For its first three billion years, Earth’s atmosphere was vastly different from what we experience today. There was no free oxygen, and while nitrogen dominated—as it still does—the levels of other gases were strikingly different. Carbon dioxide was far more abundant, possibly up to 100 times higher than modern concentrations. The atmosphere also contained water vapor, trace amounts of hydrogen and carbon monoxide, and notably higher levels of methane.
Methane played a critical role during this early period, largely because it was produced by some of Earth’s first life forms known as methanogens. These microbes generated energy in an oxygen-free environment, using metabolic processes that released methane as a byproduct.
This ancient atmospheric landscape changed dramatically during the Great Oxygenation Event (GOE), which began about 2.4 billion years ago. The GOE marked the rise of a new kind of microbe called cyanobacteria. Unlike methanogens, cyanobacteria used photosynthesis to produce energy, releasing oxygen rather than methane as a byproduct. Over hundreds of millions of years, the oxygen produced by cyanobacteria accumulated in the atmosphere, ultimately enabling the emergence of complex life.
The availability of oxygen allowed lifeforms to harness far more energy through respiration, which is believed to have been a key driver in the evolution of multicellular organisms. However, new research reveals that the GOE could not have occurred without another critical element: phosphorus.
### The Critical Role of Phosphorus in the GOE
A recent study titled *“Marine phosphorus and atmospheric oxygen were coupled during the Great Oxidation Event,”* published in *Nature Communications*, sheds light on the indispensable role phosphorus played during this transformational time. The lead author, Dr. Matthew Dodd from the University of Western Australia’s School of Earth and Oceans, explains the interconnectedness of phosphorus availability, biological productivity, and oxygen levels during the GOE.
While carbon is often highlighted as essential for life—giving rise to the term “carbon-based life”—all the elements in the acronym CHNOPS (carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur) are vital. Of these, phosphorus acts as a throttle on biological activity: more phosphorus means greater biological productivity, while less phosphorus restricts it.
Earth’s crust contains abundant phosphorus; it is the 11th most abundant element and the backbone of DNA. However, up to 99% of Earth’s phosphorus is locked away in the core, bound in metal alloys and thus unavailable to support life. The new research shows that during the GOE, enough phosphorus was periodically released into the oceans to trigger blooms of photosynthetic microbes. These microbial blooms, fueled by phosphorus, increased organic carbon burial and led to significant oxygen accumulation in the atmosphere.
“By fueling blooms of photosynthetic microbes, these phosphorus pulses boosted organic carbon burial and allowed oxygen to accumulate in the air, a turning point that ultimately made complex life possible,” Dodd said in a press release.
### Tracing Phosphorus in Ancient Oceans
Investigating phosphorus levels from billions of years ago is no simple task. The GOE was a complex, multilayered event with multiple fluctuations in ocean chemistry across different regions.
One of the strongest pieces of evidence supporting the GOE comes from banded iron formations—alternating layers of iron oxides and iron-poor chert. These sedimentary features formed in seawater as a result of oxygen produced by photosynthetic bacteria.
The researchers’ findings are based on analyses of ancient carbonate rocks—sedimentary rocks like limestone and dolomite that form predominantly in marine environments. Carbonate minerals incorporate elements from seawater into their crystal structures in proportion to their ambient concentrations.
The team utilized a proxy called carbonate-associated phosphate (CAP) to estimate ancient ocean phosphate levels. CAP records phosphorus concentrations as preserved within carbonate minerals, factoring in seawater chemistry such as pH, alkalinity, temperature, and mineralogy.
Dodd and colleagues discovered that CAP variations tracked tightly with carbon isotope signatures that reflect biological activity and carbon burial. Their thousands of simulations revealed that transient spikes in oceanic phosphorus coincided with rapid oxygenation events and specific isotopic fingerprints in marine sediments.
“Using the carbonate-associated phosphate proxy, we reconstructed oceanic phosphorus concentrations during the GOE from globally distributed sedimentary rocks,” the authors state. “We find that CAP and the inorganic carbon isotope composition of marine sediments co-varied during the GOE, suggesting synchronous fluctuations in marine phosphorus, biological productivity, and atmospheric O₂.”
### What Was the Source of Phosphorus?
The Precambrian oceans had phosphate present but often chemically locked away. Iron in seawater scavenged phosphate, making it unavailable to life. Additionally, low sulfate concentrations limited efficient recycling of phosphorus bound in organic matter.
However, during parts of the GOE, these limitations eased, releasing pulses of bioavailable phosphorus. This surge in phosphorus availability accelerated photosynthesis in the oceans and allowed free oxygen to accumulate in the atmosphere.
“Oxygen is the hard currency of complex life, and when phosphorus levels rose in the early oceans, photosynthesis revved up,” Dodd explained. “When more organic carbon was buried, it resulted in oxygen being free to build in the atmosphere—that’s how Earth took its first big breath.”
### Implications for Astrobiology and the Search for Life
Understanding Earth’s oxygenation and nutrient cycles holds valuable clues for the search for life beyond our planet. Astrobiologists often use oxygen as a key biosignature because, on Earth, oxygen rose due to biological activity. Yet oxygen can also be produced abiotically, complicating interpretations.
This new research highlights phosphorus as a potentially critical biosignature because of its role in regulating biological productivity and oxygen buildup.
“Astronomers increasingly treat oxygen-rich atmospheres as prime targets in the search for life beyond Earth, but oxygen can, in principle, arise without biology,” said Dodd. “By identifying a nutrient throttle that couples oceans, biology, and the atmosphere, we offer a testable, biological pathway for creating and sustaining oxygen on living worlds.”
“We also provide a framework for interpreting oxygen detections on planets outside our solar system,” he concluded.
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This groundbreaking study enriches our understanding of the intricate interplay between Earth’s geology, oceans, biosphere, and atmosphere during one of the most pivotal transitions in planetary history. It underscores the essential role of phosphorus in powering the rise of oxygen and complex life on our planet—a story that may well echo across other worlds in the cosmos.
https://www.universetoday.com/articles/phosphorus-prepared-earth-for-complex-life-and-could-be-a-valuable-biosignature
