At the core of almost every plant, algae and green pond spots on Earth have a molecular motor to harvest sunlight. Its only emissions are oxygen, a gas we can all be incredibly grateful for today.
Were it not for the evolution of this very common form of photosynthesis (also known as oxygen), complex life as we know it would almost certainly never have arisen, at least not in the form it did.
But knowing exactly who to thank for this precious gift is not easy. Most efforts to determine 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 that spilled into our oceans and atmosphere.
It is likely that more primitive forms of photosynthesis existed, although the ability to extract oxygen from water would have truly given an advantage to phototropic organisms, implying that this oxygen-producing version was a late adaptation.
Molecular biologist at Imperial College London, Tanai Cardona, argues that we could have it all wrong, suggesting that oxygen photosynthesis might have been when life was just beginning about 3.5 billion years ago.
“We had previously shown that the biological system for performing oxygen production, known as photosystem II, was extremely old, but so far we had not been able to place it in the chronology of life history,” says Cardona. .
Several years ago, Cardona and colleagues compared genes in two distance-related bacteria; one that was able to do photosynthesis without producing oxygen, called Heliobacterium modesticaldum, and a phototropic microbe called cyanobacterium.
They were surprised when, despite sharing 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 incredibly similar.
H. modesticaldum’s the ability to divide water suggested that microbes may have been able to generate oxygen from photosynthesis much earlier than contemporary models suggest.
This latest study takes their research a step further, estimating how quickly photosystem II essential proteins have evolved over the years, allowing the team to calculate a time in history when they could have a functional version of the system has emerged.
“We used a technique called Ancestral Sequence Reconstruction to predict the protein sequences of ancestral photosynthetic proteins,” says Thomas Oliver, first author of the study.
“These sequences provide us with information on how the ancestral photosystem II would have worked and we have been 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 evolution of the ‘enzyme’.
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 existed during these “fundamental” enzymes, placing them among the earliest forms of microbial life about 3.5 billion years ago.
“We now know that Photosystem II shows patterns of evolution that are generally only attributed to the oldest known enzymes, which were crucial for life itself to evolve,” says Cardona.
The degree of functioning of these enzymes is a task for future research. Without the signs of rising oxygen levels so far back in time, it is unlikely to have been an efficient process or necessarily bring a huge advantage.
Knowing that the building blocks were in place, however, could affect the way we determine priorities in the search for life on other planets, suggesting that oxygen on a planet only a billion years old may be signs of life.
The discovery also provides researchers with a starting point for designing synthetic forms of photosynthesis.
“We now 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 completely powered by light.”
This research was published in BBA-Bioenergetics.