Dr. Tanai Cardona, a researcher with Imperial College London, UK, studied molecular machines responsible for oxygenic photosynthesis and found the process may have evolved as long as 3.6 billion years ago.
This image is the crystal structure of Photosystem I. Image credit: Tanai Cardona.
Photosynthesis is the process that sustains complex life on Earth — all of the oxygen on our planet comes from photosynthesis.
There are two types of photosynthesis: (i) oxygenic photosynthesis, which uses light energy to split water molecules, releasing oxygen, electrons and protons; and (ii) anoxygenic photosynthesis, which uses compounds like hydrogen sulfide or minerals like iron or arsenic instead of water, and it does not produce oxygen.
Previously, scientists believed that anoxygenic evolved long before oxygenic photosynthesis, and that the Earth’s atmosphere contained no oxygen until about 2.4 to 3 billion years ago.
However, Dr. Cardona’s work suggests that the origin of oxygenic photosynthesis may have been as much as a billion years earlier, which means complex life would have been able to evolve earlier too.
The researcher wanted to find out when oxygenic photosynthesis originated.
Instead of trying to detect oxygen in ancient rocks, which is what had been done previously, he looked deep inside the molecular machines that carry out photosynthesis — these are complex enzymes called photosystems.
Oxygenic and anoxygenic photosynthesis both use an enzyme called Photosystem I.
The core of the enzyme looks different in the two types of photosynthesis, and by studying how long ago the genes evolved to be different, Dr. Cardona could work out when oxidative photosynthesis first occurred.
He found that the differences in the genes may have occurred more than 3.4 billion years ago — long before oxygen was thought to have first been produced on Earth. This is also long before cyanobacteria — microbes that were thought to be the first organisms to produce oxygen — existed. This means there must have been predecessors, such as early bacteria, that have since evolved to carry out anoxygenic photosynthesis instead.
“This is the first time that anyone has tried to time the evolution of the photosystems,” Dr. Cardona said.
“The result hints towards the possibility that oxygenic photosynthesis, the process that have produced all oxygen on Earth, actually started at a very early stage in the evolutionary history of life — it helps solve one of the big controversies in biology today.”
One surprising finding was that the evolution of the photosystem was not linear.
Photosystems are known to evolve very slowly — they have done so since cyanobacteria appeared at least 2.4 billion years ago.
But when Dr. Cardona used that slow rate of evolution to calculate the origin of photosynthesis, he came up with a date that was older than the Earth itself.
This means the photosystem must have evolved much faster at the beginning — something recent research suggests was due to the planet being hotter.
“There is still a lot we don’t know about why life is the way it is and how most biological process originated,” the scientist said.
“Sometimes our best educated guesses don’t even come close to representing what really happened so long ago.”
Dr. Cardona reported his results in a paper in the journal Heliyon.
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Tanai Cardona. 2018. Early Archean origin of heterodimeric Photosystem I. Heliyon 4 (3): e00548; doi: 10.1016/j.heliyon.2018.e00548
