Most of us have at some point in our busy lives wished there were more hours in the day. That wish actually came true billions of years ago as the Earth’s rotation slowed down – and now a new study hypothesizes that this is why we have so much oxygen now, as photosynthesizing microbes took advantage of the productivity boost.
As vital as oxygen is to life now, there was a time when the residents of Earth would have considered it a poison. For the first half of the planet’s lifespan, carbon dioxide dominated the atmosphere, and early forms of life thrived in that environment. But about 2.4 billion years ago, cyanobacteria appeared on the scene, and began consuming carbon dioxide and releasing oxygen.
In an eerie parallel to our current climate situation, these microbes fundamentally changed the composition of the atmosphere, and wiped out a huge amount of the life that existed at the time. This Great Oxygenation Event didn’t happen overnight though – it took hundreds of millions of years, and exactly how and why it happened remain a mystery. But the new study investigated a possible cause.
“We do not fully understand why it took so long and what factors controlled Earth’s oxygenation,“ says Judith Klatt, lead author of the study. “But when studying mats of cyanobacteria in the Middle Island Sinkhole in Lake Huron in Michigan, which live under conditions resembling early Earth, I had an idea.”
In that low-oxygen environment, the team noticed a daily dance taking place between two types of bacteria forming layers in the microbial mat. Overnight, sulfur-eating bacteria sit on top, pulling nutrients from the water around them. But once the Sun comes up, the cyanobacteria beneath them rise to the top so they can sap energy from that light.
“Now they can start to photosynthesize and produce oxygen,” says Klatt. “However, it takes a few hours before they really get going, there is a long lag in the morning. The cyanobacteria are rather late risers than morning persons, it seems.”
The team realized that this lag could help explain the slow oxygenation of Earth. Days used to be much shorter than 24 hours – but over billions of years the Moon’s gravitational pull has slowed down the rotation of the planet, dragging out the day and increasing the number of daylight hours.
“When the Earth-Moon system formed, days were much shorter, possibly even as short as six hours,” says Brian Arbic, an author of the study. “Could this mean that changing daylength would have impacted photosynthesis over Earth’s history?”
The researchers found that on long-term scales, there’s a rough correlation between atmospheric oxygen levels and daylength. There’s a period of around a billion years where the rotation speed didn’t change much, and this coincided with low oxygen levels. Around 600 million years ago, the rotation started slowing again, and oxygen levels started to rise in what’s called the Neoproterozoic Oxygenation Event.
“I realized that daylength and oxygen release from microbial mats are related by a very basic and fundamental concept: During short days, there is less time for gradients to develop and thus less oxygen can escape the mats,” says Klatt.
The team modeled how sunlight dynamics influence the release of oxygen from microbial mats. They found that longer stretches of sunlight did indeed result in more oxygen being released, as the microbes have more time to be productive rather than stopping and starting.
“Intuition suggests that two 12-hour days should be similar to one 24-hour day,” says Arjun Chennu, an author of the study. “The sunlight rises and falls twice as fast, and the oxygen production follows in lockstep. But the release of oxygen from bacterial mats does not, because it is limited by the speed of molecular diffusion. This subtle uncoupling of oxygen release from sunlight is at the heart of the mechanism.”
It’s an intriguing idea, and further study could help fill in more blanks.
The research was published in the journal Nature Geoscience, and the team describes the work in the video below.
Source: Max Planck Institute
