At the core of almost all plants, algae, and green pond scum droplets on Earth is a molecular engine for harvesting sunlight. Its only emissions are oxygen, a gas for which we can all be incredibly grateful today.
If it weren’t for the evolution of this very common form of photosynthesis (also known as oxygenated), complex life as we know it would almost certainly never have arisen, at least not in the way that it did.
But knowing exactly who to thank for such a precious gift is far from easy. Most efforts to pin down the origins of an oxygen-splitting photosystem suggest a period of about 2.4 billion years ago, a time that coincided with a flood of oxygen spilling into our oceans and atmosphere.
More primitive forms of photosynthesis likely existed, although the ability to extract oxygen from water would have actually given phototropic organisms an advantage, implying that this oxygen-producing version was a late adaptation.
Imperial College London molecular biologist Tanai Cardona argues that we could be wrong, suggesting that oxygenic photosynthesis could have existed when life was just beginning about 3.5 billion years ago.
“We had previously shown that the biological system for producing oxygen, known as photosystem II, was extremely old, but until now we had not been able to place it on the timeline of life history,” says Cardona.
Several years ago, Cardona and her colleagues compared genes in two distantly related bacteria; one that was capable of photosynthesis without producing oxygen, called Heliobacterium modesticaldumand a phototropic microbe called cyanobacteria.
They were surprised to discover that despite having shared a common ancestor billions of years ago, and the fact that each bacterium collected sunlight in different ways, an enzyme critical to their respective processes was strikingly similar.
H. modesticaldum’s The ability to split water strongly suggested that microbes may have been able to generate oxygen from photosynthesis much earlier than contemporary models suggested.
This latest study takes their research one step further, estimating the rate at which the proteins essential to photosystem II have evolved over the ages, allowing the team to calculate a historical moment in which a version could have emerged. system functional.
“We use a technique called Ancestral Sequence Reconstruction to predict the protein sequences of ancestral photosynthetic proteins,” says study first author Thomas Oliver.
“These sequences give us information about how ancestral photosystem II would have functioned, and we were able to show that many of the key components required for oxygen evolution in photosystem II date back to the early stages of enzyme evolution.”
As a point of comparison, the team applied the same technique to enzymes known to be crucial for life from the start, such as ATP synthase and RNA polymerase.
They found strong evidence that photosystem II has existed for as long as these ‘fundamental’ enzymes, placing them among the earliest forms of microbial life around 3.5 billion years ago.
“Now, we know that photosystem II shows evolutionary patterns that are generally only attributed to the oldest known enzymes, which were crucial to the evolution of life,” says Cardona.
How well these enzymes would have worked is a task for future research. With no signs that oxygen levels have risen that far back in time, it is unlikely that it was an efficient process or necessarily conveyed a great deal of advantage.
However, knowing that the building blocks were in place could affect how we prioritize in the search for life on other planets, suggesting that oxygen on a planet just a billion years old may constitute 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, adapting 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 could carry out new complex, ecological and sustainable chemical reactions, powered entirely by light.”
This research was published in BBA-Bioenergetics.