At the core of almost every plant, algae & blob of green pond scum life on Earth sits a molecular engine for harvesting sunlight. Its only emissions are oxygen, a gas we might all be incredibly thankful for today.
If not for the evolution of this vastly common form-of-photosynthesis (also referred to as oxygenic), complex life as we all know it almost certainly would never have emerged, at-least of not in-shape it did.
But knowing exactly whom to thank for such a precious gift is far-more from straightforward. Most efforts to pin down the origins of an oxygen-splitting photosystem suggest a period around 2.4 billion years ago, a time that coincided with a flood of oxygen spilling into our oceans & atmosphere.
More primitive type of photosynthesis are likely to possess existed, though the capability to pluck oxygen from water would have truly given phototropic organisms a foothold , implying this oxygen-producing version was a late adaptation.
Imperial College London molecular-biologist Tanai Cardona argues we’d have it all wrong, suggesting oxygenic photosynthesis might have-been around when life was just getting started around 3.5 billion years ago.
“We had previously shown that the biological system for performing oxygen-production, referred to as photosystem II, was extremely old, but so far we hadn’t been ready to place it on the timeline of life’s history,” says Cardona.
Several years ago, Cardona & his colleagues compared genes in two distantly related bacteria; one that was capable of photosynthesizing without producing oxygen, called Heliobacterium modesticaldum, & a phototropic microbe called a cyanobacterium.
They were surprised to seek out that in spite of last sharing a same-ancestor billions of years ago, and therefore the fact each bacterium harvested sunlight in several ways, an enzyme critical to their respective processes was uncannily similar.
H. modesticaldum’s ability to separate water strongly suggested microbes may need been capable of generating oxygen from photosynthesis far before-than contemporary models suggested.
This latest study takes their research a step further, estimating rate at which proteins essential to photosystem II have evolved over the ages, allowing the team to calculate back to a flash in history when a functional version of the system may arisen.
“We used a method called Ancestral Sequence Reconstruction to predict the protein sequences of ancestral photosynthetic proteins,” says the study’s first author, Thomas Oliver.
“These sequences give us information about how the ancestral photosystem II would have worked and that we were ready to show that a lot of of the key components required for oxygen evolution in photosystem II are often traced to the earliest stages in-evolution of the enzyme.”
As point-of-comparison, the team applied an same-technique to enzymes known to be crucial to life from the onset, like ATP synthase & RNA polymerase.
They found strong evidence that photosystem II has been around for as long as these foundation enzymes, placing them among the first-ever microbial life forms around 3.5 billion years ago.
“Now, we all know that photosystem II shows patterns of evolution that are usually only attributed to the oldest known enzymes, which were crucial for life’ itself to evolve,” says Cardona.
Just how well these enzymes would have functioned may be a task for future research. Without signs of oxygen levels rising thus far back in time, it’s unlikely to possess been an efficient process or one that necessarily conveyed an enormous advantage.
Knowing the building blocks were in situ , however, could affect the way we determine priorities in checking out life on other planets, suggesting oxygen on a planet barely a billion years old may constitute signs of life.
The discovery also provides researchers with a first place for designing synthetic form-of photosynthesis.
“Now we’ve better sense of how photosynthesis proteins evolve, adapting to a changing world, we might use ‘directed evolution‘ to find out the way to change them to produce new type of chemistry,” says Cardona.
“We could develop photosystems that would perform complex new green & sustainable chemical reactions entirely powered by light.”
This research was published in BBA-Bioenergetics.