BioEd Online

Their home lies further beneath sea level than Everest's peak rises above it. And yet tiny organisms have been found living at the very bottom of the Pacific Ocean's deepest trench, the remotest spot on the globe.

The microscopic organisms, called foraminifera, live in mud at the bottom of Challenger Deep in the Mariana Trench, almost 11 kilometres beneath the waves of the western Pacific Ocean. The pressure at this depth is a crushing 1,090 times that at the surface.

Researchers led by Hiroshi Kitazato of the Japan Agency for Marine-Earth Science and Technology in Yokosuka collected samples of sediment from the trench using the Kaiko submarine. This remote-controlled deep-sea vessel made its first trip to the Mariana Trench in 1995; it was lost at sea on a mission in May 2003.

From the samples, Kitazato's team recovered numerous bacteria and 432 living foraminifera. The latter measure a few dozen micrometres across; hundreds could crowd on to a pinhead. Most of the foraminifera were soft-walled, either spherical or needle-shaped, and coloured brown.The researchers report their discovery in Science¹.

Shell shortage

Foraminifera are perhaps the most abundant ocean life after bacteria, says Alan Hughes, who studies the creatures at the Southampton Oceanography Centre, UK. Most have intricate shells made of calcium carbonate.

But these shells are absent from the foraminifera found in Challenger Deep. Instead, 85% of the researchers' haul belongs to a soft-walled group called allogromiids, which make up only 5-20% of other foraminiferan communities nearer the surface.

This is probably because the deepest ocean has little calcium carbonate, meaning that microbes may not be able to build a shell. "As you go deeper and deeper you get to a depth called the calcium compensation point," Hughes explains. "Below that you tend to get soft-walled foraminifera."

Kitazato's group speculates that soft foraminifera thrive in the Mariana mud because they are among the few creatures that can withstand the huge pressures there. Geologists think that the trench was created over the past 6 million to 9 million years, during which time the ecosystem's less hardy species may literally have been crushed to death.