Life Discovered in the Deepest Ocean

Researchers probing the deepest ocean have found a surprisingly high concentration of microbes, the latest evidence of organisms thriving in inhospitable environments that is reshaping scientists' understanding of the conditions necessary for life.

The bacteria were found nearly 6.8 miles below sea level, on the floor of the Pacific Ocean's Mariana Trench, according to the researchers, whose findings were published online Sunday by the journal Nature Geoscience. No light reaches that part of the ocean, where the temperature is an estimated 36 degrees Fahrenheit.

"Even in these extreme conditions we have bacterial communities," said Ronnie Glud, a Danish scientist who led the project. Although researchers expected to find some microbes, Dr. Glud said the presence of so much activity was a surprise.

At such depths, little is left of the plant matter on the ocean surface that, as it drifts downward, sustains fish and other ocean life. Filmmaker James Cameron, who last year dived in a submersible to the floor of the Mariana Trench, called it a "sterile, almost desert-like place."

Yet scientists have made a steady string of discoveries of life in such extreme environments that have stretched notions about the temperatures, pressures and other conditions possible for life. In February, bacteria were found a half-mile below the Antarctic ice, thriving without sunlight. Previously, jellyfish and tubeworms were discovered near underwater chimneys that heat the surrounding water to 635 degrees Fahrenheit.

"Nobody realized there is so much biodiversity," said Anna-Louise Reysenbach, a microbiologist at Portland State University's Center for Life in Extreme Environments who wasn't affiliated with the study. Discovery of life in unexpected places, she said, "helps inform us what is possible elsewhere in the solar system."

The team led by Dr. Glud, of the University of Southern Denmark, probed the sediments of the Mariana Trench in late 2010. The scientists dropped a submersible outfitted with sensors to gauge oxygen levels, a way to measure how much microbes were eating and digesting of the scraps of food left in the soil. The submersible also collected sediment samples from the trench floor and took hours of video footage.

The researchers explored the deepest part of the Mariana Trench, an area called the Challenger Deep. In addition to bacteria, the probes turned up single-cell organisms called archaea, said Dr. Glud. His team is now sequencing the DNA of these organisms to gain an understanding of how they can live at such depths, he said.

Both kinds of microbes, Dr. Glud said, were feeding on the remnants of algae and other plants that hadn't been gobbled up by other sea creatures during the descent to the ocean floor. That process helps turn such matter into carbon dioxide, which is important in keeping the planet's oxygen level up.

A shallower part of the ocean, about four miles below sea level, had less than half as much microbial activity, the researchers found. That suggests the geography of Challenger Deep made it a landing spot for more organic matter than other deep parts of the ocean, perhaps because it is so far down and maybe because mudslides helped ferry the matter to the spot.