Before altering the air, microbes oxygenated large swaths of the sea

Ancient oxygen-making microbes may have oxygenated large swaths of Earth’s seafloor hundreds of millions of years before the element filled the atmosphere.

Geochemical analysis of sediments deposited roughly 2.6 billion years ago reveals that pulses of oxygen may have swept through large regions of the ocean, researchers report April 26 in Nature Geoscience. The findings suggest that cyanobacteria, the microorganisms responsible for oxygenating Earth’s atmosphere, were more widespread at the time than previously believed.

This shows that not only had cyanobacteria already evolved, but they were around in vast numbers and had even oxygenated the seafloor, says geochemist Kurt Konhauser of the University of Alberta in Edmonton, Canada, who was not involved in the study. And that, he says, means aerobic organisms might have evolved on the seabed long before oxygen permeated the sky.

Roughly 2.4 billion years ago, atmospheric oxygen levels soared for the first time due to the photosynthetic activities of cyanobacteria. This profound change was called the Great Oxidation Event, and it would forever alter the trajectory of life’s evolution.

Studies of chemicals in primitive seafloor sediments suggest cyanobacteria had evolved to photosynthesize hundreds of millions of years before the Great Oxidation Event. Some scientists believe that during that interval, the microbes were restricted to sites in the ancient ocean known as oxygen oases.  However, it has remained unclear exactly how extensive these aquatic nurseries of photosynthetic life may have been.

For the new study, geochemist Xinming Chen of Shanghai Jiao Tong University in China and colleagues studied ancient shales from Australia and South Africa, focusing on concentrations of the element thallium. In oxygen-rich seawater, manganese oxides form and strip the water of thallium’s heavier forms, or isotopes. This leads to less of the heavy thallium making its way into shale layers forming on the seafloor.

But for this signature to form, oxygen must be present in and along the seabed as well. So, by measuring abundances of thallium isotopes in ancient shales, Chen’s team aimed to find evidence of deep, regional oxygenation in the sea.

The thallium record revealed that the ocean became at least regionally oxygenated around 2.65 billion and 2.5 billion years ago. These bouts were separated by an interval that was devoid of oxygen. Oxygen levels oscillated in the ancient seas, Chen explains. “It’s not just continuous or just in one direction.”

What’s more, the 2.5-billion-year-old bout of oceanic oxygenation that Chen’s team detected coincided with oxygenation discovered by another group of researchers in a different Australian shale formation. “We’re about 1,000 kilometers away,” Chen says. That suggests the oxygen pulse encompassed a broad area, probably in a shallow, near-shore setting along a continental shelf, he says.

The method that Chen’s team used to search for ancient oxygen on Earth could also assist the search for life on other planets. If the formation of manganese oxides remains the only known process that can generate these thallium signatures, Konhauser says, “this could be potentially a really interesting biosignature.”