At the base of each plant, algae and green manure stains on the ground is a molecular engine for harvesting the sun. Its only emissions are oxygen – a gas for which we can all be incredibly grateful today.
Were it not for the evolution of this extremely common form of photosynthesis (also known as oxygen), complex life as we know it would almost certainly never have occurred, at least not in the form it did.
But knowing exactly who to thank for such a precious gift is far from simple. Most efforts to identify the origins of an oxygen-sharing photosystem suggest a period of about 2.4 billion years ago, a period that coincided with a flood of oxygen spilled into our oceans and atmosphere.
There may have been more primitive forms of photosynthesis, although the ability to snatch oxygen from water would have really given an edge to phototropic organisms, which means that this oxygen-producing version was a late adaptation.
Tanai Cardona, a molecular molecular biologist at Imperial College London, says we may be wrong, suggesting that oxygen photosynthesis may have existed when life just began about 3.5 billion years ago.
“We previously showed that the biological system for oxygen production, known as photosystem II, was extremely old, but so far we have not been able to place it on the chronology of life history,” says Cardona.
A few years ago, Cardona and colleagues compared genes in two related bacteria; one that was able to photosynthesize without producing oxygen, called Heliobacterium modesticaldum, and a phototropic microbe called cyanobacteria.
They were surprised to find that, despite sharing the last common ancestor billions of years ago, and the fact that each bacterium harvested sunlight in different ways, an enzyme critical to their respective processes was extremely similar.
H. modesticaldum’s the ability to divide water suggested that microbes could have generated oxygen from photosynthesis much faster than contemporary models have suggested.
This latest study takes their research a step further, estimating the rate at which photosystem II essential proteins have evolved over the centuries, allowing the team to calculate back to a time in history when a functional version of the system could have appeared.
“We used a technique called Ancestral Sequence Reconstruction to predict the protein sequences of ancestral photosynthetic proteins,” says the study’s lead author, Thomas Oliver.
“These sequences provide information about how the ancestral photosystem II would have worked, and we were able to show that many of the key components needed for the evolution of oxygen in photosystem II can be traced back to the early stages of the enzyme’s evolution.”
As a point of comparison, the team applied the same technique to enzymes that are known to be crucial for life from the beginning, such as ATP synthase and RNA polymerase.
They found strong evidence that photosystem II existed as long as these “foundation” enzymes, placing them among the first microbial life forms about 3.5 billion years ago.
“Now, we know that Photosystem II has evolutionary patterns that are usually attributed only to the oldest known enzymes, which were crucial for life itself to evolve,” says Cardona.
How well these enzymes would work is a task for future research. With no signs of rising oxygen levels so far back in time, it is unlikely to have been an efficient process or one that would necessarily convey a huge advantage.
Knowing that the building blocks were in place could affect how we set priorities in search of life on other planets, suggesting that oxygen on a planet a billion years old may be signs of life.
The discovery also provides researchers with a starting point for designing synthetic forms of photosynthesis.
“Now we have a good sense 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 perform new, environmentally friendly, sustainable chemical reactions powered entirely by light.”
This research was published in BBA-Bioenergetics.