Virtually all oxygen on Earth was and is produced by photosynthesis, which was invented by tiny organisms, the cyanobacteria, when our planet was still a rather uninhabitable place. Cyanobacteria evolved more than 2.4 billion years ago, but Earth only slowly transformed to the oxygen-rich planet we know today. “We do not fully understand why it took so long and what factors controlled Earth’s oxygenation,“ said geomicrobiologist Judith Klatt. “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.”
Cyanobacteria are late risers
Klatt worked together with a team of researchers around Greg Dick from the University of Michigan. The water in the Middle Island Sinkhole, where groundwater seeps out of the lake bottom, is very low in oxygen. “Life on the lake bottom is mainly microbial, and serves as a working analog for the conditions that prevailed on our planet for billions of years”, says Bopi Biddanda, a collaborating microbial ecologist from the Grand Valley State University. The microbes there are mainly purple oxygen-producing cyanobacteria that compete with white sulfur-oxidizing bacteria. The former generate energy with sunlight, the latter with the help of sulfur.
In order to survive, these bacteria perform a tiny dance each day: From dusk till dawn, the sulfur-eating bacteria lie on top of the cyanobacteria, blocking their access to sunlight. When the sun comes out in the morning, the sulfur-eaters move downwards and the cyanobacteria rise to the surface of the mat. “Now they can start to photosynthesize and produce oxygen,” explained 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.” As a result, their time for photosynthesis is limited to only a few hours each day. When Brian Arbic, a physical oceanographer at the University of Michigan, heard about this diel microbial dance, he raised an intriguing question: “Could this mean that changing daylength would have impacted photosynthesis over Earth’s history?”
Daylength on Earth has not always been 24 hours. “When the Earth-Moon system formed, days were much shorter, possibly even as short as six hours,” Arbic explained. Then the rotation of our planet slowed due to the tug of the moon’s gravity and tidal friction, and days grew longer. Some researchers also suggest that Earth’s rotational deceleration was interrupted for about one billion years, coinciding with a long period of low global oxygen levels. After that interruption, when Earth’s rotation started to slow down again about 600 million years ago, another major transition in global oxygen concentrations occurred.
After noting the stunning similarity between the pattern of Earth’s oxygenation and rotation rate over geological timescales, Klatt was fascinated by the thought that there might be a link between the two – a link that went beyond the “late riser” photosynthesis lag observed in the Middle Island sinkhole. “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,” Klatt hypothesized.
From bacterial mats to global oxygen
Overall, the two major oxygenation events (jumps in oxygen concentration) in Earth’s history – the Great Oxidation Event more than two billion years ago and the later Neoproterozoic Oxygenation Event – might be linked to increasing daylength. Hence, increasing daylength could have boosted benthic net productivity sufficiently to impact atmospheric oxygen levels. “Juggling with this wide range of temporal and spatial scales was mind-boggling – and lots of fun,” Klatt concludes.
IMAGE CREDIT: Phil Hartmeyer, NOAA Thunder Bay National Marine Sanctuary