Photosynthesis was developed by microscopic creatures called cyanobacteria when our planet was still a pretty inhospitable place, generated nearly all of the oxygen on Earth, and continues to do so now.
Cyanobacteria first appeared around 2.4 billion years ago, but Earth only gradually developed into the oxygen-rich world we see today.
"We don't know why it took so long or what variables governed Earth's oxygenation," said Judith Klatt, a geomicrobiologist.
"However, I had an idea when researching cyanobacteria mats at the Middle Island Sinkhole in Lake Huron, Michigan, which thrive in the circumstances similar to those of early Earth."
Klatt collaborated with a group of University of Michigan researchers led by Greg Dick. The Middle Island Sinkhole, where groundwater leaks out of the lake's bottom, has extremely low oxygen levels.
According to Bobi Biddanda, a cooperating microbial ecologist from Grand Valley State University, "life on the lake bottom is mostly microbial and offers us a working analog for the circumstances that existed on our planet for billions of years."
Purple oxygen-producing cyanobacteria compete with white sulfur-oxidizing bacteria in this environment. The former uses sunlight to create energy, whereas the latter uses sulfur.
Until life, these bacteria engage in a daily dance: from nightfall to dawn, sulfur-eating bacteria lie on top of cyanobacteria, limiting their access to sunlight. The sulfur-eaters migrate lower when the sun rises in the morning, and the cyanobacteria ascend to the mat's surface.
Klatt stated, "Now they can start photosynthesizing and producing oxygen. However, it takes a few hours for them to really start rolling in the morning; there is a significant lag. The cyanobacteria appear to be more of a night owl than a morning person."
As a result, their photosynthetic period is restricted to a few hours every day. So when Brian Arabic, a physical oceanographer at the University of Michigan, learned about this diel microbial dance, he wondered aloud, "Could this indicate that altering day length would have influenced photosynthesis across Earth's history?"
Earth's day duration has not always been 24 hours. "Days were significantly shorter when the Earth-Moon system originated, maybe as short as six hours," Arabic added. Days became longer when our planet's spinning slowed due to the drag of the Moon's gravity and tidal friction.
Some experts believe Earth's rotational slowdown was halted for nearly a billion years, coinciding with a protracted period of low worldwide oxygen levels. Another significant change in global oxygen concentrations happened after that hiatus, some 600 million years ago when Earth's rotation began to slow down again.
Possible Relationship Between Earth's Rotation and Oxygenation
Klatt was interested in the idea that there may be a relationship between the pattern of Earth's oxygenation and rotation rate throughout geological periods, a link that went beyond the "late riser" photosynthetic lag seen in the Middle Island sinkhole.
"I discovered that day length and oxygen release from microbial mats are linked by a very basic and fundamental concept: there is less time for gradients to develop during short days, and hence less oxygen can leave the mats," Klatt theorized.
Klatt joined up with Arjun Chennu, who worked at the Max Planck Institute for Marine Microbiology at the time and now runs his own lab at Bremen's Leibniz Centre for Tropical Marine Research (ZMT).
They studied how solar dynamics connect to oxygen release from the mats using open-source software created by Chennu for this work. "My intuition tells me that two 12-hour days are equivalent to one 24-hour day.
The sun rises and sets twice as rapidly, and the creation of oxygen keeps pace. The release of oxygen from bacterial mats, on the other hand, does not, since molecular diffusion speed is limited. "At the heart of the mechanism is a slight uncoupling of oxygen release from sunlight," Chennu explained.
Klatt and her colleagues incorporated these findings into global oxygen levels to better understand how daily processes affect long-term oxygenation. According to the results, the enhanced oxygen release caused by the shift in day length may have improved oxygen levels globally. But, again, it's a connection between the actions of tiny creatures and the activities that take place on a larger scale.
"From molecular diffusion to planetary mechanics, we connect principles of physics functioning at very different scales. We show that day duration has a basic relationship with how much oxygen ground-dwelling microorganisms may release," Chennu added.
"It's a lot of fun. Therefore, the dance of the molecules in the microbial mat is linked to the dance of our planet and its Moon."
Largest Oxygenation Episodes
Overall, the two largest oxygenation episodes in Earth's history - the Great Oxidation Event, which occurred more than two billion years ago, and the Neoproterozoic Oxygenation Event, which occurred later - may be connected rising daylength
As a result, increasing day length may have increased benthic net production to affect atmospheric oxygen levels. Klatt writes, "Juggling with this wide variety of temporal and geographic scales was mind-boggling - and lots of fun."
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