Arsenic-Breathing Microbes Found In The Open Ocean

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Underwater air bubbles.

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As air-breathers, humans require oxygen to drive the production of ATP, or energy, from sugar molecules. However, large pockets of the ocean lack oxygen entirely – forcing the organisms that live there to find different means to produce energy.

Until recently, the microorganisms that call these ‘oxygen-deficient zones’ home were thought to primarily use elements like sulphur and nitrogen as energy drivers. However, a recent study published in the Proceedings of the National Academy of Sciences by researchers at the University of Washington shows that microbes living in oxygen-free pockets of the Pacific Ocean are using arsenic, a highly toxic element, to breathe more frequently than previously thought.

“Thinking of arsenic as not just a bad guy, but also as beneficial, has reshaped the way that I view the element,” said lead author Dr. Jaclyn Saunders, who is now a post-doc at Woods Hole Oceanographic Institute (WHOI).

Microbes that use arsenic have previously been studied in other environments rich in the toxic element, such as the polluted soils of the Niger Delta and California’s Mono Lake.

Mono Lake, California, where arsenic-using bacteria were discovered by Dr. Felisa Wolf-Simon of the Lawrence Berkeley National Laboratory.

However, part of what makes this discovery surprising is the minimal amount of arsenic available in modern oxygen-deficient zones. “We’ve known for a long time that there are very low levels of arsenic in the ocean,” said co-author Gabrielle Rocap, a UW professor of oceanography. “But the idea that organisms could be using arsenic to make a living — it’s a whole new metabolism for the open ocean.”

Although it is unclear when oxygen first filled the oceans, we know the early oceans of 3 billion years ago lacked oxygen completely – meaning the entire ocean was an oxygen-deficient zone. What’s more, fossilized arsenic-rich masses from 2.7 billion years ago discovered in Western Australia suggest arsenic metabolism was quite common in prehistoric oceans, when arsenic was more abundant.

Limestone concretions, called tumbiana stromatolites (left), formed by the activity of microorganisms ~2.7 billion years ago, are abundant in arsenic (center). The arsenic (As) is present in globules of organic matter (right, in red), which represent fossilized bacterial cells.

Pascal Philippot

Prior to this recent discovery, the use of arsenic was thought to be an ancient survival strategy rarely used today due to the relatively small amount of arsenic available in modern oceans. But with this new discovery, scientists are re-thinking their understanding of modern arsenic metabolism.

“We found the genetic signatures of pathways that are still there, remnants of the past ocean that have been maintained until today,” said Saunders. “[This discovery] opens up the boundaries for where we could look for organisms that are respiring arsenic, in other arsenic-poor environments.”

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