Amongst the huge variety of life on Earth, there are some mighty resilient microorganisms out there. Now, scientists have tracked down an ocean microbe that survives by breathing arsenic – the chemical element we humans regard as a notorious poison.

The researchers think this is an ancient survival strategy used in the distant past when oxygen was less abundant on our planet, but it's somewhat of a surprise to see it still in use under the sea today.

The team from the University of Washington in Seattle says it could be a sign of how life in the ocean will continue to alter in the face of climate change: if underwater oxygen starts to become scarce again, ecosystems may have to adapt.

"We've known for a long time that there are very low levels of arsenic in the ocean," says one of the team, oceanographer Gabrielle Rocap. "But the idea that organisms could be using arsenic to make a living – it's a whole new metabolism for the open ocean."

The hardy microorganisms were discovered in the Pacific Ocean off the coast of Mexico, in a patch of water classed as anoxic – having virtually no oxygen available.

While past research has shown tiny marine lifeforms surviving on nitrogen and sulphur in the absence of oxygen, this is the first time researchers have detected microbes breathing arsenic in this environment. We've seen a few microbes like this before, but never in the ocean.

Based on a detailed DNA analysis of the samples collected, the scientists were able to identify two genetic pathways able to convert arsenic-based molecules to generate the energy necessary to sustain life.

Specifically, the study shows how arsenate molecules can be converted into arsenite, in areas where oxygen isn't readily available.

And not only does that chemical reaction help explain life in oxygen-deficient zones (ODZs) in the ocean, it might also help us in the search for extraterrestrial life on other planets: perhaps alien bugs don't need as much oxygen to live as we thought.

"Thinking of arsenic as not just a bad guy, but also as beneficial, has reshaped the way that I view the element," says one of the researchers, Jaclyn Saunders.

Based on the collected seawater that formed the basis of the study, these arsenic-sucking microorganisms could account for around one percent of the marine microbial community, but learning more about them can help our understanding of the complex chemical reactions going on in the ocean as a whole.

The next step is to try and grow these microbes in the lab where their metabolisms can be studied more closely: one of the clever tricks they pull off is being able to survive on just small amounts of arsenic in the water.

"What I think is the coolest thing about these arsenic-respiring microbes existing today in the ocean is that they are expressing the genes for it in an environment that is fairly low in arsenic," says Saunders.

"It opens up the boundaries for where we could look for organisms that are respiring arsenic, in other arsenic-poor environments."

The research has been published in PNAS.