Scuba divers observe the purple, green and white microbes that cover rocks in Lake Hurons Middle Island Sinkhole. Credit: Phil Hartmeyer (NOAA Thunder Bay National Marine Sanctuary).The remarkable diversity of animal life was possible because of the rapid rise in oxygen levels at the beginning of Earth's history. Scientists have been trying to understand the mechanisms behind this gradual, stepwise evolution for nearly two billion years.A team of international researchers has proposed that the increasing length of days on Earth's early Earth may have contributed to an increase in the oxygen release by photosynthetic bacteria. This could have affected the timing of Earth’s oxygenation.The study of current-day microbial communities at the bottom 80 feet below the surface of Lake Huron's submerged sinkhole inspired their conclusion. The Middle Island Sinkhole water is rich in sulfur, low in oxygen, so the brightly colored bacteria that flourishes there can be considered an analog for single-celled organisms billions of years ago that created mat-like colonies that covered both land and seafloors.Researchers found that photosynthetic microbes release more oxygen when the day is longer. This finding points out a previously unknown link between Earth's oxygenation rate and Earth's history. Although the Earth spins on its axis now once per 24 hours, the Earth's initial days were only 6 hours long.The findings of the team are due to be published in Nature Geoscience on Aug. 2.Judith Klatt, of the Max Planck Institute for Marine Microbiology, and Arjun Chennu, of the Leibniz Centre for Tropical Marine Research are the lead authors. Klatt was a former postdoctoral researcher at the University of Michigan Geomicrobiologist Gregory Dick's lab. He is also one of the corresponding authors. Other co-authors include U-M and Grand Valley State University.Dick stated, "An enduring query in the Earth sciences was how Earth's atmosphere got its oxygen and what factors controlled when it occurred." Dick spoke from the deck on the R/V Storm. This 50-foot NOAA research vessel carried a team consisting of scientists and divers. The trip took them from Alpena, Michigan to the Middle Island Sinkhole.Dick, a professor at the U-M Department of Earth and Environmental Sciences, stated that "our research suggests the Earth's spinning rate, or in other words, its daylengthmay have had an impact on the timing and pattern of Earth's oxygenation."Researchers simulated Earth's slowing rotation rate. They found that longer days would have increased the oxygen release by early cyanobacterialmats. This helps to explain the planet's two major oxygenation events.Brian Arbic, a U-M Department of Earth and Environmental Sciences physical oceanographer, saw a lecture on Klatt's work and realized that day length variations could have a significant role in the photosynthesis story Dick was creating.Because they are responsible for the toxic and unsightly algal blooms that have plagued Lake Erie and other water bodies all over the globe, Cyanobacteria is often given a bad name.But these microbes, formerly known as blue-green algae, have been around for billions of years and were the first organisms to figure out how to capture energy from sunlight and use it to produce organic compounds through photosynthesisreleasing oxygen as a byproduct.These simple organisms, which lived in primordial seas, are responsible for releasing oxygen that allowed the rise of multicellular animals. Slowly, the planet went from being one with very low oxygen levels to one that has around 21% today.Purple oxygen-producing Cyanobacteria are competing with white sulfur-oxidizing Bacteria at the Middle Island Sinkhole, Lake Huron. They use sulfur as their primary energy source.As the environment changes, filmy sheets made of white and purple microbes compete for position in a daily microbial dance at the Middle Island Sinkhole. The purple cyanobacteria are physically covered by the white sulfur-eating bacteria in the mornings and evenings, blocking sunlight and stopping them from oxygen-producing photosynthesis.However, when the sunlight level reaches a critical threshold, sulfur-oxidizing bacteria returns to the photosynthesis cyanobacteria and starts producing oxygen.Photo taken June 19, 2019, by NOAA Thunder Bay National Marine Sanctuary. It shows purple microbial mud mats at the Middle Island Sinkhole, Lake Huron, Mich. These mats are formed by gases such as methane or hydrogen