At 140 to 212 degrees Fahrenheit (60 to 100 degrees Celsius), the microscopic life forms are probably also the hottest life-forms yet found in seafloor sediments, according to study co-author R. John Parkes, a microbiologist at Cardiff University in the United Kingdom.
(Related: "Hottest Life-Form Found: Microbe Thrives When Boiling" [May 21, 2004].)
The scientists examined core samples of sediments in the North Atlantic Ocean and found microbes known as prokaryotes.
Many of the prokaryotes share characteristics in common with "extremophiles," which live in hot springs, both under the sea and in areas such as Yellowstone National Park.
The microbes appear to make their livings by metabolizing methane and other hydrocarbons created as the Earth's interior heat warms organic material in the sediments, Parkes said.
"That's what we think they're using as an energy source."
The organisms do not appear simply to have been dormant microbes trapped in the sediments, Parkes added, but instead appeared to be thriving.
The discovery supports predictions that as much as 70 percent of the Earth's prokaryotes may live in seabed sediments, some of which can be several miles thick.
All told, Parkes said, these prokaryotes could amount to 10 to 30 percent of the world's total living matter.
Life on Mars?
The find is also significant for the search for life on Mars and other planets. Our "surface centric" view of life on Earth, Parkes said, may mean we're looking in the wrong places for life elsewhere.
"There are [nonbiological] sources that can produce methane [and related chemicals]," he said. "Therefore there might be a biosphere on other planets that may not require" the ability to harness sunlight on a planet's surface for energy.
Other scientists agree.
"The more places we look for life, the more places we find it," said Dennis Geist, a University of Idaho geologist who was not involved in the study. "This [new study] furthers the notion that the days of limiting our search for new life to surface conditions are long gone," he continued by email.
"The findings of this work push the limits in terms of both pressure and temperature."
In fact, Geist notes, the drilling for the core samples used in the study began on a seafloor 2.8 miles (4.5 kilometers) beneath the waves, where the water temperature is barely above freezing. The drilling ended at a point beneath the ocean bed where the temperature is nearly equal to that of a boiling pot of water.
"The range of physical conditions is enormous."