The decrease in the rotational speed of our planet could have significantly influenced the development of life, researchers report: As the days got longer, the pioneers of photosynthesis were able to release more oxygen and thereby enrich the earth’s atmosphere with the elixir of life. This suggests research results from communities of oxygen-producing cyanobacteria and sulfur-oxidizing microbes in Lake Huron, which presumably still interact with one another in a similar way as the life forms of prehistoric times.
The air we breathe is shaped by the process of biological energy generation from sunlight: the earth’s oxygen was and is almost exclusively produced by photosynthesis. The cyanobacteria are considered to be the developers of this process. Their traces can be traced back to more than 2.4 billion years ago – a time when the oxygen content of the earth’s atmosphere increased significantly. However, it then took another 1.8 billion years for the oxygen content to rise to the same level as we know it today. “It is still not entirely clear why it took so long and what exactly controlled the oxygenation on earth,” says Judith Klatt from the Max Planck Institute for Marine Microbiology in Bremen.
As she and her colleagues report, research results from an archaic-looking microbial community in the so-called Middle Island Sinkhole in Lake Huron in Michigan shed light on this question. The microbes live there in mats on the bottom of the lake. Some of the residents are purple-colored cyanobacteria that extract energy from sunlight and produce oxygen in the process. They are associated with white bacteria, whose basis of life is the sulfur present there. “This habitat is considered to be a reflection of the conditions that also prevailed on Earth for billions of years,” explains co-author Bopi Biddanda from Grand Valley State University.
Bacteria mats with shift change
As the researchers report, the two competing groups of bacteria perform a kind of dance every day: At night, the sulfur-eating bacteria push themselves over the cyanobacteria. When the sun rises in the morning, however, the cyanobacteria move to the surface of the mat. “There they then carry out photosynthesis and produce oxygen,” explains Klatt. “However, it takes a few hours before they really get started.” In light of this system, the scientists finally had an idea: “We asked ourselves the question: Could it be that the change in day length has influenced photosynthesis over the course of the earth’s history?” Says co-author Brian Arbic University of Michigan at Ann Arbor .
Background: It is well known that the days on earth were not always 24 hours long. “When the earth-moon system came into being, they were much shorter, maybe even only six hours,” explains Arbic. Subsequently, however, the rotation speed of the earth slowed down – caused by the gravitational pull of the moon and the braking effect of the tides. So the days got longer. There are indications that this slowdown did not always proceed evenly, but was characterized by surges or delays due to resonance effects. As the researchers report, they encountered similarities in the pattern of the geological traces of the oxygen content and the development of the rotational behavior of our planet. “It became clear to me that the length of the day and the production of oxygen in microbial mats could be linked by a very basic and fundamental concept: If the days are shorter, there is less time in which a gradient – an oxygen concentration gradient – can form . This means that less oxygen can escape from the mats, ”so Arbic’s assumption.
Long days bubbled oxygen
To support this hypothesis, the researchers developed model simulations of how the dynamics of sunlight could be related to the release of oxygen from the mats. “Intuitively, one would think that two 12-hour days have the same effect as one 24-hour day. Because the sun rises and sets twice as fast, and oxygen production follows in step, ”says co-author Arjun Chennu from the Leibniz Center for Tropical Marine Research in Bremen. But as he explains, due to the special processes, less oxygen actually escapes from the bacterial mats during short days. “This subtle decoupling of the oxygen release from sunlight is at the heart of the mechanism now presented,” says Chennu.
The researchers then inserted their results into global models of the evolution of the earth’s oxygen conditions. This made it clear that the increased oxygen release caused by longer days could actually have increased the global oxygen content of the atmosphere. It thus seems possible that the two major phases of oxygenation in Earth’s history – the Great Oxygenation Event more than two billion years ago and the later Neoproterozoic Oxygenation Event about 600 million years ago, were related to the patterns of day lengthening. “Ultimately, we were able to link processes in the microbial mat with the dance of our planet and its moon,” concludes Chennu.
Source: Max Planck Society, University of Mischigan, specialist article: Nature Geoscience, doi: 10.1038 / s41561-021-00784-3