Evidence of oxygen-breathing microbial life has been found in places where scientists did not think life could exist – such as more than 200 feet below the ocean floor that is more than 12,000 feet below sea level in the Pacific Ocean – according to a team of international researchers that includes a Texas A&M University research scientist.
Carlos Alvarez-Zarikian, expedition project manager and staff scientist with the International Ocean Discovery Program (IODP) at Texas A&M, and other members of the international research team have had their work published in the current issue of Nature Geoscience.
The team found oxygen and oxygen-breathing microbes all the way through the sediment from the seafloor to the igneous basement at seven sites in the South Pacific gyre, considered the “deadest” location in the ocean. Their findings contrast with previous discoveries that oxygen was absent from all but the top few feet of sediment in biologically productive regions of the ocean.
“Our objective was to understand the microbial community and microbial habitability of sediment in the deadest part of the ocean,” says Professor Steven D’Hondt of University of Rhode Island, the lead researcher on the project.
“Our results overturn a 60-year-old conclusion that the depth limit to life is in the sediment just meters below the seafloor in such regions. We found that there is no limit to life within this sediment. Oxygen and aerobic microbes hang in there all the way to the igneous basement, to at least 220 feet below the seafloor.”
Based on the researchers’ predictive model and core samples they collected in 2010 with the IODP’s research ship JOIDES Resolution, the researchers believe that oxygen and aerobic microbes occur throughout the sediment in up to 37 percent of the world’s oceans and 44 percent of the Pacific.
They found that the best predictors of oxygen penetration to the igneous basement are a low sediment accumulation rate and a relatively thin sediment layer. Sediment accumulates at just a few inches to feet per million years in the regions where the core samples were collected.
In the remaining 63 percent of the ocean, most of the sediment beneath the seafloor is expected to lack dissolved oxygen and to contain anaerobic communities.
While the research team found evidence of life throughout the sediment, it did not detect a great deal of it. The team found extremely slow rates of respiration and quantities that were nearly undetectable by previous techniques. “It’s really hard to detect life when it’s not very active and in extremely low concentrations,” explains D’Hondt.
“The findings show the lowest cell concentrations and lowest rates of microbial activity ever encountered in marine sediment,” Alvarez-Zarikian adds.
“It shows that there is no limit within the sediment in these deep environments that can reach 18,000 feet or greater below the ocean surface. The research team used new instruments and techniques that have never been utilized before on drilling expeditions such as this one.”
The research team includes 35 scientists from 12 countries. Contributing to the research from Texas A&M are research scientists Alvarez-Zarikian and Helen Evans, both with the IODP, and graduate student Claire McKinley from the Department of Oceanography.
Full text of the research paper is available at http://dx.doi.org/10.1038/ngeo2387.
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