Life as we know it was only able to flourish when the Earth began to spin more slowly and the days became longer, a study has shown — because the microbes that produced our planet’s oxygen are late risers.
While the Earth now rotates on its axis once every 24 hours, the length of a day was possibly as brief as six hours during the planet’s infancy.
The new research suggests that a slower rotation, which caused longer days, was key because it allowed bacteria to use sunlight to produce oxygen through photosynthesis.
“An enduring question in the Earth sciences has been how did Earth’s atmosphere get its oxygen, and what factors controlled when this oxygenation took place,” Gregory Dick, a professor in geomicrobiology at the University of Michigan, said. “Our research suggests that the rate at which the Earth is spinning — in other words, its day length — may have had an important effect on the pattern and timing of Earth’s oxygenation.”
Earth’s rotation rate has been slowly decreasing for billions of years, largely due to the tug of the moon’s gravity, which creates tidal friction. The researchers argue that this had an effect on the organisms credited with providing Earth’s oxygen.
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Cyanobacteria, also known as blue-green algae, are often spoken of unfavourably because they are the main culprits behind toxic algal blooms. But they have existed for more than 2.4 billion years and were the first organisms to harvest energy from sunlight through photosynthesis, which results in oxygen being released.
Earth was transformed from a planet with vanishingly small amounts of oxygen to one where it forms about 21 per cent of the atmosphere.
To gain an insight into conditions on the early Earth, the researchers looked at the Middle Island Sinkhole in Lake Huron, Michigan. In this deep depression in the lake bed, purple oxygen-producing cyanobacteria compete with white sulphur-oxidising bacteria that use sulphur, not sunlight, as their main energy source.
Life on the lake bottom is mainly microbial, and resembles the conditions that prevailed on our planet for billions of years, Bopi Biddanda, of Grand Valley State University, said.
In a microbial dance repeated daily at the bottom of the sinkhole, the purple and white microbes jockey for position as the day progresses.
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The white sulphur-eating bacteria physically cover the purple cyanobacteria in the morning and evening, blocking their access to sunlight and preventing them from producing oxygen via photosynthesis.
When sunlight levels increase to a critical threshold, the sulphur-oxidising bacteria sink below the photosynthetic cyanobacteria, enabling them to start producing oxygen.
The researchers suggest that a similar competition took place on the early Earth. They suggest that a lengthening of the days, caused by the planet’s slowing rotation, helped the cyanobacteria flourish and, with them, other life.
The longer days were important as it takes the cyanobacteria a few hours to get going. “There is a long lag in the morning,” Judith Klatt, of the Max Planck Institute for Marine Microbiology, said. “The cyanobacteria are rather late risers than morning people, it seems.”
The study is published in the journal Nature Geoscience.
