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Early life on Earth limited by enzyme

4 September 2019

A single enzyme found in early single-cell life forms could explain why oxygen levels in the atmosphere remained low for two billion years during the Proterozoic eon, preventing life colonising land, suggests a 香港六合彩中特网-led study.

The enzyme鈥搉itrogenase鈥揷an be traced back to the universal common ancestor of all cells more than four billion years ago.

Found only in bacteria today, nitrogenase is nevertheless essential for the production of oxygen from water in photosynthesis, making it instrumental in how aquatic bacteria produced Earth鈥檚 first molecular oxygen 2.5 billion years ago.

鈥淔or half of Earth鈥檚 4.6 billion year existence, the atmosphere contained only carbon dioxide and nitrogen, with no oxygen, but this changed when cyanobacteria, also known as聽blue-green algae,聽started producing the first oxygen using nitrogenase. This led to the Great Oxidation Event,鈥 explained study author Professor John Allen (香港六合彩中特网 Genetics, Evolution & Environment).

鈥淏ut instead of rising steadily, atmospheric oxygen levels stabilised at 2% by volume for about two billion years before increasing to today鈥檚 level of 21%. The reasons for this have been long debated by scientists and we think we鈥檝e finally found a simple yet robust answer.鈥

A study, published today in聽Trends in Plant Science聽by researchers from 香港六合彩中特网, Queen Mary University of London and Heinrich-Heine-Universit盲t D眉sseldorf, proposes for the first time that atmospheric oxygen produced using nitrogenase blocked the enzyme from working.

This negative feedback loop prevented further oxygen from being made and initiated a long period of stagnation in Earth鈥檚 history about 2.4 billion years ago.

Lasting nearly two billion years, the Proterozoic Eon saw very little change in the evolution of life, ocean and atmosphere composition and climate, leading some to call it the 鈥榖oring billion鈥.

鈥淭here are many ideas about why atmospheric oxygen levels stabilised at 2% for such an incredibly long period of time, including oxygen reacting with metal ions, but remarkably, the key role of nitrogenase has been completely overlooked,鈥 said study co-author Professor William Martin (Heinrich-Heine-Universit盲t D眉sseldorf).

鈥淥ur theory is the only one that accounts for the global impact on the production of oxygen over such a sustained period of time and explains why it was able to rise to the levels we see today, fuelling the evolution of life on Earth.鈥

The team says that the negative feedback loop ended only when plants conquered land about 600 million years ago.

When land plants emerged, their oxygen producing cells in leaves were physically separated from nitrogenase containing cells in soil. This separation allowed oxygen to accumulate without inhibiting nitrogenase.

This theory is supported by evidence in the fossil record that shows cyanobacteria had begun to protect nitrogenase in dedicated cells called heterocysts about 408 million years ago, once oxygen levels were already increasing from photosynthesis in land plants. 聽聽

鈥淣itrogenase is essential for life and the process of photosynthesis as it fixes聽nitrogen in the air into ammonia, which is used to make proteins and nucleic acids,鈥 said co-author Mrs Brenda Thake (Queen Mary University of London).聽聽

鈥淲e 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鈥檛 been suggested as a driver behind one of Earth鈥檚 great mysteries, until now.鈥

The work was funded by the Leverhulme Trust, European Research Council, Volkswagen Foundation and the German Research Foundation (DFG).

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Image

  • Anabaena cylindrica, a filamentous cyanobacterium, 1946 watercolour by G. E. Fogg, FRS