The Breath of Sterile Soil

For fifteen years, Sébastien Fontaine, a biochemist affiliated with the French National Institute for Agriculture, Food, and Environment, pursued a scientific objective that seemed virtually impossible: to kill dirt. His primary goal was to determine the precise quantity of carbon released by soil samples that were entirely devoid of living organisms. To achieve this state of absolute sterility, Fontaine’s research team sealed soil samples inside glass jars and subjected them to intense gamma radiation. This high-energy process was designed to obliterate every living microbe, thereby effectively sterilizing the soil substrate.

After the radiation treatment, the scientists entered a period of careful observation. They meticulously monitored the sealed jars for carbon dioxide, a gas typically released by living organisms as a byproduct of respiration. If the soil had been truly lifeless, the release of this gas should have ceased almost immediately. Instead, the experimental results defied all expectations. The soil continued to emit carbon dioxide for weeks, then for months, and eventually for years. Under microscopic examination, the irradiated soil revealed absolutely no signs of biological life. There were no bacteria, no fungi, and no visible organisms. Yet, the soil would not cease its apparent "breathing."

Fontaine’s laboratory repeated these experiments numerous times to ensure that the results were not the product of experimental error. Convinced that their experimental setup was correct and that their sterilization methods were thoroughly effective, they began to investigate the source of this mysterious respiration in dead soil. In a 2025 paper published in the journal Science Advances, Fontaine and his colleagues reported a startling finding: their soil samples continued to consume oxygen and release carbon dioxide for six years. They proposed that a metabolic process, which typically powers living cells, can also occur outside of living bodies. Their experiments suggest that the complex chemistry associated with life is not exclusive to life itself.

Joseph Moran, an organic chemist at the University of Ottawa who was not involved in the research, noted that these experiments reveal what happens to biological molecules when they are left alone. "It’s the chemistry of geology," he said. This perspective suggests that some biochemical reactions, such as those that release energy from sugar, may not be unique to living things. Fontaine believes that these reactions could even predate the emergence of life on Earth.

Fontaine’s accidental discovery occurred while he was attempting to establish a baseline for carbon content in lifeless soil. His team used a sterile syringe to sample the air inside sealed jars containing soil. They measured the carbon content using a mass spectrometer. After the radiation killed the microbes, the rate of carbon emission dropped quickly. However, it did not disappear completely. The emissions remained stable for over one hundred days.

When Fontaine shared these initial results with other researchers, they advised him to ignore them. Colleagues suggested that the results were an experimental error, or an "artifact," and were not worth investigating. But Fontaine refused to give up. He needed to understand whether a metabolic process was occurring in sterile soil. Metabolism is a precise sequence of chemical reactions that usually requires enzymes, which are biological catalysts that speed up chemical processes.

To test this hypothesis, his team added enzymes extracted from yeast to the soil. Immediately, the carbon emissions spiked. Fontaine speculated that the enzymes had speeded up a reaction that was already happening on its own. This was a crucial clue. It suggested that the soil contained the necessary chemical components for the reaction, even without living cells present.

Convincing the scientific community was difficult. When Fontaine submitted his findings to journals, reviewers were divided. Some were positive, while others were highly suspicious about the sterility of the soil. The results were finally published in the journal Biogeosciences in 2013. Despite this publication, Fontaine remained unsatisfied. He wanted to definitively prove that his soil samples were completely free of life, leaving no room for doubt.

Over the next decade, his lab worked tirelessly to eliminate any possibility of contamination. They employed more extreme methods to kill the soil, including higher levels of radiation, greater pressure, and increased heat. Despite these efforts, the soil continued to emit carbon for months. At one point, a graduate student named Benoit Kéraval found cells in the irradiated soil under an electron microscope. However, staining tests showed no RNA or DNA in these cells, indicating they were definitely dead. When the team added live microbes to the soil, the cells rapidly recolonized the area and released much more carbon dioxide. This confirmed that the low-level emissions in the sterilized samples were not due to leftover living organisms.

