The Earth's oxygen levels are increasing? The reason is that its rotation is slowing down! How is this possible?

More sunlight helps oxygen escape from the microbial layer.

A bobot fish rests on rocks covered with purple and white microbial mats in a sinkhole on Middle Island in Lake Huron.

(Photo credit: Phil Hartmeyer, National Oceanic and Atmospheric Administration, Thunder Bay National Marine Sanctuary)

Now there's a new idea for how Earth became an oxygen-rich planet: As the planet's rotation slowed, microbes were bathed in sunlight for longer periods of time, which sped up the rate at which they released oxygen into the atmosphere.

You can breathe freely because billions of years ago, the first life on Earth, dense colonies of cyanobacteria, began producing oxygen through photosynthesis. But scientists are still unsure what triggered the two transformative oxygenation events that transformed Earth from a low-oxygen planet to an oxygen-rich world where complex life could evolve and diversify.

Now, researchers have identified an important factor that may have spurred the release of oxygen produced by microbes: the slowing of Earth's rotation that began about 2.4 billion years ago. When Earth was new, it spun faster, completing a full rotation in a few hours, but over hundreds of millions of years, it gradually slowed down. Day length reaches a certain threshold, and it may be during those critical oxygenating hours that longer periods of light allow more oxygen molecules to diffuse from areas of high concentration (bacterial mats) to areas of lower concentration (the atmosphere), according to a new study.

Scientists recently found clues to that connection in a sinkhole at the bottom of Lake Huron, one of the world's largest freshwater lakes bordered by Michigan and Ontario, Canada. The Middle Island sinkhole in the lake is 300 feet (91 meters) in diameter and lies about 80 feet (24 meters) below the water's surface. There, sulfur-rich water nourishes colorful microbes that thrive in low-oxygen environments, much like the earliest bacteria on Earth.

In the frigid depths of the sinkhole live two types of microbes: purple cyanobacteria that produce oxygen through photosynthesis and seek out sunlight, and white bacteria that consume sulfur and release sulfate. These microbes jockey for position throughout the day, and in the mornings and evenings, the sulfur-eating bacteria cover their purple neighbors, blocking the purple microbes from reaching the sun. However, when daylight is strongest, the white microbes avoid the light and burrow deeper into the ground, exposing the purple cyanobacteria so they can photosynthesize and release oxygen.

Billions of years ago, similar competition may have occurred among microbial communities, with oxygen-producing bacteria's microbial neighbors blocking their access to sunlight, the researchers wrote. Then, as days grew longer on Earth, oxygen-makers spent more time in sunlight and released more oxygen into the atmosphere.

"We realized that there is a fundamental connection between photodynamics and oxygen release, based on the physics of molecular diffusion," when thermal changes cause molecules to migrate from areas of high to low concentrations, says lead author Judith Kratt, a research scientist at the Max Planck Institute for Marine Microbiology in Bremen, Germany.

"Even if the same amount of oxygen is produced each hour, a shorter day would allow less oxygen to escape the mat," Kratt told Live Science in an email.

Purple microbial mats in the Center Island sinkhole in Lake Huron in June. Hills and "fingers" like the one on the mat are caused by gases like methane and hydrogen sulfide bubbling up beneath them. (Image credit: Phil Hartmeyer, Thunder Bay National Marine Sanctuary, NOAA)

Spin period

Today, Earth completes a full rotation on its axis every 24 hours, but four billion years ago, a day lasted only about six hours, researchers report. For billions of years, Earth's ongoing dance with the moon was slowed by a process called tidal friction, which slowed the planet's rotation.

As Earth spins, the moon (and to a lesser extent the sun) pull on the planet's oceans. This stretches the oceans, moves them away from the center of the planet, sucking energy from the spin and slowing it down, said study co-author Brian Arbic, a professor in the Department of Earth and Environmental Sciences in the U-M College of Literature, Science and the Arts.

The slowdown was tiny, but over hundreds of millions of years it added a few hours of daylight; the slowdown continues today, Arbic told Live Science in an email.

"Tidal friction will continue to slow down the rotation - and the days will continue to get longer over geological time," Abick said.

A diver observes purple, white and green microorganisms covering the rocks at the Middle Island Sinkhole in Lake Huron. (Image credit: Phil Hartmeyer, NOAA/Thunder Bay National Marine Sanctuary)

Get some fresh air

The researchers simulated different scenarios for day length and oxygen escape from the microbial mat. When they compared their model with an analysis of competing microbial mats sampled from the Nakajima cenote, they found that their predictions were confirmed: photosynthetic bacteria released more oxygen when the days were longer.

This isn't because the microbes are photosynthesizing more; rather, it's because longer hours of sunlight mean more oxygen escapes from the mats throughout the day, says study co-author Arjun Chennu, a research scientist at the Leibniz Centre for Tropical Marine Research in Bremen.

"A subtle uncoupling of oxygen release from sunlight lies at the heart of the mechanism," Chennu said in a statement.

Earth's atmosphere took shape after the planet formed and cooled about 4.6 billion years ago, and is primarily composed of hydrogen sulfide, methane and carbon dioxide (CO2) - 200 times the amount of CO2 present in the atmosphere today, according to the Smithsonian Environmental Research Center.

That all changed after the Great Oxygenation Event (GOE) about 2.4 billion years ago, followed by the Neoproterozoic Oxidation Event about 2 billion years later, which boosted atmospheric oxygen to today's levels of about 21%. Both oxygenation events were previously thought to be linked to the activity of photosynthetic cyanobacteria, and this new evidence suggests that another factor may have been that Earth's days -- "a factor that had been largely unconsidered before" -- became long enough to trigger microbial mats to release more oxygen, "in parallel to other previously proposed drivers of oxygenation," Kratt said.

BY: Mindy Weisberger

FY: Yin Yiting

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