Billions of years before oxygen, highly toxic arsenic may have been the compound that brought new life to Earth.

In Laguna La Brava, an area of Chile's Atacama Desert, scientists have been studying a purple photosynthetic microorganism that lives in permanently oxygen-free, high-Salt Lake water.

Geologist Pieter Visscher from University of Connecticut says, “I’ve been studying this underwater microbial mat for about 35 years.”

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“This is the only system on earth where I can find a microbial mat that can survive in complete anaerobic conditions.”

Microbes have been living on earth for at least 3.5 billion years, fossilized to form stromatolites. But for the first billion years that microbial mat existed, there was no oxygen on Earth for them to photosynthesize.

How to survive for these life forms under such extreme conditions is unknown, but by studying stromatolites and living extremophiles, researchers have managed to come up with some possible answers.

While iron, sulfur and hydrogen elements have long been suggested as possible substitutes for oxygen, the discovery of the arsenotrophy microorganism in California's salty Lakes Searle and Mono lakes has made arsenic a contender.

Since then, stromatolites in the Tumbiana geological formation in Western Australia have revealed that collecting light and arsenic was an efficient model for photosynthesis in the Precambrian period. Iron or sulfur elements are different.

Just last year, researchers discovered a rich life form that also breathes arsenic in the Pacific Ocean.

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Even La Brava's life forms resemble the purple sulfur bacteria Ectothiorhodospira SP., which was recently found in an arsenic-rich lake in Nevada. They seem to photosynthesize by oxidizing arsenic compounds into arsenates.

While more research is needed to approve if the La Brava microbes metabolize arsenic, preliminary studies have found that the turbulent water around these microbial mats is rich in hydrogen sulphide and arsenic.

La Brava microbes are indeed "breathing" arsenic, so these life forms would be the first to do so on a permanent and completely oxygen-free microbial cushion, which is very similar to what we think of as a Precambrian environment.

So, the microbial mat which it exists is an excellent model for understanding some of the possible life forms of early Earth.

Although genetic studies have shown that La Brava microbes have tools to metabolize arsenic and sulfur elements, the author says that its arsenic reduction appears to be more effective than sulfur reduction.

In any case, they say there is strong evidence that both pathways exist and these could have supported a large number of microbial mats in the early years of life on Earth.

If the team’s discovery is right, then perhaps we should broaden our study of life forms elsewhere.

Visscher said, “When looking for evidence of life on Mars, scientists will look for iron and maybe arsenic.”

It's really more than toxicant.