Arsenic is generally viewed as a life-ending element, but new research shows how some organisms rely on it to breathe.
Certain microorganisms in the Pacific Ocean respire arsenic, according to a new study from the University of Washington. The findings are quite surprising as, although arsenic-based respiration has been documented in ancient and current organisms, it is extremely rare on the planet. Moreover, ocean water just doesn't have that much arsenic, to begin with.
"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 team analyzed Pacific seawater samples taken from water layers at depth intervals where oxygen is almost absent. Given the lack of oxygen here, organisms had to adapt and seek other sources of energy, the team writes. The results are interesting and may become very important in our understanding of marine ecosystems, as these areas -- known as oxygen-deficient zones, ODZs or oxygen minimum zones, OMZs -- will likely expand under climate change, according to other recent research.
The most common alternatives to oxygen that biology draws upon today are nitrogen and sulfur. However, previous research carried out by Jaclyn Saunders, this paper's first author, suggested that arsenic might also do the trick. She was curious to see whether this was the case, which spurred the present paper.
The samples used in this study were collected during a 2012 research cruise to the tropical Pacific, off the coast of Mexico. Analysis of eDNA material recovered from the samples showed two genetic pathways that process arsenic-based molecules to extract energy. Two different forms of arsenic seem to be targeted by these pathways, leading the authors to believe that we're looking at two organisms that cycle arsenic back and forth between the different forms. Which, as far as ecosystems are concerned, is quite a nifty trick.
"Thinking of arsenic as not just a bad guy, but also as beneficial, has reshaped the way that I view the element," said 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.
While arsenic might be beneficial, it's certainly not very popular. Only about 1% of the microbe population in the samples seems to breathe arsenic, judging by the ratios of genetic material. Most likely, these strains are loosely-related to arsenic-breathing microbes found in hot springs or contaminated sites on land. Saunders recently collected samples from the same region and is now trying to grow the arsenic-breathing marine microbes in a lab in order to study them more closely.
"Right now we've got bits and pieces of their genomes, just enough to say that yes, they're doing this arsenic transformation," Rocap said. "The next step would be to put together a whole genome and find out what else they can do, and how that organism fits into the environment."
"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."
Arsenic respiration is most likely a 'retro' type of respiration, passed down over the eons. When life first sprung up on Earth, oxygen was very scarce both in the air and in the ocean (as oxygen is very reactive and forms chemical bonds readily). Until photosynthesizing plants became widespread, there simply wasn't enough output of this gas to maintain any meaningful levels available for organisms to use up. As such, early life had to use something else for energy -- and arsenic was likely common in the oceans at that time.
Climate change may, sadly, breathe new life into arsenic-breathing life. Low-oxygen regions are projected to expand as thermal imbalances shift water currents, and dissolved oxygen is also predicted to drop across the board in marine environments.
The paper "Complete arsenic-based respiratory cycle in the marine microbial communities of pelagic oxygen-deficient zones" has been published in the journal Proceedings of the National Academy of Sciences.