Researchers studying the unique environment of Lake Huron in Michigan have used it to effectively look at the ancient Earth and how changing daylight length might affect the oldest existing life forms: microorganisms called cyanobacteria.
Also called blue-green algae, cyanobacteria evolved over 2.4 billion years ago, producing oxygen when Earth was still inhospitable. Scientists have struggled to explain why it has taken so long for oxygen levels on Earth to gradually rise over the course of nearly two billion years — until now.
Cyanobacteria do not have a good reputation these days because they are associated with toxic algal blooms in Lake Erie and other bodies of water. But these bacteria have been around for longer than any other life form on Earth, and they were the first to convert light into energy through photosynthesis — releasing oxygen as a byproduct.
Researchers are beginning to wonder how the increased day length on early Earth might have allowed cyanobacteria to create more oxygen and lead to a diversification of animal life.
“When the Earth-Moon system formed, the days were much shorter, perhaps even six hours,” study co-author Brian Arbeck, a University of Michigan physicist oceanographer, said in a statement. “Could this mean that changing the length of the day will affect photosynthesis over Earth’s history?”
As the Moon became Earth’s satellite, the gravitational pull of the Moon slowed our planet’s rotation rate, resulting in longer days. More sunlit hours will have a positive effect on the photosynthetic activity of cyanobacteria.
The answers in the sink
Underneath Lake Huron is a bedrock formed by the ancient seas that once covered the North American continent. This rock includes limestone, dolomite and gypsum, and over time, groundwater dissolved some of it. This has formed fissures and caverns, both of which create submerged holes.
The central island sinkhole in Lake Huron is where cold, sulfur-rich and oxygen-poor groundwater seeps from the lake bed. While most plant and animal life avoids this area, microbes have found a home in this harsh environment 80 feet (24.4 meters) below the surface of the water. The brightly colored bacteria form colonies called microbial mats, the perfect counterpart for researchers wanting to study similar colonies that once existed on the land and sea floor billions of years ago.
Today, two types of cyanobacteria form competing colonies here. One is a blue-purple bacteria that produces oxygen, while the other is a white bacteria that generates energy with the help of sulfur.
Sulfur bacteria lie on top of the cyanobacteria from dusk until dawn, blocking their access to sunlight. But as soon as the sun appears, the upper bacteria colony moves down and allows the purple cyanobacteria to start photosynthesis to produce oxygen.
“However, it takes a few hours before it really starts, there is a long delay in the morning. The cyanobacteria seem to be somewhat later than they get up in the morning,” said study author Judith Klatt, a microbiologist at the Max Planck Institute. for Marine Microbiology of Germany, in a statement.
“I realized that the length of the day and the release of oxygen from the microbial mats are linked to a very basic and basic concept: During short days, there is less time for gradients to develop, and therefore less oxygen can escape from the mats,” Klatt said.
By modeling the link between sunlight and oxygen production, Klatt and her colleagues discovered that the release of oxygen during two 12-hour days early on Earth would not match a 24-hour day. The study team’s findings showed a direct link between day length and the amount of oxygen that microbes could release.
Simply put, there is less time for oxygen to leave the rug on shorter days,” Klatt said.
This suggests that two major leaps in oxygen on Earth, including the Great Oxidation Event over 2 billion years ago and the Neoproterozoic Oxygenation event between 800 and 540 million years ago, could be related to longer Earth days.