Microbes Used to Breathe Arsenic Before Oxygen Arrived on Earth

Oxygen 'only' appeared in our planet around 2.4 billion years ago.
Fabienne Lang

When you think of planet Earth, you imagine it with oxygen. After all, this is all we've known so far. However, our planet wasn't always so hospitable and oxygen only emerged some 2.4 billion years ago. 

A recent study carried out by researchers at the University of Connecticut (UConn) has explained that before the days of oxygen, microbes on Earth likely survived thanks to arsenic. You know, the material that can cause cancer and be toxic over long-term use, per the World Health Organization (WHO).

The study was published in the journal Nature Communications Earth & Environment on Tuesday.


Pre-oxygen days

The oxygen cycle that plants and some microbes use in our times take water, sunlight, and CO2, then convert them into carbohydrates and oxygen. These are consumed by other organisms that require oxygen to breathe, and the cycle continues. 

But as the study pointed out, half of our Earth's lifetime was devoid of oxygen. So how did these other organisms survive? 

The lead author of the study and UConn professor of Marine Sciences and Geosciences, Pieter Visscher, and his team set out to answer just that question. 

There were multiple already-existing theories as to how life on Earth continued in the days before oxygen was available, but Visscher wasn't convinced. 

Arsenic had been on the list of potentialities since 2008 after the evidence was found. And in 2014, Visscher and his colleagues discovered further proof of arsenic-based photosynthesis. 

Then, the team found an active microbial mat in the Laguna La Brava, in the Atacama Desert in northern Chile that offered a breakthrough. These mats inhabit an environment that is completely devoid of oxygen, at high altitudes, where they are exposed to harsh conditions and plenty of UV rays. Perfect for a comparison to early-Earth days' conditions. 

Visscher explained that "The water is very high in arsenic as well. The water that flows over the mats contains hydrogen sulfide that is volcanic in origin and it flows very rapidly over these mats."

"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," he continued.

What the team discovered was that the mats were creating carbonate deposits and making a new generation of stromatolites, layered sedimentary deposits — in many ways thanks to arsenic.