Researchers working off the coast of Mexico have discovered evidence of arsenic-breathing life in oxygen-starved waters. These resilient microbes are a vestige of Earth’s ancient past, but they could also be a sign of things to come under the influence of climate change.
Billions of years ago, when the Earth was still very young, the first organisms to emerge did not have the benefit of abundant oxygen. Instead, these pioneering microbes likely exploited other elements to get their energy, including nitrogen, sulphur and, perhaps surprisingly, arsenic — a compound typically associated with poison.
Eventually, our planet became rich in oxygen owing to the effects of photosynthesising organisms, which converted carbon dioxide into oxygen.
Plentiful amounts of oxygen made those early microorganisms obsolete — or at least that’s what we thought. New research published this week in Proceedings of the National Academy of Sciences suggests some organisms with an arsenic-based respiratory system are still around, and they’re found pretty much where you’d expect them to be: Low-oxygen environments.
More specifically, they live in marine oxygen-deficient zones (ODZs) — a middle layer of tropical ocean where oxygen exists in trace amounts.
The traces of these atavistic microbes were discovered by a team led by Jaclyn Saunders, a postdoctoral fellow at the Woods Hole Oceanographic Institution and the Massachusetts Institute of Technology, off the coast of Mexico in the Eastern Tropical South Pacific ODZ. The new research shows that this ancient survival strategy is still in use some low-oxygen, or anoxic, marine ecosystems.
“We’ve known for a long time that there are very low levels of arsenic in the ocean,” Gabrielle Rocap, a co-author of the study and a University of Washington professor of oceanography, said in a press statement. “But the idea that organisms could be using arsenic to make a living — it’s a whole new metabolism for the open ocean.”
The creatures that live in ODZs, she said, “have to use other elements that act as an electron acceptor to extract energy from food”.
Given the ongoing effects of climate change, the new finding grimly suggests an expansion of habitat for these microbes, as ODZs are produced by sensitive imbalances between the amount of oxygen available in the atmosphere and the decay of organic matter.
More encouragingly, however, the finding also holds implications in the search for extraterrestrial life. Astrobiologists can now include low-oxygen, arsenic-friendly environments in their hunt for microbial alien life.
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For the study, Saunders’ team analysed seawater pulled from the ETSP ODZ. Among the bits and pieces of DNA found floating in the solution were genetic pathways associated with arsenic.
As Saunders explained to Gizmodo in an email, the pathways her team identified all appear to be bacterial in nature, as the sequences derived came from a prokaryotic, or single-celled, metagenome (that is, all the genetic material present in an environmental sample).
“We filtered seawater through a 30-micron mesh first — so we’re only looking at small organisms — and then we passed that seawater through a 0.2 micron filter which captured the microbial cells,” said Saunders. “The DNA from all the organisms captured was then extracted, cut up into little pieces, and sequenced.”
These short sequences were then put together in a “sequence puzzle” to create a long, contiguous stretch of DNA called a contig, she explained. From there, the arsenic-related genes were identified on the contigs assembled from the metagenome sequence data.
This form of DNA sequencing “has really accelerated our understanding of environmental communities that aren’t amenable to classical microbiological isolation techniques,” said Saunders.
Two species of microorganism, the researchers suspect, are cycling two forms of arsenic in what is now a newly detected respiratory cycle, where respiration is essentially the transformation of chemical energy into biological energy. Less than one per cent of the total microbe population found in these waters are capable of breathing arsenic, the researchers estimated.
The scientists also speculated that these water-borne microbes might be distantly related to similar microbes found in hot springs and contaminated, arsenic-rich sites on land.
The existence of this cycle “may be underestimated in the modern ocean” and potentially a “significant contributor to biogeochemical cycles in the anoxic ancient oceans when arsenic concentrations were higher,” the researchers wrote in the study.
In terms of next steps, Saunders is hoping to culture the arsenic-gobbling microbes in her lab so they can be studied further.
“I have also returned to the location where this DNA sample was collected to try to observe microbial cycling of arsenic through coupled chemical and genetic sequencing analyses,” she said. Indeed, a logical next step is to put together a whole genome to better characterise these microbes and determine how they fit into the larger marine environment.
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Saunders agreed that her finding is relevant to the search for extraterrestrial life.
“There are ocean worlds — planetary bodies that have liquid water oceans — in our own solar system,” she told Gizmodo. “Enceladus is a moon of Saturn that has a rocky core, a liquid water ocean, and a thick ice crust on the surface. It is one of the most promising locations for finding life elsewhere in the universe.”
The identification of these arsenic-friendly organisms in the oxygen-poor open ocean water column, she said, expands the limits at which scientists would traditionally search for such microbes.
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