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Scientists found an explanation, why the Proterozoic eon was the ‘boring billion’

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Posted August 23, 2019

The fact that there is life on Earth is akin to a miracle. At the beginning of the history of our planet conditions were not very hospitable. For example, oxygen levels in the atmosphere during the Proterozoic eon were so low that life couldn’t colonize Earth entirely. Now a new UCL-led study suggests that one of the reasons why for billions of years oxygen levels were rather low is a single enzyme found in early single-cell life forms.

Cyanobacteria were the first life forms on Earth to start producing oxygen using nitrogenase. Image credit: Nesnera via Wikimedia (CC BY-SA 4.0)

The name of that enzyme is nitrogenase and it hasn’t disappeared, but now it is only found in bacteria. Regardless of its rarity, it is actually essential for oxygen production from light and water through a process of photosynthesis. Simple aquatic life forms billions of years ago were producing oxygen – the role now assumed by green plants – using the nitrogenase on their surface. And this could offer the best explanation why at the beginning of the history of life on Earth oxygen levels in the atmosphere were so low.

Earth is around 4.6 billion years old, but for half of its existence it’s been inhabitable, because its atmosphere was not much more than carbon dioxide and nitrogen. When cyanobacteria, also known as blue-green algae, appeared, and started producing oxygen using nitrogenase. This, of course, started the rise of oxygen, but instead of its concentration growing steadily, it reached 2 % and stabilized. Why it didn’t continue to climb was a great puzzle for scientists.

Nitrogenase was essential for that initial production of oxygen, but it is possible, as this new study suggests, that that 2 % oxygen concentration in the atmosphere pretty much blocked nitrogenase from working. The Proterozoic Eon was very stable and slow in terms of the development of life and growing concentration of oxygen. Scientists think that a negative feedback loop sort of blocked nitrogenase, preventing enzyme from being used to produce more oxygen. This “boring billion” period ended only when plants conquered land about 600 million years ago, because they produced oxygen without using nitrogenase.

Brenda Thake, co-author of the study, said: “We know from studying cyanobacteria in laboratory conditions that nitrogenase ceases to work at higher than 10% current atmospheric levels, which is 2% by volume, as the enzyme is rapidly destroyed by oxygen. Despite this being known by biologists, it hasn’t been suggested as a driver behind one of Earth’s great mysteries, until now”.

Knowing this information scientists can start looking differently at the evolution of the Earth and history of life on it. Matching nitrogenase with oxygen production could help explaining why the Proterozoic eon was so boring in terms of the development of life.

 

Source: UCL

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