Arsenic is toxic to almost all life forms, but now researchers at the University of Washington have discovered that some microbes in the Pacific Ocean not only tolerate the stuff, but actively breathe it. The discovery has implications for how life may adapt to a changing climate, as well as where we might find it on other planets.

Oxygen plays a vital role in every cell in your body, as well as the bodies of many other organisms. We use it to chemically change the food we eat and the liquids we drink into energy that then powers every other function in the body. But in environments where there isn't much oxygen to spare, some crafty creatures have evolved to make use of other elements, most commonly nitrogen or sulfur. In this case, that environment was a particular part of the ocean.

"In some parts of the ocean there's a sandwich of water where there's no measurable oxygen," says Gabrielle Rocap, co-author of the study. "The microbes in these regions have to use other elements that act as an electron acceptor to extract energy from food."

The discovery was made in water samples gathered in the Pacific Ocean off the coast of Mexico. After conducting genetic analyses on DNA from those samples, the team found two genetic pathways that are known to help organisms gain energy by converting one form of arsenic molecule into another, and back again.

Arsenic-breathing microbes have previously been found in hot springs or lakes with high arsenic levels, but finding them in the ocean, where there isn't all that much arsenic to begin with, is quite strange.

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

That said, it does seem to be a very small population – less than one percent of the microbes in these waters. They appear to be distantly related to the other arsenic-respiring species on land and in lakes, which may suggest that this survival strategy is a holdover from an ancient time, when the levels of arsenic were naturally much higher.

"We found the genetic signatures of pathways that are still there, remnants of the past ocean that have been maintained until today," says Jaclyn Saunders, first author of the study. "What I think is the coolest thing about these … is that they are expressing the genes for it in an environment that is fairly low in arsenic. It opens up the boundaries for where we could look for organisms that are respiring arsenic, in other arsenic-poor environments."

And with climate change projected to reduce the total amount of dissolved oxygen in the oceans, perhaps this kind of respiration system is due for a comeback. The next steps for the team are to try to culture these microbes in the lab, to try to learn more about them.

The research was published in the journal Proceedings of the National Academy of Sciences.