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The appearance of oxygen in Earth's atmosphere was a turning point in the history of our planet, forever transforming the environment and setting the stage for complex life. This event, known as the Great Oxidation Event (GOE), occurred roughly 2.1 to 2.4 billion years ago.

Yet, although oxygenic photosynthesis in cyanobacteria is thought to have evolved hundreds of millions of years earlier than this event, oxygen levels in the atmosphere remained low for a prolonged period. Scientists have debated why Earth's oxygenation was delayed, exploring factors ranging from volcanic gases to microbial activity. However, a complete explanation has been elusive.

A recent study set out to address that mystery by examining an often overlooked but crucial factor, the influence of trace elements and compounds, specifically nickel and urea, on cyanobacterial growth.

Lead researcher, Dr. Dilan M. Ratnayake from the Institute for Planetary Materials, Okayama University, Japan (current address is Department of Geology, University of Peradeniya, Sri Lanka), explained, "Generating oxygen would be a massive challenge if we are ever to colonize another planet. Therefore, we sought to understand how a tiny microbe, cyanobacteria, was capable of altering Earth's conditions to make them suitable for the evolution of complex life, including our own. The insights gained from this study will also provide a new framework for the sample analysis strategies for future Mars sample return missions."

Professor Ryoji Tanaka and Professor Eizo Nakamura from the Institute for Planetary Materials, Okayama University, also contributed to the study. The findings of the study were published in the journal on August 12, 2025.

To explore the role of trace elements and compounds in early Earth environments, the researchers conducted a two-part experimental study simulating Archean conditions (approximately 4–2.5 billion years ago). In the first set of experiments, mixtures of ammonium, cyanide, and iron compounds were exposed to ultraviolet light (UV)-C radiation, mimicking the UV that likely reached Earth's surface before the formation of the ozone layer. These trials tested whether urea, an essential nitrogen source, could form abiotically under prebiotic conditions.

In the second part, cyanobacterial cultures (Synechococcus sp. PCC 7002) were grown under controlled light–dark cycles with varying urea and nickel concentrations in their growth media. Growth was tracked by optical density and chlorophyll-a level to assess how these compounds influenced proliferation.

From these findings, the researchers propose a new theoretical model of Earth's oxygenation. In the early Archean, high nickel and urea concentrations acted as bottlenecks, keeping cyanobacterial blooms rare and short-lived. As Dr. Ratnayake explains, "Nickel has a complex yet fascinating relationship with urea regarding its formation as well as its biological consumption, while the availability of these at lower concentrations can lead to the proliferation of cyanobacteria." This sustained expansion ultimately drove long-term oxygen release and triggered the GOE.

The real-world implications of this work are far-reaching. "If we can clearly understand the mechanisms for increasing the atmospheric oxygen content, it will shed light upon the biosignature detection in other planets," shares Dr. Ratnayake. "The findings demonstrate that the interplay among inorganic and organic compounds played crucial roles in Earth's environmental changes, deepening our understanding of the evolution of Earth's oxygen and hence the life on it."

Beyond Earth, the results may guide strategies for detecting biosignatures, as chemical markers like nickel and urea could influence oxygen buildup, and thus the potential for life on exoplanets.

This study uncovers how nickel and urea shaped the timing of Earth's oxygen evolution. By experimentally confirming urea production under Archean conditions and demonstrating its dual role, both as a nutrient and as a potential inhibitor at high levels, the research reframes how we think about early life's ecological constraints. Ultimately, it shows that the decline of nickel and moderation of urea paved the way for cyanobacterial expansion and the rise of oxygen, providing a clearer picture of how Earth transitioned to a habitable world.