Ancient ocean phosphorus cycle kept Earth’s oxygen levels stable

More than two billion years ago, Earth’s atmosphere underwent one of the most profound transformations in its history. Oxygen, a reactive gas that had been almost entirely absent from the air, began to accumulate permanently during what geologists call the Great Oxidation Event, dated to roughly 2.3 billion years ago. For decades, scientists have puzzled over what happened next. Once oxygen appeared, how did the planet manage to keep it? A new study led by researchers including University of California, Riverside geologist Andrey Bekker, published in Nature Communications, offers a compelling answer: the chemistry of the ancient oceans itself created a self-sustaining feedback loop that kept oxygen levels high enough to sustain life-friendly conditions over geological timescales.

The central player in this story is phosphorus, an element that most people rarely think about but that every living organism absolutely requires. Phosphorus forms the backbone of DNA and RNA, sits at the heart of energy-carrying molecules like ATP, and builds the structural molecules of cells. As Bekker explained, living things cannot grow or function properly without phosphorus. In the oceans, the amount of this nutrient available to microscopic organisms effectively sets the ceiling on how much biological productivity can occur. When marine organisms die and sink, their organic carbon is buried in sediments, and a crucial side effect of burying that carbon is the release of oxygen into the atmosphere. More available phosphorus therefore means more life, more carbon burial, and ultimately more oxygen accumulating in the air.

The research team found that as oxygen entered Earth’s oceans during and after the Great Oxidation Event, the concentration of sulfate, a chemical form of sulfur, also increased in seawater. This shift mattered enormously for the nutrient cycle. Microbes living in the sediments used the abundant sulfate to break down organic matter more efficiently through sulfate-driven respiration. In the process of decomposing that organic material, the microbes released phosphorus back into the seawater, where it became available once again to fuel new biological growth. The cycle thus reinforced itself: oxygen raised sulfate levels, sulfate boosted phosphorus recycling, recycled phosphorus fueled more life, more life buried more organic carbon, and that burial pumped additional oxygen into the atmosphere.

Demonstrating that this feedback loop operated billions of years ago required a methodological breakthrough. Until now, scientists could measure only the total amount of phosphorus preserved in ancient rocks, which made it extremely difficult to determine how much of that phosphorus had actually been available to support life when the rocks formed. Some phosphorus is bound in minerals that living systems can use, while other phosphorus is locked away in forms that are essentially inaccessible to organisms. A total measurement conflates the two, blurring the picture of nutrient conditions in ancient oceans.

To overcome this limitation, Bekker and his collaborators analyzed ancient rocks from South Africa using a new technique that separates phosphorus according to the different types of minerals it is attached to. By dissolving each mineral type one at a time, the method reveals whether the phosphorus in a given sample was available for biological functions or tied up in minerals that made it unavailable to living systems. The result, as Bekker put it, is that researchers can now separate the phosphorus that was available to organisms from phosphorus that was essentially locked away, giving a much clearer picture of nutrient levels in ancient oceans than was previously possible.

The findings carry implications that extend well beyond the chemistry of a single nutrient. They suggest that oxygen levels fluctuated far more dramatically after the Great Oxidation Event than scientists once believed. Rather than rising smoothly and stabilizing, atmospheric oxygen apparently swung through pronounced variations, and those swings likely reshaped ocean chemistry over tens of millions of years. The way nutrients cycled through the marine environment changed in response, which in turn fed back on biological productivity and atmospheric composition. Earth’s early atmosphere and oceans, in this view, were a dynamically coupled system rather than a simple one-way progression toward modern conditions.

These oscillations also bear directly on one of the deepest questions in the history of life: why did complex organisms take so long to appear? For much of the twentieth century, a popular explanation held that oxygen remained scarce for more than a billion years after the Great Oxidation Event, placing a hard physiological limit on the size and complexity that living things could achieve. The new evidence complicates that narrative. If oxygen remained abundant for extended periods, Bekker noted, then other environmental or biological factors, rather than a severely limited oxygen supply alone, may have slowed the emergence of more complex organisms. The search for those additional constraints, whether ecological, nutritional, or evolutionary, now becomes a central task for researchers studying the Proterozoic Eon.

The research also resonates with urgent questions about the modern ocean. Today, climate change is causing parts of the ocean to lose oxygen, a phenomenon visible in expanding dead zones and shrinking habitable volumes for fish and other marine animals. The ancient record suggests a worrying mechanism: if oxygen loss continues, phosphorus could once again become less available in seawater, reducing marine productivity and making marine ecosystems less resilient. In other words, the same feedback loop that helped stabilize Earth’s early oxygenated atmosphere could, when run in reverse, amplify the stresses that modern ocean life faces. Understanding the ancient coupling between oxygen and nutrients provides a long-term baseline against which present-day changes can be judged.

There is even an astrobiological dimension to the work. Scientists searching for life beyond Earth increasingly focus on ocean-bearing worlds, from icy moons in our own solar system to exoplanets orbiting distant stars. Because the new study shows how oxygen, nutrients, and life evolved together on our planet, it offers a template for recognizing similar coupled conditions elsewhere. Detecting an atmosphere rich in oxygen is one thing; understanding whether a planet’s oceans can recycle the nutrients needed to sustain a biosphere over billions of years is another. Earth’s history demonstrates that these connections matter, and Bekker emphasized that grasping them gives scientists a more nuanced perspective on our own planet’s future and on what to look for on other planets.

What makes the study remarkable is how a single element, measured with new precision in rocks that are billions of years old, can tie together so many threads of Earth’s story: the rise of the air we breathe, the productivity of the seas, the timing of life’s increasing complexity, the vulnerability of modern oceans, and the search for habitable worlds beyond our own. The Great Oxidation Event marked the moment oxygen became a permanent feature of the atmosphere, but this research shows that permanence was not guaranteed. It was earned and maintained by an oceanic chemical cycle in which microbes, sulfate, phosphorus, and buried carbon conspired, over immense stretches of time, to keep the planet breathing.

Subject of Research: The role of ancient ocean phosphorus and sulfate chemistry in sustaining atmospheric oxygen after the Great Oxidation Event

Article Title: How ocean chemistry helped life keep breathing

Article References: How ocean chemistry helped life keep breathing. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: Great Oxidation Event, ocean chemistry, phosphorus, sulfate, oxygenation, Andrey Bekker, UC Riverside, Nature Communications, Proterozoic, nutrient cycling, marine productivity, astrobiology