Evidence of microbial life breathing arsenic in the Pacific Ocean

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Baja California and the Mexican coast seen from the ISS.

Researchers working off the Mexican coast have discovered evidence of arsenic respiration in oxygen-free waters. These resilient microbes are a remnant of the Earth's ancient past, but they could also be a sign of the influence of climate change.

Billions of years ago, while the Earth was still very young, the first emerging organisms did not benefit from abundant oxygen. Instead, these pioneering microbes probably exploited other elements to get their energy, including nitrogen, sulfur, and, perhaps surprisingly, arsenic, a compound typically associated with a poison. Finally, our planet became rich in oxygen as a result of photosynthetic organisms, which converted carbon dioxide into oxygen.

The team extracts and analyzes anoxic water from the Pacific Ocean.

Abundant amounts of oxygen made these early microorganisms obsolete – or at least that's what we thought. A new study published this week in the Proceedings of the National Academy of Sciences suggests that some organisms with an arsenic-based respiratory system are still present and that they meet roughly where you expect : a low oxygen environment.

More specifically, they live in oxygen deficient marine areas (ZOD), an intermediate layer of a tropical ocean where oxygen exists in trace amounts. 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 ODZ region of the eastern tropical Pacific South. New research shows that this old survival strategy still uses some low oxygen or anoxic marine ecosystems.

"We have known for a long time that there are very low levels of arsenic in the ocean," said Gabrielle Rocap, co – author of the study and professor of oceanography. at the University of Washington, in a press release. "But the idea that organisms could use arsenic for a living is a whole new metabolism for the ocean". The creatures that live in the ODZ, she said, "must use other elements that act as electron acceptors to extract the energy from the food."

Given the lingering effects of climate change, the new discoveries dramatically suggest an expansion of the habitat of these microbes, as ZDOs are produced by significant imbalances between the amount of oxygen available in the atmosphere. and the decomposition of organic matter. More encouragingly, this discovery also has implications for the search for extraterrestrial life. Astrobiologists can now include low oxygen and arsenic-free environments in their hunting of microbial extraterrestrials.

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For the study, the Saunders team analyzed the seawater extracted from the ETSP ODZ. Among the fragments of DNA floating in the solution were genetic pathways associated with arsenic. As Saunders explained to Gizmodo in an email, the pathways identified by his team all appear to be bacterial in nature, as the derived sequences originated from a prokaryotic, or unicellular, metagenome (that is, ie all the genetic material present in an environmental sample).

"We first filtered the seawater through a mesh of 30 microns – so we were interested only in small organisms – then we passed this seawater through a 0.2 micron filter that captured the microbial cells, "said Saunders. "The DNA of all captured organisms was then extracted, cut into small pieces and sequenced."

These short sequences were then put together in a "sequence puzzle" to create a long contiguous DNA segment called contig, she explained. From there, arsenic-related genes were identified on assembled contigs from the metagenome sequence data. This form of DNA sequencing "has really accelerated our understanding of environmental communities that do not lend themselves to conventional microbiological isolation techniques," said Saunders.

Researchers suspect two species of microorganisms to transform two forms of arsenic into a newly detected respiratory cycle, where breathing essentially involves turning chemical energy into biological energy. The researchers estimated that less than 1% of the total microbial population found in these waters was able to breathe arsenic. Scientists have also hypothesized that these waterborne germs may be distantly related to similar microbes found in hot springs and contaminated arsenic-rich terrestrial sites.

The existence of this cycle, "may be underestimated in the modern ocean" and potentially "significant in biogeochemical cycles in ancient anoxic oceans when arsenic concentrations were higher," wrote the researchers in the study.

As for the next steps, Saunders hopes to grow the arsenic-eating microbes in his lab so he can study them more thoroughly.

"I also went back to where this sample of DNA was taken to try to observe the microbial cycle of arsenic through coupled analyzes of chemical sequencing. and genetics, "she said. Indeed, a logical step is to build a complete genome to better characterize these microbes and determine their integration into the broader marine environment.

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