Buried under the seafloor for 86 million years, a bacterial community lives so slowly it's still surviving on a "lunch box" from dinosaur days, a new study says. (See marine-microbe pictures.)
It's been known since the 1990s that microbes can live trapped in ocean sediments for millions of years, but until now it's been a mystery how these organisms make a living.
To find out, scientists collected mud-dwelling bacteria from 11 spots, each tens of meters below the seafloor of the North Pacific Gyre, a circular current that encompasses much of the Pacific.
The gyre "just turns round and round like a huge closed pot, without exchanging much water with the rest of the ocean," said study leader Hans Røy, a geomicrobiologist at Aarhus University in Denmark.
As a result, vast swaths of ocean around the gyre—and the sediments below—are among Earth's most nutrient starved.
Probing the mud with oxygen sensors, Røy's team found that the deep-sediment bacteria are consuming oxygen at extremely sluggish rates. What's more, the team discovered the microbes are living off the same supply of organic carbon that got trapped along with them.
"They left the surface world when the dinosaurs walked the planet—and they are still eating the same lunch box that they got back then," Røy said.
And they're not alone: Such microbes may be the most common organisms on the planet, making up about 90 percent of Earth's single-celled life, recent research suggests.
Deep-Sea Bacteria Hard to Decode
Scientists don't know much about these deep-seafloor bacteria—they can't even be named yet, since their DNA doesn't match that of any known bacterial species, Røy said.
It's like having a set of fingerprints form a crime scene but no others to compare it to, he said. At the genetic level, the deep bacteria "don't look like anyone we know, so it limits the usefulness of DNA work."
Further complicating matters, in the lab "we're not able to cultivate the majority of the organisms" found deep under the seafloor, said marine microbiologist Danny Ionescu, who wasn't involved in the new study.
"Very often we're trying to overfeed them [when] they're used to poor conditions," said Ionescu, of the Max Planck Institute in Germany.
Because the organisms are so deprived, they've also adapted to reproduce very slowly, unlike other bacterial species, many of which multiply by the millions within a few days.
By contrast, trying to grow deep-sea bacteria in a petri dish is like "looking at a tree to see if it grows," said study leader Røy said, whose team's work will be published Friday in the journal Science.
Life in Slow Motion
Luckily, it wasn't hard to figure out the life spans of North Pacific Gyre bacteria. "Nature made the experiment for us," Røy said.
For starters, it's easy to determine the ages of mud layers under the North Pacific Gyre, since sediments there accumulate very slowly—about a millimeter every thousand years.
The team used their oxygen sensors to determine how much oxygen is "missing" from various sediment layers due to bacteria using the gas.
"It's a simple calculation—measuring what oxygen got in, compared with what didn't get out."
From that data, the scientists determined that each of the bacteria reproduce between once every few hundred years and once every few thousand years. This doesn't break any records—many organisms, such as sponges, can live longer.
What makes the research "fascinating," Max Planck's Ionescu said, is that it shows how life can survive on so little.
The species' slow-motion lifestyle, he added, "breaches the limits of life as we know it."