Researchers have pinpointed how trace levels of nickel and urea hindered the buildup of oxygen in Earth's atmosphere for over a billion years. Their experiments show these compounds limited cyanobacterial growth until conditions changed, triggering the Great Oxidation Event around 2.1 to 2.4 billion years ago. The findings offer insights into early Earth chemistry and potential biosignatures on other planets.
The Great Oxidation Event (GOE), which occurred approximately 2.1 to 2.4 billion years ago, transformed Earth's atmosphere by allowing oxygen to accumulate and support complex life. However, oxygenic photosynthesis by cyanobacteria had evolved hundreds of millions of years earlier, yet atmospheric oxygen levels remained low for an extended period. Scientists long puzzled over this delay, exploring factors like volcanic emissions and chemical sinks, but none fully accounted for it.
A team led by Dr. Dilan M. Ratnayake from the Institute for Planetary Materials at Okayama University, Japan—now at the Department of Geology, University of Peradeniya, Sri Lanka—investigated the role of trace compounds nickel and urea in cyanobacterial growth. Collaborators included Professors Ryoji Tanaka and Eizo Nakamura. Their study, published in Communications Earth & Environment in 2025, recreated Archean Earth conditions from 4 to 2.5 billion years ago through lab experiments.
In the first experiment, mixtures of ammonium, cyanide, and iron compounds were exposed to UV-C light, simulating pre-ozone layer radiation, to test natural urea formation. The second involved growing cyanobacteria (Synechococcus sp. PCC 7002) under varying light cycles with different nickel and urea concentrations, measuring growth via optical density and chlorophyll-a levels.
Results indicated that high nickel and urea in the early Archean restricted cyanobacterial blooms, preventing sustained oxygen release. As Dr. Ratnayake explained, "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." Declining nickel and stabilizing urea allowed cyanobacteria to thrive, driving oxygen accumulation and the GOE.
Dr. Ratnayake noted broader implications: "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 the Earth's conditions to make them suitable for the evolution of complex life, including our own." He added, "If we can clearly understand the mechanisms for increasing the atmospheric oxygen content, it will shed light upon the biosignature detection in other planets. 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."
These insights could inform Mars sample return missions and the search for life on exoplanets by highlighting how chemical balances influence oxygenation.