Samples drilled from 3.5-million-year-old seafloor rocks have yielded the strongest evidence yet that a variety of microorganisms live deeply buried within the ocean's crust. These microbes make their living by consuming methane and sulfate compounds dissolved in the mineral-rich waters flowing through the immense networks of fractures in the crust. The new find confirms that the ancient lavas formed at midocean ridges and found throughout deep ocean basins are by volume the largest ecosystem on Earth, scientists say.
Many teams have found hints of microbial life in rock samples drilled from the ocean floor. Just as often, questions have arisen about whether the microbes were actually living in the rocks when samples were obtained. For example, many scientists argue that DNA and other organic compounds could simply be remnants of past life, says Mark Lever, a geomicrobiologist at Aarhus University in Denmark. Many others, he notes, suggest that samples are too easily contaminated during drilling by microbes that live in overlying sediments, or by inadequate precautions while handling the samples once they've been retrieved from the sea floor.
In their latest study, Lever and his colleagues tackle such arguments head-on. The samples of ancient lava that they analyzed came from a heavily fractured zone of crust between 351 and 583 meters below the sea floor a few hundred kilometers off the shore of Washington state. There, about 100 kilometers east of the submerged Juan de Fuca Ridge, rocks are about 3.5 million years old, Lever says. After taking the samples back to the lab, the team sterilized the outside of the rocks, carved away their outer layers, then incubated the exposed samples in 65°C waters similar to those infiltrating the sea floor at the site—poor in oxygen but rich in chemicals such as dissolved hydrogen, sulfates, acetates, methanol, and dimethyl sulfide. After 2 years—enough time for some of the typically slow-growing rock-dwelling microbes to reproduce—samples of the fluid were transferred into another container of simulated seafloor waters, along with sterilized samples of rock that microbes could grow upon, and then incubated for another 5 years.
Analyses revealed that the long-incubated fluids contained low concentrations of methane with a lower-than-normal concentration of the carbon-13 isotope—a sure sign that methane-producing microorganisms were living in the fluid, the researchers report online today in Science. "These cultivations in the lab show that the organisms were alive," Lever says. Another set of tests revealed that five of the 10 rock samples analyzed contained genetic material from methane-producing microbes, bolstering the notion that such organisms were living deep beneath the sea floor. Additional tests indicated that a separate group of microbes, ones that make their living by consuming sulfates, was also inhabiting the heavily fractured rocks. Similar organisms—or at least some that have the same methane-producing or sulfate-consuming lifestyles—can be found in other environments, including some unusual, oxygen-poor or oxygen-free habitats at Earth's surface.
The team's findings are "very exciting," says James Cowen, a microbial geochemist at the University of Hawaii, Manoa, in Honolulu. "This is very strong work," he notes. "To get relatively uncontaminated material from the ocean floor is quite difficult."
It isn't clear when the deep-sea rocks were colonized by the microbes, but it could have occurred soon after the rocks were formed at the Juan de Fuca Ridge millions of years ago, Lever says. Or, it's possible that the rocks were colonized in recent times, long after they cooled. Also, although the microorganisms certainly influence the chemistry of the rocks and the seawater that flows through them, their precise effect within the crust isn't yet known. Their effect on the chemistry of the ocean is unknown as well.
"We don't have enough information about these organisms or their growth rates," says Katrina Edwards, a geomicrobiologist at the University of Southern California in Los Angeles. Nor do scientists have a good handle on how much microbial biomass exists in the ocean crust—an ecosystem that according to some estimates covers about 60% of Earth's surface. Previous studies suggest that each single cubic centimeter of water flowing through the seafloor crust could contain 10,000 microbes, about the same number found in deep waters just above the sea floor in many regions.