There's life deep under the sea, but not as much as we thought.
Scientists looking at the abundance of microbes in the sediments under the seafloor — the so-called deep biosphere once thought to be teeming with more than 300 billion tons of life — have found that the vast subterranean world is not quite so crowded.
The new research followed up on landmark findings from 15 years ago, when researchers from the University of Georgia estimated that subseafloor sediments housed 35.5 x 10^29 microbes (that's 1 followed by 29 zeroes), comprising 334 billion tons (303 x 10^12 kilograms) of carbon.
The new study calculates that a mere 4.1 x 10^29 microbes, made up of 4.5 billion tons (4.1 x 10^12 kilograms) of carbon, live in the subseafloor. The new tally is 92 percent smaller than the earlier estimate, but still indicates a significant amount of subseafloor microbial biomass.
Low levels of life
The difference comes from a larger sampling of ocean sediment environments, the researchers say.
"Basically all the sites that were used in the previous studies were in high-productivity areas, so all estimates had to be skewed towards higher values," said Jens Kallmeyer, a geomicrobiologist at the University of Potsdam in Germany, who led the study. "The earlier cell counts are correct — we also used them for our model — but they do not cover the whole range of conditions in the world's oceans."
Sediments in upwelling zones — where nutrient-rich waters from greater depths circulate into upper ocean layers — and areas near the shore typically house the greatest number of microbes because they are the most nutrient-rich areas of the ocean floor. Farther from shore, however, fewer nutrients reach the bottom of the sea and microbial cell counts are typically much lower — up to five times lower, Kallmeyer's team found. [Strangest Places Where Life Is Found on Earth]
As much as 40 percent of the ocean floor can be classified as ultra-oligotrophic, or extremely nutrient-poor, Kallmeyer said. In these areas, ocean sediments contain oxygen throughout their entire depth. Since microbial metabolism typically removes oxygen from ocean sediments, high oxygen levels are a telltale sign of very low levels of microbial life.
Really small, hard to count
Microbial cells found in deep subsurface environments are often extremely small — close to the theoretical limit of how small such life-forms can be — thanks to extremely limited levels of nutrients, Kallmeyer said, which makes counting them difficult. His team had to sample large volumes of sediment to find enough microbes to count under the microscope.
"There isn't much life down there," Kallmeyer told OurAmazingPlanet.
According to the team's findings, detailed online Aug. 27 in the journal Proceedings of the National Academy of Sciences, the subseafloor houses about the same number of microbes as the planet's soil and seawater environments, though Kallmeyer thinks researchers may soon be spurred to re-examine those numbers.
"Given how drastically we changed the subseafloor sedimentary cell abundance by using just a larger dataset, I can't help but ask myself how well the numbers for other environments are actually constrained," he said.
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