The Wood-Ljungdahl pathway, also known as the reductive acetyl-coenzyme A pathway, is a metabolic route found in certain microorganisms. It allows these organisms to convert simple one-carbon molecules into more complex organic compounds. This pathway uniquely fixes carbon dioxide and carbon monoxide, serving as a fundamental process for life in oxygen-free environments. It provides a self-sustaining mechanism for growth and energy acquisition.
What the Pathway Does
The Wood-Ljungdahl pathway’s core function is carbon fixation, converting carbon dioxide (CO2) and carbon monoxide (CO) into organic compounds. It operates by reducing two CO2 molecules to form acetyl-CoA, a two-carbon compound that serves as a building block for cell components and a precursor to acetate. Hydrogen (H2) often provides electrons for these reduction reactions.
The pathway proceeds through two main branches that converge. One branch reduces carbon dioxide into a methyl group, while the other reduces carbon dioxide into carbon monoxide. These one-carbon units are then joined. Specifically, the enzyme carbon monoxide dehydrogenase converts CO2 to CO, and acetyl-CoA synthase combines CO with the methyl group and coenzyme A to produce acetyl-CoA.
Acetyl-CoA can be further converted into acetate, a two-carbon organic acid. This conversion produces a usable organic compound and allows microbes to generate energy, enabling them to thrive in environments where other life forms might struggle to find suitable energy and carbon sources.
Microbes That Use It
A variety of microorganisms, primarily strict anaerobes, utilize the Wood-Ljungdahl pathway. Among the most well-known groups are acetogens, bacteria that produce acetate as their main fermentation product from single-carbon substrates like carbon dioxide, formate, or carbon monoxide.
Methanogens, a group of archaea, also employ a version of this pathway for carbon dioxide fixation, though they ultimately produce methane rather than acetate. While the overall scheme is conserved in both bacteria and archaea, there are differences in the specific carriers and enzymes involved in their methyl branches.
These microbes are commonly found in oxygen-free environments, such as deep-sea vents, anaerobic sediments, and animal digestive tracts. The pathway allows these organisms to grow autotrophically, synthesizing their own food from inorganic carbon. Their capacity to utilize simple gases like H2, CO, and CO2 as sole carbon and energy sources enables them to occupy ecological niches inaccessible to most other forms of life.
Importance in Nature and Beyond
The Wood-Ljungdahl pathway holds significance in natural ecosystems and offers promising avenues for human applications. Ecologically, it plays a role in global carbon cycling, particularly in anaerobic settings, by converting carbon dioxide and carbon monoxide into organic compounds. This process helps influence the levels of these gases in the environment, with an estimated 10^8 tons of carbon monoxide removed from the lower atmosphere and Earth annually by bacteria using this pathway.
The pathway is also responsible for the production of a large amount of acetate in nature, with estimates suggesting around 10^10 tons produced annually through acetogenesis. This conversion of one-carbon gases into organic matter contributes to biomass formation and nutrient cycling in anaerobic food webs. The pathway’s ancient origins suggest its involvement in early Earth’s carbon cycle, potentially preceding oxygen-dependent life forms.
Beyond its natural ecological functions, the Wood-Ljungdahl pathway is gaining interest for biotechnological applications. Its ability to convert waste gases, such as those found in industrial emissions (syngas, which contains CO, H2, and CO2), into valuable chemicals and fuels presents a sustainable approach. Researchers are exploring its use for producing biofuels like ethanol and butanol, as well as industrial chemicals such as acetate. This pathway could contribute to carbon capture and waste valorization strategies by transforming harmful emissions into useful products, potentially reducing reliance on fossil resources.