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At some point in Earth’s early history, the planet took a turn towards habitability when a group of enterprising microbes known as cyanobacteria developed oxygenated photosynthesis – the ability to transform light and water into energy, releasing oxygen in the process.
This evolutionary moment allowed oxygen to eventually accumulate in the atmosphere and oceans, triggering a domino effect of diversification and shaping the unique habitable planet we know today.
Now scientists at MIT have an accurate estimate of when cyanobacteria and oxygenated photosynthesis first appeared. Their results appear today in the Proceedings of the Royal Society B.
They developed a new technique for genetic analysis that shows that all species of cyanobacteria living today can be traced back to a common ancestor that evolved around 2.9 billion years ago. They also found that the ancestors of cyanobacteria branched out from other bacteria around 3.4 billion years ago, with oxygenated photosynthesis likely evolving over the intervening half billion years, over the eon Archean.
Interestingly, this estimate places the onset of oxygenated photosynthesis at least 400 million years before the great oxidation event, a period in which the Earth’s atmosphere and oceans first experienced an increase in oxygen. This suggests that the cyanobacteria may have developed the ability to produce oxygen early on, but it took some time for this oxygen to really take hold in the environment.
“In evolution, things always start small,” says lead author Greg Fournier, associate professor of geobiology in the Department of Earth, Atmospheric and Planetary Sciences at MIT. “Even though there is evidence for early oxygenated photosynthesis – which is the most important and amazing evolutionary innovation on Earth – it still took hundreds of millions of years for it to take off.”
Fournier’s MIT co-authors include Kelsey Moore, Luiz Thiberio Rangel, Jack Payette, Lily Momper, and Tanja Bosak.
Slow fuse or forest fire?
Estimates of the origin of oxygenated photosynthesis vary considerably, as do methods for tracking its evolution.
For example, scientists can use geochemical tools to search for traces of oxidized elements in ancient rocks. These methods revealed that oxygen was present 3.5 billion years ago – a sign that oxygenated photosynthesis could be the source, although other sources are also possible.
The researchers also used molecular clock dating, which uses the genetic sequences of today’s microbes to trace changes in genes through evolutionary history. Based on these sequences, researchers then use models to estimate the rate at which genetic changes occur, to trace when groups of organisms first evolved. But molecular clock dating is limited by the quality of the ancient fossils and the rate model chosen, which can produce different age estimates, depending on the rate assumed.
Fournier says that different age estimates can involve conflicting evolutionary accounts. For example, some analyzes suggest that oxygenated photosynthesis evolved very early and progressed “like a slow wick”, while others indicate that it appeared much later and then “took off like wildfire” to trigger the great oxidation event and the accumulation of oxygen in the biosphere. .
“In order for us to understand the history of habitability on Earth, it is important for us to distinguish between these assumptions,” he says.
Horizontal genes
To accurately date the origin of cyanobacteria and oxygenated photosynthesis, Fournier and his colleagues associated molecular clock dating with horizontal gene transfer, an independent method that does not rely entirely on fossils or rate assumptions.
Normally, an organism inherits a gene “vertically” when it is passed on from the parent of the organism. In rare cases, a gene can also pass from one species to another, distant species. For example, one cell can eat another and in so doing incorporate new genes into its genome.
When such a history of horizontal gene transfer is discovered, it is clear that the group of organisms that acquired the gene is evolutionarily younger than the group that gave rise to the gene. Fournier believed that such cases could be used to determine the relative ages between certain bacterial groups. The ages of these groups could then be compared to ages predicted by various molecular clock models. The closest model would probably be the most accurate and could then be used to accurately estimate the age of other bacterial species, especially cyanobacteria.
Following this reasoning, the team looked for cases of horizontal gene transfer across the genomes of thousands of bacterial species, including cyanobacteria. They also used new cultures of modern cyanobacteria taken by Bosak and Moore, specifically to use fossil cyanobacteria as calibrations. In the end, they identified 34 clear cases of horizontal gene transfer. They then found that one in six molecular clock models consistently matched the relative ages identified in the team’s horizontal gene transfer analysis.
Fournier used this model to estimate the age of the “crown” group of cyanobacteria, which encompasses all species living today and known to exhibit oxygenated photosynthesis. They found that during the Archean Aeon, the crown group appeared around 2.9 billion years ago, while cyanobacteria as a whole branched out from other bacteria around 3.4 billion years. This strongly suggests that oxygenated photosynthesis was already occurring 500 million years before the Great Oxidation Event (GOE) and that cyanobacteria were producing oxygen for quite a long time before it accumulated in the atmosphere.
The analysis also found that shortly before GOE, around 2.4 billion years ago, cyanobacteria experienced an explosion of diversification. This implies that a rapid expansion of cyanobacteria may have tipped the Earth into the GOE and projected oxygen into the atmosphere.
“This new article sheds essential new light on the history of the oxygenation of the Earth by bringing together, in an unprecedented way, the fossil record of genomic data, including horizontal gene transfers”, explains Timothy Lyons, professor of biogeochemistry at the University of California at Riverside. “The results speak to the beginnings of biological oxygen production and its ecological importance, in a way that provides vital constraints on models and controls on the first oxygenation of the oceans and subsequent accumulations in the atmosphere.”
Fournier plans to apply horizontal gene transfer beyond cyanobacteria to pinpoint the origins of other elusive species.
“This work shows that molecular clocks incorporating horizontal gene transfers (HGT) promise to reliably provide group ages across the entire tree of life, even for ancient microbes that left no records. fossil… which was previously impossible, ”Fournier explains.
This research was funded, in part, by the Simons Foundation and the National Science Foundation.
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