Seawater could have provided the phosphorus needed for emergent life
Their findings, published in the journal Nature Communications, show that seawater could be the missing source of phosphate, meaning it could have been available on a scale large enough for life without requiring special environmental conditions.
“It could really change the way we think about the environments in which life originated,” said co-author Professor Nick Tosca of Cambridge’s Department of Earth Sciences.
The study, which was led by Matthew Brady, a doctoral student from Cambridge’s Department of Earth Sciences, shows that early seawater could have contained a thousand to ten thousand times more phosphate than it had. previously estimated – as long as the water contained a lot of iron.
Phosphate is an essential ingredient in the creation of the building blocks of life – forming a key component of DNA and RNA – but it is one of the least abundant elements in the cosmos relative to its importance organic. In its mineral form, phosphate is also relatively inaccessible – it can be difficult to dissolve in water for life to use.
Scientists have long suspected that phosphorus is a part of early biology, but they have only recently begun to recognize phosphate’s role in directing the synthesis of molecules necessary for life on Earth. “Experiments show it does amazing things – chemists can synthesize crucial biomolecules if there’s a lot of phosphate in solution,” Tosca said.
But the exact environment needed for phosphate production has been discussed. Some studies have suggested that when iron is plentiful, phosphate should actually be even less accessible to life. This is however controversial as early Earth would have had an oxygen-poor atmosphere where iron would have been prevalent.
To understand how life came to depend on phosphate and what type of environment this element would have formed in, they performed geochemical modeling to recreate early conditions on Earth.
“It’s exciting to see how simple experiments in a bottle can disrupt our thinking about the conditions that were present on early Earth,” Brady said.
In the lab, they made seawater with the same chemistry that was thought to have existed early in Earth’s history. They also conducted their experiments in an oxygen-deprived atmosphere, just like on ancient Earth.
The team’s results suggest that seawater itself could have been a major source of this essential element.
“This doesn’t necessarily mean that life on Earth began in seawater,” Tosca said, “it opens up a lot of possibilities for how seawater could have provided phosphate to different environments – for example, lakes, lagoons or shores where sea spray could have transported phosphate to land.
Previously, scientists had proposed a range of ways to generate phosphate, with some theories involving special environments such as acidic volcanic springs or alkaline lakes, and rare minerals found only in meteorites.
“We had a hunch that iron was the key to phosphate solubility, but there just wasn’t enough data,” Tosca said. The idea for the team’s experiments came when they examined the waters that bathe the sediments deposited in the modern Baltic Sea. “It’s unusual because it’s high in phosphate and iron – we started to wonder what was so different about those particular waters.”
In their experiments, the researchers added different amounts of iron to a range of synthetic seawater samples and tested how much phosphorus it could hold before crystals formed and the minerals separated from the liquid. They then incorporated these data points into a model that could predict how much phosphate ancient seawater might contain.
The pore waters of the Baltic Sea provided a set of modern samples which they used to test their model. “We could replicate this unusual water chemistry perfectly,” Tosca said. From there, they continued to explore seawater chemistry before any biology.
The findings also have implications for scientists trying to understand the possibilities of life beyond Earth. “If iron helps put more phosphate into solution, that might be relevant for early March,” Tosca said.
Evidence of water on ancient Mars is abundant, including ancient riverbeds and flood deposits, and we also know that there was a lot of iron on the surface and the atmosphere was sometimes low in oxygen, Tosca said.
Their simulations of surface water filtering through rocks on the Martian surface suggest that iron-rich water may have provided phosphates in this environment as well.
“It’s going to be fascinating to see how the community uses our findings to explore new, alternative pathways for the evolution of life on our planet and beyond,” Brady said.
Matthew P. Brady et al. “Marine Phosphate Availability and the Chemical Origins of Life on Earth.” Communication Nature (2022). DOI: 10.1038/s41467-022-32815-x