Tiny ocean ingredients held back Earth's first breath of oxygen

Two tiny ingredients in ancient seawater – nickel and urea – may explain why oxygen arrived late on Earth. A team in Japan tested this idea and found a chemical choke point that held back early oxygen-making microbes.

The Great Oxidation Event lifted oxygen in the air between about 2.4 and 2.1 billion years ago. The new study argues that trace chemistry controlled that timing in a direct, testable way.

The study was led by Dr. Dilan Ratnayake of Okayama University. His team focused on cyanobacteria – oxygen producing bacteria that transformed Earth’s surface chemistry.

“Here, we show the critical role of urea as a nitrogen source for cyanobacteria, the cascading impact of nickel on abiotic urea production, and their combined effects on the proliferation of cyanobacteria leading to the Great Oxidation Event,” said Dr. Ratnayake.

The nickel and urea balance

Urease, a nickel-using enzyme that converts urea into ammonia, sits at the center of this idea. Because urease needs nickel to work, any shift in oceanic nickel could slow or speed cyanobacterial growth.

Urea mattered not only inside cells; nickel also influenced the chemistry that produced urea outside them, tightening the link between metal supply and nutrient flow.

At lower levels, nickel lets urease run and urea feeds the cells. At higher levels, nickel and urea can stress cells and shut growth down.

That push and pull gives a simple handle on bloom timing in shallow waters. It blends metabolism with seawater chemistry without calling on unusual conditions.

Testing early Earth chemistry

The team generated urea by shining UV-C – short wave ultraviolet light between 200 and 280 nanometers – onto simple cyanide and ammonium mixtures. That exposure is plausible for early Earth without a shielding ozone layer.

In cultures of Synechococcus sp. PCC 7002, urea supported growth up to a clear upper bound. Beyond roughly 2 millimoles per liter, cells bleached and stalled, indicating a hard limit under those conditions.

Varying nickel at a fixed urea level revealed a fast growth peak near 136 nanomoles per liter. Above that point, growth slowed, consistent with stress from excess nickel.

On agar surfaces, cells tolerated much higher urea concentrations with less bleaching. That contrast points to reduced bioavailability on sticky surfaces, allowing mats to persist where free-floating cells could not.

Nickel’s role in Earth’s oxygen rise

Geological records indicate that ancient oceans held hundreds of nanomoles of dissolved nickel early on, then fell below about 200 nanomoles by 2.5 billion years ago.

That decline would have eased the metal pressure on cyanobacteria while also reshaping other parts of the carbon cycle.

As nickel dropped, methane-making microbes likely lost a key micronutrient. That loss would have trimmed methane in the air and reduced a shield that had kept oxygen in check on Earth.

Evidence for very ancient methanogenesis, biological methane-making from simple carbon and hydrogen, reaches back about 3.5 billion years in hydrothermal minerals. Those organisms would have felt the metal changes recorded in banded iron formations.

With urea forming photochemically and feeding cyanobacteria when nickel stayed moderate, local oxygen oases could last longer. As both urea and nickel drifted toward gentler levels, blooms could persist long enough to lift global oxygen.

Sunlight, nickel, and urea reactions

Sunlight in the shortwave band can power reactions that build small nitrogen-containing compounds. The lab work shows that those compounds emerged readily under controlled exposure and simple starting materials.

Inside cells, the nickel site of urease must be assembled with care to avoid free metal toxicity. Biochemists have mapped that assembly line across bacteria, with dedicated carrier proteins steering nickel to the active site.

Early seas did not need extreme settings to toggle oxygen production. They needed trace metal levels and small nitrogen supplies to slip into the right window.

Once oxygen on Earth climbed, UV-C waned as ozone formed, and urea production likely slowed. That feedback would have nudged cyanobacteria toward other nitrogen sources as ecosystems matured.

What it means for life elsewhere

Scientists often look for biosignatures, a chemical or pattern that signals life, on planets beyond Earth. Oxygen is one sign, yet it depends on hidden controls.

This work adds a different target list for telescopes and field teams. Trace nickel and urea proxies could show whether oxygen-rich atmospheres are even possible on young worlds.

It also offers a plan for returned samples from Mars. Measurements of nickel-rich phases and nitrogen-bearing residues could flag past chemical pathways, even without intact cells.

No single marker will settle the case. Together, metal budgets, small nitrogen compounds, and oxygen tracers can narrow the search.

The study is published in the journal Communications Earth & Environment.

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