“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.”
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 measureable oxygen,” Rocap said. “The microbes in these regions have to use other elements that act as an electron acceptor to extract energy from food.”
The most common alternatives to oxygen are nitrogen or sulfur. But Saunders’ early investigations suggested arsenic could also work, spurring her to look for the evidence…
Results suggest that arsenic-breathing microbes make up less than 1% 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.
“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.”
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. Paper. (paywall) – Jaclyn K. Saunders, Clara A. Fuchsman, Cedar McKay, Gabrielle Rocap. Complete arsenic-based respiratory cycle in the marine microbial communities of pelagic oxygen-deficient zones. Proceedings of the National Academy of Sciences, 2019; 201818349 DOI: 10.1073/pnas.1818349116 More.
A friend tells us this looks like a new find and not to be confused with a previous one that didn’t hold up.
What’s amazing is the way life, once started, exploits every avenue, and yet how does it get started?