by John Roach, National Geographic , Thu Feb 3, 2005
At the ocean’s deepest point, the water pressure is the equivalent of having about 50 jumbo jets piled on top of you. Yet even here life thrives, according to scientists who have pulled a plug of dirt from the seafloor.
The sample was taken from the Challenger Deep, which is nearly 7 miles (11 kilometers) deep. The soil was packed with a unique community of mostly soft-walled, singled-celled organisms that are thought to resemble some of the world’s earliest life forms.
They’re called foraminifera, single-celled protists that construct shells. Protists are a kingdom of celled organisms distinct from animals, plants, and fungi. Other types of protists include algae and slime molds.
There are an estimated 4,000 species of living foraminifera. They inhabit a wide range of marine environments, mostly on the ocean bottom, though some live in the upper 300 feet (100 meters) or so of the ocean. A few species are found in fresh water and on land.
However, the discovery of 432 foraminifera living in dirt from the Challenger Deep surprised Hiroshi Kitazato, a program director at the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) in Yokosuka.
“We are surprised that so many [bottom dwelling] foraminiferaâ€”in particular so [many] soft-shelled formsâ€”live in the Challenger Deep, because former reports gave us the impression the world’s deepest point is scarce in any [celled organisms],” he said.
At 36,201 feet (11,034 meters) below sea level, Challenger Deep is the lowest part of the Pacific Ocean’s Mariana Trench, located just east of the Philippines.
The sediment core was collected with KAIKO, JAMSTEC’s remotely operated vehicle. Kitazato and colleagues report the discovery in tomorrow’s issue of the journal Science.
Brian Huber is the curator of foraminifera at the Smithsonian Institution’s National Museum of Natural History in Washington, D.C. He said the discovery of abundant soft-walled foraminifera in Challenger Deep is “one more example of how you can find life in the most extreme environments.”
In recent years scientists have found bacteria living miles beneath the Earth’s surface and snuggled up to scalding hydrothermal vents on the ocean floor. There is even evidence that bacteria live in Antarctic lakes that have been covered by ice for thousands of years.
Jere Lipps, a foraminifera expert in the Department of Integrative Biology at the University of California, Berkeley, is not surprised by the find. He believes foraminifera have lived at such great depths for millions of years and thus are adapted to life there.
However, he said, the discovery raises more questions than it answersâ€”questions “about the distribution, evolution, adaptation to high pressures,” and past distribution of foraminifera.
An earlier DNA analysis of similar forms suggests to Kitazato and colleagues that the Challenger Deep foraminifera may resemble some of the first life-forms on Earth.
The newfound specimens are primitive organisms that may be part of a lineage that is thought to date back to Precambrian times. The Precambrian is the earliest era in Earth’s history. It came to an end some 542 million years ago, when shelled organisms first appeared in the seas.
But the University of California’s Lipps is not so sure just how primitive the new specimens are. He said that DNA analysis of similar foraminifera is needed before conclusions are drawn.
Most surprising to Kitazato and his colleagues is the high percentage of tube-shaped, soft-walled foraminifera at Challenger Deep. They account for 85 percent of the assemblage. In most deep-sea environments, they make up 5 to 20 percent of the living assemblage. The rest of the assemblage was made up of spherical and clumped-together forms.
Hard-shell species are unable to survive at such depths, the researchers note in Science.
Though Kitazato and colleagues thought some life would exist at Challenger Deep, the abundance caught them off guard. Kitazato said there are likely two reasons for the abundance.
First, soft-walled foraminifera house waste pellets that the scientists believe may be a source of nutrients in the otherwise nutrient-poor environment. “Second is that Challenger Deep is scarce in predators for foraminifera, [allowing] soft-shelled foraminifera to live safely,” Kitazato said.
The foraminifera found at Challenger Deep are most closely related to forms that live at an average depth of 16,400 feet (5,000 meters). Kitazato and his colleagues theorize that these forms adapted to the steady increases in pressure as the Challenger Deep formed and deepened over the last six to nine million years.
As the depth increased, hard-shelled foraminifera would have been weeded out, as they are unable to survive at great depth, Kitazato said.
But according to Lipps, the foraminifera’s adaptations to the deep sea may have taken place before the Challenger Deep formed. “[Groups of life-forms] move around a lot in the ocean, especially over the geologic ages, so this might be an even more reasonable speculation,” he said.
Scientists say that foraminifera are of crucial importance to ocean biology and geology. Among creatures found in the deep sea, they make up more than half of all living matter and fill the lower part the food chain.
In general, foraminifera are useful for reconstructing past environments, studying ancient climate change, and dating sediments, the Smithsonian Institution’s Huber said. But the Challenger Deep foraminifera are of little help in these efforts.
“However, they are certainly one more example of how foraminifera have adapted to occur in a huge range of environments,” he said.