Evrim Yazgin
Cosmos science journalist
An accidental discovery could provide clues to a chicken-or-egg question which has puzzled scientists for decades: what came first – oxygen production by photosynthesis or oxygen consumption by aerobic metabolism?
Roughly 2.4 billion years ago, a major shift occurred that would change our planet and its life forms forever. Known as the Great Oxidation Event, the evolution of photosynthesising cyanobacteria led to the sudden influx of oxygen into Earth’s atmosphere over hundreds of millions of years.
It is believed this triggered the evolution of aerobic metabolism. In other words, photosynthesis came first followed by oxygen breathing.
But other discoveries, including the earliest sign of aerobic metabolism 2.9 billion years ago, have cast doubt on this assumption and raised the question again of which came first.
A missing-link molecule could provide the answer.
The discovery was made by accident when researchers at Harvard University in the US, were looking for specific molecules in research not related to aerobic metabolism.
Findings from the research are published in the Proceedings of the National Academy of Sciences.
The team was looking at a type of bacteria called Nitrospirota which have a nitrogen-based metabolism. Nitrospirota oxidise nitrite to nitrate, rather than the oxidisation of carbon-based carbohydrates in aerobic metabolism.
An unexpected molecule appeared in the screening of the Nitrospirota.
Methyl-plastoquinone is a variation of a molecule type called a quinone. Quinones are found in all life forms. They are thought to appear in 2 ways: aerobic quinones that require oxygen, and anaerobic ones that do not.
Aerobic quinones also come in 2 types – ones used by plants in photosynthesis and another used by bacteria and animals to breathe oxygen.
But the quinone found in the Nitrospirota used to breathe oxygen was similar to the quinones used by plants to perform photosynthesis. Methyl-plastoquinone is a third type of quinone – a possible missing link between the 2 others.
This discovery suggests that aerobic metabolism had already evolved before cyanobacteria began producing oxygen. In other words, “the chicken and the egg were at the same time”, lead author of the new study Felix Elling tells the Harvard Gazette. Elling was formerly at Harvard and is now at the University of Kiel in Germany.
“We think what we found is the primary or ancestral form of this molecule that then later was adapted to have 2 forms – one with specific functions in the algae and plants, and the alternative form in mitochondria that we have today,” Elling explains.
“This molecule is a time capsule. A living fossil of a molecule that has survived over more than 2 billion years.”
