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Researchers have developed a system in acetogenic bacteria that can perform effective carbon fixation by solely relying on light-induced electrons from cadmium sulfide nanoparticles.
In the mid 18th century, a scientist named John Tyndall coined the term “the greenhouse effect” (Tyndall 1861). His discovery did not catch international attention until the late 20th century when scientists started worrying about global warming and rapidly rising CO2 levels in the atmosphere. As human industry continued to grow, CO2 emissions and global warming rates have rapidly risen (Keeling 1997). Since the late 1900s, scientists have become fixated on how to reduce carbon gas emission and how to convert these harmful gasses back into useful forms of carbon.
This is where acetogenic bacteria comes into play. Acetogenic bacteria can convert gasses with one carbon, like CO2 or CO, into acetate through a unique metabolic pathway called the Wood-Ljungdahl pathway. This pathway converts CO2 (a waste gas) into a usable form of carbon (acetate) by using energy typically formed by breaking down hydrogen gas (H2). (This process is called carbon fixation, and it occurs during photosynthesis.) Carbon fixation pathways have attracted the attention of many researchers and are at the forefront of combating climate change (Fusch 1986).
Jin et al., in Acetogenic bacteria utilize light-driven electrons as an energy source for autotrophic growth, studied an acetogenic bacteria called C. autoethanogenum (2021). The researchers’ main goal was to determine how they could artificially improve the effectiveness of C. autoethanogenum’s carbon fixation pathway (Jin et al. 2021).
The researchers started by chemically synthesizing different cadmium sulfide nanoparticles, which are electron acceptors that can be attached to a cell’s surface in order to increase the electron acceptance rate (Martínes-Alonso 2014). Electron acceptance is critical for photosynthesis because electrons provide the energy needed to perform carbon fixation. Next, the researchers selected their most effective synthesized nanoparticles because the quicker electrons are accepted and transferred, the faster carbon fixation can take place.
With the effective nanoparticles, the researchers made an artificial photosynthesis system called the ‘hybrid system’. After running tests to see if the hybrid system functioned properly, researchers confirmed that their system consumed CO2 and produced acetate by using the artificially inserted nanoparticles. Interestingly, the researchers only needed to add light into their hybrid system to make it function rather than an energy source like H2. This was an incredible discovery because in the hybrid system, carbon fixation could take place solely relying on the energy from light.
The researchers’ next step was to compare gene expression in the hybrid system with expression in the H2 driven system (no nanoparticles) during carbon fixation. In both environments, there were similarities and differences in expression levels. Genes needed to conduct carbon fixation in the C. autoethanogenum were both highly expressed in each environment. This makes sense because in order to perform carbon fixation, we need the genes enabling carbon fixation.
Where gene expression differed was in genes that coded for energy conservation and electron acceptors. In the hybrid system, genes coding for energy conservation factors were expressed more than in the H2 driven system. Energy conservation systems are used to produce energy comparable to the energy produced from H2. This trend in expression makes sense because the hybrid system needs a form of energy to function while the H2 driven system already has H2 as an energy source.
Another key difference in gene expression was that the hybrid system had increased expression in electron acceptors (metal ions) and genes that code for electron transfer protein. Electron acceptors are critical for carbon fixation because it allows low-energy electrons to bind to the metal ions and leave the system. This allows a new energized electron to enter the system, which can power carbon fixation. The researchers also noticed the most crucial electron transfer protein coding gene was highly upregulated in the hybrid system compared to the H2 environment. If more electrons are exiting and entering the hybrid system at faster rates compared to the H2 driven system, then the hybrid system not only encourages faster electron transfer, but it also has an increased carbon fixation rate.
When the artificial hybrid system was placed under light, researchers saw an overall upregulation in the genes that enhanced the speed of carbon fixation. With more gene expression, the researchers saw a rise in the rate of carbon fixation. These results indicate the researchers accomplished their goal of improving C. autoethanogenum’s carbon fixation system.
Each C. autoethanogenum bacterium is incredibly small, but this study makes one wonder about the possibility of rapid carbon fixation conducted through millions of C. autoethanogenum bacteria. If scientists continue to improve C. autoethanogenum’s carbon fixation pathway, it could provide a way to counter the dangerous and growing greenhouse gas effect.
While increasing efficiency of carbon fixation will be helpful, it will by no means independently stop or significantly slow global warming. A reduction in carbon emissions paired with an increase in carbon fixation is the combination required to slow the consequences of a warming planet.
Walker A. Willis is currently enrolled in Davidson College. Contact him at wawillis@davidson.edu.
References
Fusch, G. “CO2 fixation in acetogenic bacteria: Variations on a theme.” Federation of European Microbiology Societies. 1986 August 1; 2(3) 181-213. https://doi.org/10.1111/j.1574-6968.1986.tb01859.x.
Keeling CD. “Climate change and carbon dioxide: An introduction.” Proceedings of the National Academy of Sciences of the United States of America. 1997 August 5; 94(16) 8273-8274. https://doi.org/10.1073/pnas.94.16.8273.
Martínes-Alonso C, Rodríguez-Castañeda CA, Moreno-Romero P, Coria-Monroy CS, and Hu H. “Cadmium Sulfide Nanoparticles Synthesized by Microwave Heating for Hybrid Solar Cell Applications.” International Journal of Photoenergy. 2014 August 19; 2014 Article ID: 453747. https://doi.org/10.1155/2014/453747.
Tyndall J. “On the absorption and radiation of heat by gases and vapours, and on the physical connexion of radiation, absorption, and conduction.” The Royal Society. 1861 January 1; 151:1-36. http://doi.org/10.1098/rstl.1861.0001.
