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Researchers Discover An Ancient Survival Strategy Still Used In The Pacific
August 12, 2019

According to information provided by the University of Washington (UW), a team of researchers has discovered that an ancient survival strategy is still being used in low-oxygen parts of the marine environment. In a large area of the Pacific Ocean, the researchers discovered that microorganisms are breathing arsenic, a deadly poison for most living things, as a substitute for sulphur or nitrogen to get energy in oxygen-deficient environments.

“Thinking of arsenic as not just a bad guy, but also as beneficial, has reshaped the way that I view the element,” said first author Jaclyn Saunders, who did the research for her doctoral thesis at the UW and is now a postdoctoral fellow at the Woods Hole Oceanographic Institution and the Massachusetts Institute of Technology.

The study, “Complete arsenic-based respiratory cycle in the marine microbial communities of pelagic oxygen-deficient zones,” was published in the Proceedings of the National Academy of Sciences.

“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.”

With funding provided by a graduate fellowship from NASA and a research grant from the National Science Foundation, the researchers analyzed 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.

“In some parts of the ocean there’s a sandwich of water where there’s no measurable oxygen,” Prof. Rocap said. “The microbes in these regions have to use other elements that act as an electron acceptor to extract energy from food.”

The UW team analyzed samples collected during a 2012 research cruise to the tropical Pacific. 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 the authors believe that the pathways occur in two organisms that cycle arsenic back and forth between different forms.

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

Believed to be a remnant behavior from the Archean until the late Proterozoic eons in Earth’s early history when both atmospheric and oceanic oxygen was scarce, organisms needed a strategy to extract energy using other elements, such as arsenic, which was likely more common in the oceans at that time. Not until the late Proterozoic eon, some 2.4 billion years ago, did photosynthesis become widespread and the conversion of carbon dioxide gas into oxygen make the Earth’s atmosphere abundant with oxygen.

“We found the genetic signatures of pathways that are still there, remnants of the past ocean that have been maintained until today,” Saunders said.

“For me, it just shows how much is still out there in the ocean that we don’t know,” Prof. Rocap said.

Co-author Clara Fuchsman collected the samples and led the DNA sequencing effort as a UW postdoctoral research scientist and now holds a faculty position at the University of Maryland. The other co-author is Cedar McKay, a research scientist in the UW School of Oceanography.

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