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RandomFactor writes:

Arsenic breathing bacteria have been located in the depths of the Tropical Pacific Ocean.

Arsenic is a deadly poison for most living things, but new research shows that microorganisms are breathing arsenic in a large area of the Pacific Ocean. A University of Washington team has discovered that an ancient survival strategy is still being used in low-oxygen parts of the marine environment.

Arsenic displays a raft of toxic effects ranging from DNA Methylation (adding a methyl group to DNA nucleotides) to replacing Phosphorous in various metabolic reactions.

The most common metabolic alternatives to Oxygen are Nitrogen and Sulfur, however Arsenic can also work.

Some humans have evolved significantly higher tolerances to Arsenic, and arsenic breathing microbes have been known to exist in hot springs and contaminated sites on land, but until now it was not known that this capability still existed in the open ocean.

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," [first author Jaclyn Saunders, who did the research for her doctoral thesis at the University of Washington] said.

The microbes were found in what are called marine oxygen-deficient zones (ODZs), which are naturally occurring layers in the ocean with no measurable oxygen.

We identified the presence and expression of genes for both arsenic reduction and oxidation in marine ODZs, suggesting the microbial community in these waters is also cycling arsenic for respiratory gain. The existence of an arsenic respiratory cycle in pelagic waters suggests microbial arsenic metabolisms may be underestimated in the modern ocean and were likely an even more significant contributor to biogeochemical cycles in the anoxic ancient oceans when arsenic concentrations were higher.

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