Did phosphorus-rich lakes help kickstart life on Earth?

Lake with tall white pillars and clouds in sky above.
Mono Lake in Eastern California. This salty lake – with salt pillars – is rich in carbonates and phosphorus, similar to lakes that scientists involved in the new study think helped with the evolution of life on Earth billions of years ago. Image via Matthew Dillon/ Flickr/ University of Washington.

How did life begin on Earth? That’s one of the most fundamental questions of science. Much progress has been made, but there are still many puzzles. Now, researchers at the University of Washington (UW) present some intriguing new findings involving phosphorus, one of the chemical ingredients essential for life as we know it. They explored how high levels of phosphorus in carbonate-rich lakes could have helped life begin on our planet.

The new peer-reviewed study was published December 30, 2019, in Proceedings of the National Academy of Sciences.

The study focuses on what scientists call the phosphate problem.

That is, the building blocks of life on Earth are created by chemical reactions that require large amounts of phosphorus. But phosphorus is rather scarce on our planet overall, so where did enough of it come from to fuel the development of life? As Jonathan Toner of UW, first author on the new paper, said in a statement:

For 50 years,the phosphate problem has plagued studies on the origin of life.

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Chemical symbol P next to small green two-leaf seedling emerging from brown soil.
Phosphorus is one of the six main chemical elements needed for the building blocks of life as we know it. Image via Roanoke Landscapes.
Conical pile of dark grains on a square.
A sample of black phosphorus on a microchip. Image via MUO.

The answer, it seems, may be in certain kinds of lakes, ones that are rich in carbonates, chemical compounds derived from carbonic acid or carbon dioxide. Found on all seven continents, these lakes are found in depressions in dry environments. They are also known as alkaline or soda lakes and have high evaporation rates. This evaporation concentrates the water into salty and alkaline – high-pH – solutions. The researchers studied various carbonate-rich lakes, including Mono Lake in California, Lake Magadi in Kenya and Lonar Lake in India.

It turned out that not only are the lakes rich in carbonates, but also phosphorus. The concentration can vary depending on when and where the samples were taken, but the researchers found that these lakes can contain up to 50,000 times more phosphorus than seawater, rivers and other types of lakes. This would seem to make them ideal locations for life to originate and evolve. And indeed, these lakes also host a wide variety of life, from microbes to flamingoes.

The lakes also help to answer another question about how there could have been an abundance of phosphorus on the very early Earth, when it was still lifeless. The researchers found it’s due to the carbonates in the water. In such carbonate-rich waters, the carbonate outcompetes phosphate to bind with calcium, leaving some of the phosphate unattached. Lab tests showed that when the ingredients were combined at different concentrations, the calcium binds to the carbonates and leaves the phosphate freely available in the water. That phosphate would then be available for life to use.

Shallow lakeshore with trees, an industrial processing refinery, and mountain.
Lake Magadi in Kenya. Image via Wikipedia.
Circular lake surrounded by trees and bushes.
Lonar Lake in India. Image via Wikipedia.

Toner said:

It’s a straightforward idea, which is its appeal. It solves the phosphate problem in an elegant and plausible way.

It’s even possible that the phosphorous levels could climb even higher – up to a million times the levels found in seawater – when the lake waters evaporated during dry seasons, along shorelines, or in pools separated from the main body of the lake. According to study co-author David Catling at UW:

The extremely high phosphate levels in these lakes and ponds would have driven reactions that put phosphorus into the molecular building blocks of RNA, proteins, and fats, all of which were needed to get life going.

The lakes themselves would have been created by the predominately carbon dioxide atmosphere on Earth a few billion years ago (before life appeared), much different from today. Carbon dioxide would have dissolved in the water, which in turn released phosphorus from rocks in the lakes.

Glass bottle labeled Poison containing yellowish fluid.
These lakes also naturally produce cyanide, which although poisonous to humans, is essential for the formation of amino acids and nucleotides, which are the building blocks of proteins, DNA and RNA. Image via Chemistry World.

From the paper:

Phosphate-rich lakes may have preferentially formed on the prebiotic Earth because of carbonic acid weathering under CO2-rich atmospheres and the absence of microbial phosphate consumption. This specifically points to an origin of life in carbonate-rich lakes, and so defines aqueous conditions that prebiotic chemists should consider.

As Toner explained:

The early Earth was a volcanically active place, so you would have had lots of fresh volcanic rock reacting with carbon dioxide and supplying carbonate and phosphorus to lakes. The early Earth could have hosted many carbonate-rich lakes, which would have had high enough phosphorus concentrations to get life started.

Man with blue shirt in front of gray background.
Jonathan Toner at the University of Washington, first author of the new study. Image via UW.

An earlier study had also found that these lakes are beneficial to the emergence of life in another way as well, by producing cyanide. That may not sound like a good thing, but cyanide actually helps support the formation of amino acids and nucleotides, which are the building blocks of proteins, DNA and RNA.

The scenario outlined in the study seems rather simple, but may go a long way to explain how large enough quantities of phosphorus were created at a time on Earth when life was still absent, and how that crucial chemical helped to kickstart the evolution of biology. It is another significant step in understanding how life originated and evolved on our planet.

Bottom line: A new study from University of Washington shows the chemical ingredient phosphorus originated in carbonate-rich lakes and helped life to begin on Earth.

Source: A carbonate-rich lake solution to the phosphate problem of the origin of life

Via University of Washington

January 17, 2020

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