources and industrial plants as part of a BMBF-funded cooperative project (Zero Carbon Foot Print, ZeroCarbFP). In fact, among the thousands of microorganisms investigated, they found ones that produced substances from CO₂ which can be used as building blocks for chemical compounds. The team was confident that they would find what they were looking for, as the ratio of oxygen to CO₂ (0.5% oxygen and approx. 20% CO₂) on Earth about 1 billion years ago was almost exactly the opposite of what we know today. And the only life on the planet at that time was microorganisms that coped magnificently with these conditions.

After studies on the metabolism of the bacteria and many other properties, the next step was to get the microorganisms to metabolize CO₂ more efficiently and, above all, faster. The project participants used biotechnological tools for this purpose – and normal CO₂ consumers became CO₂ super-consumers. The Südzucker project is currently in the 3rd funding phase and thus in the pilot phase.

Energy from hydrogen for energy-free CO₂

Since CO₂ was previously called the end of the carbon cycle and, as a completely oxidized molecule, is considered "energetically dead", preventing further reaction, the greatest task of the researchers was to find an energy source that could bring the CO₂ back to a reduced (energetically usable) state. To this end, experiments were conducted with sulfur, copper and, most recently, hydrogen. Hydrogen was selected as the best energy source. (This can be obtained by electrolyzing water – a process that can be performed cost-effectively with solar or wind energy in a so-called electrolyzer, for example).

In biotechnological applications, the CO₂-utilizing microorganisms quantitatively metabolize the element carbon from the CO₂ into an organic intermediate compound, which is needed, for example, in the chemical industry as a building block for further compounds. The building block can now replace the "old" building blocks that were previously obtained from fossil resources such as petroleum. Thus, the use of these microorganisms creates a kind of "biorefinery" for various processes for the production of a variety of substances. The new biotechnological processes have therefore promoted the trend in the chemical industry towards the use of bio-based building blocks.

Added value based on biological resources and renewable energies: implemented in this way, the use of CO₂ from industrial by-products described here corresponds to the principle of the bio-economy.

→ This article was published in the "Heads of Change" section of the BMBF online platform bioökonomie.de. The platform is part of the "Science Year 2020 – Bioeconomy" and an initiative of the German Federal Ministry of Education and Research (BMBF).