Arsenic-breathing microbes discovered in Pacific Ocean

Scientists have discovered microorganisms breathing arsenic -- a deadly poison for most living things -- in the low-oxygen parts of the Pacific Ocean. The researchers from the University of Washington (UW) in the US analysed seawater samples from a region below the surface where oxygen is almost absent, forcing life to seek other strategies. These regions may expand under climate change, according to the study published in the journal Proceedings of the National Academy of Sciences.

"Thinking of arsenic as not just a bad guy, but also as beneficial, has reshaped the way that I view the element," said Jaclyn Saunders, who conducted the research for her doctoral thesis at the UW. "We've known for a long time that there are very low levels of arsenic in the ocean," said Gabrielle Rocap, a UW professor.

"But the idea that organisms could be using arsenic to make a living -- it's a whole new metabolism for the open ocean," Rocap said in a statement. In some parts of the ocean there's a sandwich of water where there's no measureable oxygen.

The microbes in these regions have to use other elements that act as an electron acceptor to extract energy from food. The team analysed samples collected during a 2012 research cruise to the tropical Pacific, off the coast of Mexico.

Genetic analyses on DNA extracted from the seawater found two genetic pathways known to convert arsenic-based molecules as a way to gain energy. The genetic material was targeting two different forms of arsenic, and authors believe that the pathways occur in two organisms that cycle arsenic back and forth between different forms.

Results suggest that arsenic-breathing microbes make up less than one per cent of the microbe population in these waters. The microbes discovered in the water are probably distantly related to the arsenic-breathing microbes found in hot springs or contaminated sites on land. Biologists believe the strategy is a holdover from Earth's early history.

During the period when life arose on Earth, oxygen was scarce in both the air and in the ocean. Oxygen became abundant in Earth's atmosphere only after photosynthesis became widespread and converted carbon dioxide gas into oxygen.

Early lifeforms had to gain energy using other elements, such as arsenic, which was likely more common in the oceans at that time. "We found the genetic signatures of pathways that are still there, remnants of the past ocean that have been maintained until today," Saunders said. Arsenic-breathing populations may grow again under climate change. Low-oxygen regions are projected to expand, and dissolved oxygen is predicted to drop throughout the marine environment.