At the core of just about every plant, algae and blob of green pond foam on Earth is a molecular motor for harvesting sunlight. The only release is oxygen – a gas for which we can all be incredibly grateful today.
Without the evolution of this common form of photosynthesis (also known as oxygenic), complex life as we know it would almost certainly never have emerged, at least not in the form it was in.
But knowing exactly who to thank for such a precious gift is far from easy. Most attempts to determine the origin of an oxygen-splitting photosystem suggest a period of about 2.4 billion years ago, a time that coincided with a stream of oxygen entering our oceans and atmosphere.
More primitive forms of photosynthesis probably existed, although the ability to extract oxygen from water would really have given phototropic organisms a head start, implying that this oxygen-producing version was a late adaptation.
Molecular biologist Tanai Cardona of Imperial College London argues that we may be completely wrong, suggesting that oxygen-rich photosynthesis could have been there when life just started, about 3.5 billion years ago.
“We had previously shown that the biological system for carrying out oxygen production, known as Photosystem II, was extremely old, but until now we had not been able to place it on the timeline of the history of life,” says Cardona.
Several years ago, Cardona and colleagues compared genes in two distantly related bacteria; called one that was able to photosynthesize without producing oxygen Heliobacterium modesticaldum, and a phototropic microbe called cyanobacterium.
They were surprised to find that, despite the fact that they had last shared a common ancestor billions of years ago, and the fact that each bacteria harvested sunlight in different ways, an enzyme crucial to their respective processes, was eerily similar. seemed each other.
H. modesticaldum’s The ability to split water strongly suggested that microbes might have been able to generate oxygen from photosynthesis much earlier than contemporary models have suggested.
This latest study takes their research a step further, by estimating the rate at which proteins essential for photosystem II have evolved over the centuries, allowing the team to calculate back to a point in history when a functional version of the system could be may have arisen.
“We used a technique called Ancestral Sequence Reconstruction to predict the protein sequences of ancestral photosynthetic proteins,” said study lead author Thomas Oliver.
“These sequences provide us with information on how the ancestral photosystem II might have worked and we were able to show that many of the key components required for oxygen evolution in photosystem II can be traced back to the earliest stages of the enzyme’s evolution.”
For comparison, the team applied the same technique to enzymes known to be critical to life from the start, such as ATP synthase and RNA polymerase.
They found strong evidence that photosystem II has been around for as long as these “base” enzymes, making them among the very first microbial life forms about 3.5 billion years ago.
“Now we know that photosystem II exhibits patterns of evolution usually only attributed to the oldest known enzymes, which were crucial to the development of life itself,” says Cardona.
How well these enzymes would have functioned is a task for future research. With no signs of oxygen levels rising this far back in time, it is unlikely to have been an efficient process or one that necessarily yielded a huge benefit.
Knowing that the building blocks were in place, however, could affect the way we prioritize when looking for life on other planets, suggesting that on a planet barely a billion years old, oxygen may be signs of life.
The discovery also provides researchers with a starting point for designing synthetic forms of photosynthesis.
“Now that we have a good idea of how photosynthetic proteins evolve and adapt to a changing world, we can use ‘directed evolution’ to learn how to change them to produce new types of chemistry,” says Cardona.
“We could develop photosystems that can perform complex new green and sustainable chemical reactions, powered entirely by light.”
This research is published in BBA-Bioenergetics