sulfide rising up below them. Do you feel like the days are getting longer? According to new evidence, they are. This is a good thing as we wouldn't be able to breathe as much if it weren't. This new hypothesis was supported by scientists who tested purple bacteria in a deep Lake Huron sinkhole. Credit: Phil Hartmeyer/NOAA Thunder Bay National Marine SanctuaryIt has been seen before that sulfur-oxidizing bacteria can migrate vertically. The Nature Geoscience study is the first to connect these microbial movements and their resulting rates of oxygen production to changes in day length over Earth's history.Klatt explained that two groups of microbes inhabit the Middle Island Sinkhole mats, with sulfur-oxidizing bacteria shading the photosynthetically active Cyanobacteria. Klatt was examining a core sample taken from the Middle Island Sinkhole microbial matrix in an Alpena lab. It is possible that similar competition among microbes caused the delaying of oxygen production on the early Earth.Understanding the link between Earth's oxygenation and changing day length is key. Longer days allow for more sunlight to reach the afternoon, which allows photosynthetic cyanobacteria (photosynthesis) to produce more oxygen.Dick stated that the idea was to have a shorter day and a shorter window for high-light conditions during the afternoon so that white sulfur-eating bacteria would remain on top of the photosynthetic bacterial for longer periods of time, limiting oxygen production." Dick said as the boat rocked in choppy waters, just a few hundred yards from Middle Island.Because of the similar environment at the bottom to the Middle Island Sinkhole, the microbes from Lake Huron today are thought to be analogs for ancient organisms.The 400-million-year old limestone, dolomite, and gypsum beds that were formed from saltwater seas once covering the continent are the foundations of Lake Huron. Some of the bedrock was disintegrated over time by groundwater, creating caves and cracks. Later, these caves and cracks collapsed and created sinkholes on-land as well as submerged ones near Alpena.Today, cold, oxygen-poor and sulfur-rich groundwater seeps into Middle Island Sinkhole's bottom. This water is a threat to most animals and plants, but provides a home for certain microbes.Dick's team has been working with Bopaiah Biddanda, Annis Water Resources Institute at Grand Valley State University to study the Middle Island Sinkhole's microbial mats for many years. They have used a variety techniques. The researchers used the assistance of NOAA's Thunder Bay National Marine Sanctuary scuba divers to deploy instruments to the lake floor to study chemistry and biology.They also brought mat samples into the lab for experiments under controlled conditions.Klatt proposed that the link between oxygen release and day length can be applied to all mat ecosystems, based upon the physics of oxygen transportation. Chennu and Klatt collaborated to perform detailed modeling studies that compared microbial mat processes with Earth-scale patterns over geological times.Modeling studies showed that the day length actually shapes oxygen release from mats.Klatt stated that oxygen takes less time to leave the mat than it does in other ways.Researchers were able to see a link between increasing atmospheric oxygen levels and longer days. These models suggest that the proposed mechanism could help explain the unique stepwise pattern of Earth’s oxygenation as well as the persistence low-oxygen periods throughout most of Earth’s history.Atmospheric oxygen was scarcely available for most of Earth's history. It is thought that it has increased in two main steps. The Great Oxidation Event, which occurred around 2.4 billion years ago, is generally attributed to the first photosynthesizing Cyanobacteria. A second surge of oxygen, the Neoproterozoic Oxygenation Event, took place nearly 2 billion years later.Earth's orbit rate has been steadily decreasing since its formation around 4.6 billion years ago. This is due to the constant tug of the moon’s gravity, which causes tidal friction.Continue reading Researchers discover an oxygen spike that coincides with an ancient global extinctionMore information: Nature Geoscience (2021) Possible link between Earth's orbit rate and oxygenation www.nature.com/articles/s41561-021-00784-3 Journal information: Nature Geoscience Possible link between Earth's rotation rate and oxygenation,(2021). DOI: 10.1038/s41561-021-00784-3
