Samplings: November 2021

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Lake Huron’s Middle Island Sinkhole

Phil Hartmeyer, NOAA Thunder Bay National Marine Sanctuary

According to geological and biological evidence, appreciable quantities of free oxygen first appeared in our planet’s atmosphere and ocean about 2.4 billion years ago—the Great Oxidation Event. This was followed by a period of low-oxygen conditions until about 600 million years ago, when atmospheric oxygen levels rose dramatically—the Neoproterozoic Oxygenation Event. The mechanisms that caused this stepwise pattern of oxygenation are poorly understood—whether they were biological, tectonic, or geochemical.

A new study has proposed a novel explanation tied to day length. When the planet formed some 4.6 billion years ago, a day lasted a mere six hours. Tidal friction between the Earth and the Moon has since slowed Earth’s spin rate, stretching out the duration of the rotation that constitutes a day. The process is ongoing, though at a comparatively slow pace. This latest study hypothesizes that this gradual increase in day length spurred greater oxygen production from cyanobacteria.

The hypothesis is based on the researchers’ studies of microbes in what is known as the Middle Island Sinkhole, located about eighty feet under the surface of Lake Huron. These microbes are thought to be similar to early life on Earth. The microbial mats consist of both white, sulfur eating bacteria and purple, photosynthetic cyanobacteria. Over the course of a day, the bacteria alternate positions. In the mornings and evenings, the sulfur-eaters cover the cyanobacteria, blocking sunlight and stalling photosynthesis. During the midday hours, the purple, sun-loving bacteria take over and benefit photosynthetically from high-light conditions, pumping out more oxygen.

Besides greater oxygen production per longer days, the physics of particle movement suggest that ample sunlight should also boost oxygen diffusion from the cyanobacteria mats, because there is more time for the gas to escape into the environment.

"The biggest takeaway," said Gregory Dick, a geomicrobiologist at the University of Michigan and a senior author of the study, "is that planetary processes, like rotation rate and dynamics of the Earth-Moon system, can have profound effects on biology and chemistry in ways that we are just beginning to understand." (Nature Geoscience)