Early life on Earth ‘limited by a single enzyme’

By Nina Massey, PA Science Correspondent

A single enzyme found in early single-cell life forms could explain why oxygen levels in the atmosphere remained low for half of the Earth’s existence.

The discovery could explain why, for two billion years during the Proterozoic Eon, life was prevented from colonising land.

The enzyme – nitrogenase – can be traced back to the universal common ancestor of all cells more than four billion years ago.

Researchers said that today it is only found in bacteria, but is nevertheless essential for the production of oxygen from water in photosynthesis.

This, says the study published in the Trends in Plant Sciences journal, makes it instrumental in how aquatic bacteria produced Earth’s first molecular oxygen 2.5 billion years ago.

Author Professor John Allen, from University College London, said: “For half of Earth’s 4.6 billion-year existence, the atmosphere contained only carbon dioxide and nitrogen, with no oxygen, but this changed when cyanobacteria, also known as blue-green algae, started producing the first oxygen using nitrogenase.

“This led to the Great Oxidation Event.

“But instead of rising steadily, atmospheric oxygen levels stabilised at 2% by volume for about two billion years before increasing to today’s level of 21%.

“The reasons for this have been long debated by scientists and we think we’ve finally found a simple yet robust answer.”

Researchers from UCL, Queen Mary University of London and Heinrich-Heine-Universität Dusseldorf suggest that atmospheric oxygen produced using nitrogenase blocked the enzyme from working.

This negative feedback loop prevented further oxygen from being made, kick-starting a period of stagnation in Earth’s history about 2.4 billion years ago.

According to the study, this loop only ended 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 allowed oxygen to accumulate without inhibiting nitrogenase.

The Proterozoic Eon, which lasted almost two billion years, saw very little change in the evolution of life, ocean and atmosphere composition and climate, leading some to call it the “boring billion”.

Co-author Professor William Martin said: “Our 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.”

Co-author Brenda Thake 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.”

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