For biochemists, one big question has always been: which came first—oxygen-producing photosynthesis or oxygen-using metabolism?
A recent accidental discovery may hold the answer.
Photosynthesis is the process plants and algae use to turn sunlight, water, and carbon dioxide into energy, releasing oxygen as a byproduct.
Animals, in contrast, use oxygen to convert food into energy through aerobic metabolism, releasing carbon dioxide. Scientists have long wondered which of these processes evolved first.
A new study published in the Proceedings of the National Academy of Sciences details a surprising find by researchers, led by Felix Elling, that could provide a missing link between the two.
Elling was a postdoctoral fellow at Harvard’s Department of Earth and Planetary Sciences when he made the discovery.
Elling was not looking for answers to the oxygen mystery when he stumbled upon something unexpected. He and his team were studying bacteria for a different project when they found a molecule in a nitrogen-utilizing bacterium, Nitrospirota.
The molecule, called methyl-plastoquinone, looked like something plants would use for photosynthesis rather than a molecule typically found in bacteria.
All living organisms use quinones—molecules involved in metabolism.
Previously, scientists thought quinones came in two main types: one type used in photosynthesis by plants and another used by bacteria and animals for oxygen-based metabolism.
The discovery of methyl-plastoquinone suggests there is actually a third type, possibly an evolutionary link between the two.
The research sheds light on the Great Oxidation Event, which occurred about 2.3 to 2.4 billion years ago when cyanobacteria started producing large amounts of oxygen through photosynthesis. This event made aerobic metabolism possible.
However, the discovery of methyl-plastoquinone suggests that some bacteria may have already been using oxygen before cyanobacteria started producing it.
Simply put, oxygen metabolism and oxygen production may have evolved around the same time. “The chicken and the egg were at the same time,” said Elling.
Ann Pearson, in whose lab Elling’s research began, explained that the ability to process oxygen safely was a huge evolutionary step. Oxygen can be highly reactive and damaging to cells, so early organisms had to develop mechanisms to handle it. “This is how we learned to breathe,” Pearson said.
Traces of these ancient evolutionary changes still exist today. Different quinone structures are found in both plants and human mitochondria. Researchers believe that methyl-plastoquinone may be the original molecule that eventually split into the two types we see today—one for plants and another for animals.
“This molecule is a time capsule,” said Elling. “A living fossil that has survived for more than 2 billion years.”
Source: Harvard Gazette.
