The bottom of the sea is a biologist's wonderland, but scientists are only beginning to understand the even more mysterious organisms that live deep under its floorboards. These buried microbial communities have been thought to be simple and nearly dormant. Now, a study of sediments buried nearly half a kilometer below the Pacific reveals surprisingly complex and active populations.
The vast majority of life on Earth, at least in terms of biomass, is microscopic and hidden beneath the seafloor. Despite their large numbers, these organisms don't do much. Surveys have found that most of them either reduce sulfate or produce methane, and some estimates of their metabolic rates indicate that they reproduce as slowly as once every 100,000 years. Some scientists even think the bacteria are just plain dead.
This picture may be about to change in light of a study of deep-sea rocks and sediments led by John Parkes, a microbiologist at Cardiff University in the U.K. By visiting oil-drilling projects at two sites in the Pacific in 2002, Parkes and colleagues obtained samples as deep as 400 meters beneath the seafloor. Back on their research ship, the researchers counted the microbes and tested their metabolic activity by seeing how many nutrients they took up or broke down. They also sequenced the DNA of the organisms to estimate the populations' total genetic diversity.
Not only is there a much higher diversity of microbes under the seafloor than originally thought, large and active populations exist much deeper in the sediments than was believed, the team reports 21 July in Nature. Besides sulfate-reducers and methane-producers, the researchers found a variety of bacteria that use carbon for energy. The metabolic activity and genetic diversity peaked around sea floor layers packed with the remains of plankton called diatoms, which presumably provide the carbon and other nutrients for the more complex ecology.
Populations of microbes in the oldest sediments are estimated to have been trapped for as long as 11 million years, living in complete isolation from sun-powered life. The researchers argue that these ancient populations are still metabolically active because they contain soluble manganese, a by-product of living off minerals. Moreover, they will quickly gobble up nutrients back in the lab.
The survey is "the most comprehensive I have seen for deep subseafloor sediments," says John Baross, a microbiologist at the University of Washington at Seattle. He wonders if there are buried microbial communities much older than 11 million years, and whether any of them hide other markers of complex ecology, such as viruses.
Gallery of marine sediment bacteria