Anaerobic oxidation of propane by thermophilic archaea from marine hydrothermal sediments
This study identifies *Candidatus* Syntrophoarchaeum caldarium as the first archaeon capable of anaerobically oxidizing propane in syntrophy with a sulfate-reducing bacterium, elucidating the specific enzymatic mechanisms and metabolic pathways involved in this process.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer
Deep beneath the ocean floor, in the scalding, dark sediments where hot water meets cold rock, life finds a way to breathe without oxygen. In these extreme environments, vast reservoirs of natural gas seep upward from the Earth's crust. This gas is not just methane; it contains significant amounts of propane, a three-carbon molecule that is a major component of the fuel we use in our homes. For decades, scientists knew that certain bacteria could break down this propane in the absence of oxygen, using sulfate from the seawater to power the process. However, a crucial piece of the puzzle was missing. While researchers had recently discovered that ancient single-celled organisms called archaea could eat other small gas molecules like ethane and butane, no one had ever found an archaeon capable of consuming propane. This gap left a blind spot in our understanding of how carbon cycles through the deep Earth and how much of this potent greenhouse gas is naturally removed before it reaches the atmosphere.
A team of researchers has now filled that gap, identifying a specific type of heat-loving archaeon that thrives on propane. The discovery began with a culture of microbes originally grown to eat butane, collected from the Guaymas Basin, a hydrothermal vent field in the Gulf of California. When the scientists switched the food source from butane to propane, the community of microbes shifted dramatically. The dominant organism, a bacterium that had been eating the butane, disappeared. In its place, a different, less common archaeon took over, growing in tight, microscopic clusters with a partner bacterium. This new culture, which the researchers named Propane50, proved that this specific archaeon could not only survive on propane but could use it as its sole source of energy and carbon, driving a chemical reaction that converted sulfate into sulfide.
The organism at the heart of this process is a candidate species named Candidatus Syntrophoarchaeum caldarium. It does not work alone. It forms a symbiotic partnership with a sulfate-reducing bacterium called Candidatus Desulfofervidus auxilii. In this arrangement, the archaeon breaks down the propane, but it cannot dispose of the excess electrons that result from this breakdown. It passes these electrons directly to its bacterial partner, which uses them to reduce sulfate into sulfide. This exchange is so efficient that the two organisms grow together in dense, visible aggregates. The researchers confirmed that the archaeon is the one doing the heavy lifting of breaking down the propane, while the bacterium acts as the necessary electron sink, allowing the reaction to continue.
To understand exactly how this tiny organism eats propane, the scientists looked inside the cells. They found that the archaeon activates the propane molecule by attaching a specific helper molecule to it, creating a compound called propyl-coenzyme M. This step is the critical first move that makes the propane vulnerable to further breakdown. The researchers identified a specific enzyme complex, which they named ACR1, that performs this attachment. Interestingly, the archaeon's genome contains instructions for four different versions of this enzyme, but only one, ACR1, is actively produced in large quantities when the organism is eating propane. This suggests that ACR1 is the specialized tool for the job, capable of handling the three-carbon propane molecule with high efficiency.
Once the propane is activated, the archaeon converts it into a form that can be fully burned down to carbon dioxide. The path involves turning the propane derivative into a molecule called propionyl-CoA, and then rearranging it into acetyl-CoA, a central fuel for many life forms. Finally, the organism uses a well-known metabolic route, the reverse Wood-Ljungdahl pathway, to oxidize this fuel completely, releasing energy and carbon dioxide. The entire process is a sophisticated, multi-step assembly line that allows life to thrive in conditions that would be toxic to most other organisms.
This discovery changes how we view the underground carbon cycle. It proves that archaea are not just limited to eating methane or even-chain alkanes like ethane and butane; they can also tackle odd-chain molecules like propane. This finding suggests that these microscopic organisms play a much larger role in cleaning up natural gas seeps than previously thought. Because the way these archaea break down propane leaves a different chemical fingerprint than the way bacteria do, scientists can now use this knowledge to better interpret the composition of gases found in deep reservoirs. The work reveals a hidden layer of biological activity in the deep Earth, showing that even in the hottest, most anoxic sediments, life has evolved specialized tools to consume the very gases that fuel our modern world.
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