CO2 accounts for 60 % of all man-made greenhouse gas, and is known to be a high potential “climate killer”. However, after a three-year period of stagnation, global CO2 emissions increased in 2017 and have done so again in 2018. This was predicted by the Global Carbon Project, an international research project and partner of the World Climate Research Program. The data reveal much more—we are unlikely to solve the problem just by minimizing the use of fossil fuels, and ideas for capturing, storing, and recycling CO2 are urgently needed.
In December 2018, the global community met in Katowice at the annual United Nations Climate Change Conference COP24. Once again, the outcome was quite poor. While politicians, environmentalists, and lobbyists still discuss various scenarios and potential guidelines to govern coming decades, some scientists are exploring promising methods to reduce atmospheric CO2 levels using specialized bacteria. They intend to capture the climate-damaging CO2 and transform it into useful organic compounds.
Some bacteria can fix CO2 from the environment in the same way that plants and algae do; photosynthesis is the physiological skill they all have in common. Cyanobacteria, green sulfur bacteria, and purple sulfur bacteria belong to this group of so-called phototrophic bacteria. They use the energy of light to convert CO2 and electrons from water or another donor into sugars. Bacterial photosynthesis is an ancient phenomenon and widely spread throughout the environment. Sulfur bacteria are exclusively found in waters rich with hydrogen sulfite, but cyanobacteria populate nearly every natural habitat on earth. Phototrophic bacteria have already existed for millions of years and they have evolved to do their job very effectively. They naturally fix CO2 and thereby contribute to the global carbon cycle, along with all the other plants and algae on the planet.
Sun-lovers: the natural talents
Now, scientists are searching for ways to make full use of the potential to use phototrophic bacteria to capture CO2 and protect the climate. Using phototrophic bacteria to produce sugar as a resource for ethanol or acetic acid fermentation is well established, but the natural metabolic pathways of carbon fixation have limited efficiency. Therefore, scientists are creating artificial photosynthesis platforms and engineered organisms for optimized CO2 uptake and conversion.
Out of the lab: tailor-made micro factories Synthetic microbiology is a novel interdisciplinary field of research that creates biological systems with new properties. Biologists, chemists, and engineers work together to design unique and specialized molecules, cells, and organisms. For the consumption of CO2 this requires the engineering of artificial bacteria and novel metabolic pathways in order to biocatalyze valuable products such as hydrogen, lipids, alcohols, sugars, and other organic compounds. Today, 90 % of organic chemicals are secondary products of fossil fuels. With the help of synthetic microbiology, scientists are trying to find innovative solutions to reduce the exploitation of fossil fuels and turn CO2 from being a climate killer into a sustainable resource.
Scientists at Harvard University, for example, have successfully realized an artificial photosynthesis platform that may provide an effective and cheap way to convert CO2 into a useful product. The researchers found a way to treat bacteria to build up heavy metal semiconductor crystals on their surface. When exposed to light, water, and CO2, the crystals absorb the light energy to produce acetic acid. The reaction occurs at about 80 % efficiency, which is more than six times as efficient as natural photosynthesis. In a second fermentation step, the acetic acid can then be further processed into biofuels or bioplastics. Berkeley researchers went one step further: They worked with anaerobic bacteria that produce acetic acid because their specific selectivity for a single end-product is about 100%. The chemical reaction produces oxygen, however, which poisons the bacteria. Therefore, the scientists created a protective shield made of metallic organic frameworks, structures that have some of the highest surface areas of any material. These biohybrid bacteria may remove CO2 from power plant exhausts or from the air to produce useful chemicals. In their vision, the bacteria could even become tiny factories to produce useful compounds on future space stations or newly colonized planets.
Apart from photosynthesis, microorganisms have developed other carbon pathways. Some of these pathways and the corresponding enzymes have only recently been discovered and it can be assumed that a lot are still unknown. Researchers at the University of Dundee, for example, have developed an exciting process that allows E. coli to operate as a very efficient biological capture device. They found that E. coli bacteria produce a metal-containing enzyme when cultivated in the complete absence of oxygen. In pressurized CO2 and hydrogen gas mixtures, this enzyme can convert CO2 to formic acid at 100 % efficiency.
Creating tailor-made pathways by including modified or new enzymes is another recent approach. Most work is still in the experimental phase, and not yet efficient or reliable enough for scale-up and industrial use. However, some early findings are promising: At the Weizmann Institute of Science, for example, scientists introduce the Calvin cycle for carbon fixation and sugar production into E. coli, thereby changing the bacterium from a sugar consumer and CO2 releaser into a CO2 capturing organism. At MIT, researchers developed a two-step fermentation process to fix CO2 and transform it into lipids at a significant level of productivity. This bacterial conversion process avoids the controversial use of carbohydrate feedstocks for biodiesel production and uses steel mill exhausts instead.
The spark of life Microbial fuel cells are bio-electrochemical systems that generate current by using electroactive bacteria as catalysts. Today, they are in commercial use in wastewater treatment. By converting the chemical energy of organic substrates to electricity, they can reduce the energy demand and operating costs of wastewater treatment plants. Moreover, microbial fuel cells can be used for the process of microbial electrosynthesis. This produces basic chemicals such as acetate, butanol, acetone, or methane using electroactive microorganisms. Microbial fuel cells have already been used for the bioelectrochemical reduction of CO2 to acetate, methane, and multi-carbon compounds. In this way, it is conceivable to even convert CO2 to complex structures such as bioplastics.
Hope for the future Using atmospheric CO2 as a resource is a winner twice over—it reduces greenhouse gas levels in the atmosphere and it replace fossil fuel as the main feedstock for the chemical industry, thus saving further CO2 emissions. There is much room for optimism, and for further research and technology development.