What was the source of life before oxygen? Study says ancient microbial life on Earth relied on arsenic

For half of the time life has existed on Earth, there was no oxygen present. Researchers wondered what supported life back then

                            What was the source of life before oxygen? Study says ancient microbial life on Earth relied on arsenic
La Brava microbial mats (Visscher et al, Nature, 2020)

Billions of years ago, the Earth had no oxygen reserves, and yet some organisms thrived. But how? This question is one of the biggest unsolved mysteries we have today. New evidence suggests that ancient life likely used the poisonous arsenic to breathe and make food, according to a new study.

For half of the time life has existed on Earth, there was no oxygen present, lead author of the study, Dr Pieter Visscher, said in a statement. Back then, life on Earth was mostly just large slimy mats of microbes living in shallow water, Dr Visscher from the University of Connecticut, Dr Brendan Paul from UNSW, Dr Kimberley L Gallagher from Quinnipiac University, wrote in The Conversation. "But these microbial mats existed for a billion years before oxygen was present in the atmosphere. So what did life use instead?" they wondered.

Scientists have wondered whether other compounds like iron, sulfur, hydrogen could have supported life. But fossil records have provided no proper evidence. The primordial soup -- organic compounds present in the water bodies of the early Earth -- had low levels of these compounds. This suggests that neither iron, sulfur, nor hydrogen was the likely candidate. "That leaves arsenic," they added.

Their previous study supported the theory that arsenic could have supported ancient microbial life. The clue came from limestone rocks produced by bacteria called stromatolites. The team collected 2.72-billion-year-old stromatolites by drilling into an ancient reef in the Outback of Australia. Their analysis pointed towards the presence of two kinds of arsenic, but not iron or sulfur. 

A piece of microbial mat (Visscher et al, Nature, 2020)

"This was tantalizing, but we wanted more proof," the researchers wrote in The Conversation. So they looked for a modern analog: microbial mats. "No researchers had ever found a microbial mat community living in a place completely free of oxygen, but if we found one, it could help explain how the first stromatolites formed when our planet’s oceans and atmosphere were lacking oxygen."

Their search led them to Chile's Atacama Desert, which is not too different from the pre-oxygen world, according to the team. They zeroed in on Laguna La Brava, a very salty shallow lake deep into the harsh desert. The water is rich in unusually high amounts of arsenic, sulfur, and lithium. "The water that flows over the mats contains hydrogen sulfide that is volcanic in origin, and it flows very rapidly over these mats. There is absolutely no oxygen," Visscher explained.

The bottom of the stream appeared deep purple. Giving it this color was microbial mat communities. "I have been working with microbial mats for about 35 years or so. This is the only system on Earth where I could find a microbial mat that worked absolutely in the absence of oxygen," Visscher said. Further analysis showed that these microbes metabolize arsenic, like how we do with oxygen.

The findings have applications that could widen the search for life beyond Earth. "In looking for evidence of life on Mars, they will be looking at iron -- and probably they should be looking at arsenic also," Visscher said.

The study is published in Communications Earth & Environment.

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