Rare, 2.5-Billion-Year-Old Rocks Reveal the Origins of Sulfur-Breathing Bacteria

When you visit a hot spring or go to Yellowstone National Park, chances are you'll catch a whiff of rotten eggs. That's the smell of hydrogen sulfide gas, which is the biochemical signature of sulfur-using bacteria. Now, scientists have taken a closer look at these ancient and widespread organisms to learn a bit more about the evolution of life on our planet.

There's actually a vigorous debate about the evolution of sulfur-dependent bacteria. These organisms first appeared during a time when oxygen levels on Earth were less than one-thousandth of what they are now. These bacteria instead lived in ocean water where they breathed in sulfate, which is a sulfur and oxygen compound. But scientists have long wondered how this sulfate reached the ocean in the first place, and when it became abundant enough for living things to use.

Now, scientists may have the answer. The researchers analyzed the biochemical signals of sulfur compounds found in 2.5-billion-year-old carbonate rocks from Brazil. The rocks were formed on the ocean floor during the Neoarchaean Eon, and then surfaced when prospectors drilling for gold in Brazil bunched a hole into the bedrock.

The scientists found that the samples carried a very strong signal that sulfur compounds were consumed and altered by living organisms. In addition, the researchers used basic geochemical models to get an idea of how much sulfate was in the oceans at that time, and found that sulfate concentrations were very low-far lower than previously thought.

But how did the sulfur-dependent bacteria survive in these conditions? That's a good question. The scientists believe that the bacteria were in shallow water, where evaporation may have been high enough to concentrate the sulfate. This would have made it abundant enough to support the bacteria.

"There is an ongoing debate about when sulfate-reducing bacteria arose and how that fits into the evolution of life on our planet," said James Farquhar, one of the researchers, in a news release. "These rocks are telling us the bacteria were 2.5 billion years ago, and they were doing something significant enough that we can see them today."

The findings are published in the journal Science.