A team of marine biologists from the University of Washington has found communities of arsenic-breathing microbes in the oxygen-deficient zones of the Pacific Ocean. The findings suggest microbial arsenic metabolism may be underestimated in the modern ocean and was likely a significant contributor to biogeochemical cycles in the ancient anoxic (oxygen-free) oceans when arsenic concentrations were higher.
“In some parts of the ocean there’s a sandwich of water where there’s no measurable oxygen,” said Professor 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 most common alternatives to oxygen are nitrogen or sulfur. But the team’s earlier investigations suggested arsenic could also work, spurring her to look for the evidence.
The researchers analyzed samples collected during an April 2012 research cruise to the tropical Pacific, off the coast of Mexico, aboard R/V Thompson.
They identified 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 scientists believe that the pathways occur in two organisms that cycle arsenic back and forth between different forms.
The 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,” said Dr. Jaclyn Saunders, first author of the study.
“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.
“We found the genetic signatures of pathways that are still there, remnants of the past ocean that have been maintained until today,” Dr. Saunders said.
The findings were published this week in the Proceedings of the National Academy of Sciences.
Jaclyn K. Saunders et al. Complete arsenic-based respiratory cycle in the marine microbial communities of pelagic oxygen-deficient zones. PNAS, published online April 29, 2019; doi: 10.1073/pnas.1818349116