By 2018, when Clémentin Bouquet joined the lab, the team was confident in their findings. They were ready to dig deeper into the underlying mechanisms of this phenomenon.

For six years, Bouquet and Kéraval studied two sets of sealed, irradiated soil samples. One set consisted of normal soil, and the other was supplemented with glucose, a type of sugar. They took regular air samples for 142 days. The daily rate of carbon dioxide emissions declined but never disappeared. Then, the samples sat in an incubator for over 1,000 days. During this long period, the researchers focused on other experiments.

When they measured the samples again at 1,606 and 2,442 days, the emissions had slowed further. However, the soil was still breathing. The samples with added glucose showed higher emission rates. This strengthened Fontaine’s suspicion that non-biological materials in the soil could induce reactions similar to the metabolic breakdown of sugar.

In living cells, sugar is broken down into smaller carbon molecules. This process feeds the Krebs cycle, a series of reactions that strip high-energy electrons from carbon-rich molecules. These electrons then pass through reactions that consume oxygen. Many scientists found it difficult to believe this complex process could happen outside a cell. Cells contain enzymes that keep everything organized and increase the chances that molecules will interact efficiently.

Fontaine devised a fuel cell to detect electrons moving through the soil as an electric current. His team added soil that had been irradiated almost five years earlier and closed the circuit. A current passed through the soil that was several times higher than in a control setup with saltwater. Fontaine stated that this demonstrated that sterile soil supports a flow of electrons. This flow is indicative of processes that resemble the oxygen-dependent metabolism of the Krebs cycle.

In a 2025 preprint, Fontaine and his colleagues reported observing four of the eight intermediate molecules known to be part of the Krebs cycle in six-month-old sterile soil samples. Many of these molecules formed after the irradiation. The authors suggested that clumps of earth can indeed catalyze these reactions without the presence of life.

Joshua Schimel, a soil ecologist at the University of California, Santa Barbara, was not surprised by Fontaine’s findings. He explained that glucose naturally forms some Krebs-cycle intermediates when it is oxidized. Many soils are rich in iron oxides and aluminum oxides, which can catalyze this conversion.

The idea that metals can catalyze biochemical reactions is central to a theory about the origins of life. Metals such as iron and zinc are at the core of many ancient enzymes found in all life forms. Some researchers, including Moran, believe these metals might have catalyzed these reactions before life emerged. Studies suggest that the chemical reactions that break down glucose derivatives might have existed before the enzymes and genes that enable them in living cells.

Moran argued that we should organize our thoughts about life differently. "We should put metabolism at the base of what life is doing, and then genes are a way of controlling that at a higher level," he said.

Markus Ralser, a biochemist at Charité University Hospital in Berlin, agreed that cell-free metabolic reactions could be more common than previously thought. "This fits a bit into my thinking about how metabolism started in evolution," he said. He argued that if these processes were very hard to achieve, the planet would not be full of life now. However, this idea is complicated by the low-oxygen conditions in which life arose.

Another explanation for the results is that enzymes released from irradiated cells might still be active. Even when degraded, enzyme backbones might be capable of catalyzing reactions. Sudha Rajamani, an astrobiologist in India, noted this possibility. Ralser also agreed, stating his gut feeling is that there are still many enzymes in Fontaine’s soil samples.

To prove that metals and minerals carry out these reactions spontaneously, researchers would need to eliminate all enzymes. This is very difficult because it would require heating the soil to temperatures that would damage its structure. However, Bouquet noted that enzyme activity diminishes exponentially after leaving cells. Fontaine added that no enzyme is known to last six years. While enzymes from living or recently dead cells contribute to carbon emissions in real-world soils, Fontaine believes it is very unlikely that they caused the long-term respiration observed in his experiments.

For Bouquet, this years-long obsession has highlighted a profound truth. "Even in a context as close and familiar to us as terrestrial soil, we are not always able to distinguish or recognize processes that indicate the presence or absence of living organisms," he said. Now a researcher at the Collège de France, he is looking for prebiotic origins of other biochemical cascades. "I find it particularly interesting to imagine the survival of processes that may predate life itself, right there under our feet."