The ocean floor is home to a staggering 2.9×1029 single-celled organisms — that's 10 million trillion microbes for every human on the planet – but gigantic though it is, this figure is more than an order of magnitude lower than the previous estimate. Geomicrobiologist Jens Kallmeyer and colleagues at the University of Potsdam in Germany have created the most accurate model so far of the geographical distribution of microbes in marine sediment. Their work is published today in the Proceedings of the National Academy of Sciences1.

“For ten years everyone’s had the sneaking suspicion those numbers were wrong,” Kallmeyer says, referring to a 15-year-old estimate of the numbers of microbes at the bottom of the oceans2 by geomicrobiologist William Whitman at the University of Georgia in Athens. In recent years, Kallmeyer and his colleagues have amassed new data on microbe abundance by drilling holes in the sea floor to study microbes surviving at the extremes of nutrient starvation. They used these data to create a mathematical model that could more accurately predict the biomass hidden in kilometres of ocean sediment. The new calculations decrease previous estimates by 92%, and reduce the estimated numbers of all microbes on Earth by around 50%, from a previous figure of to between 9.2×1029 and 31.7×1029. But, says Kallmeyers, "their still gigantic number means that they play a key role in global biogeochemical cycles".

The previous estimate was probably off because it was based on work in nutrient-rich coastal upwelling zones, which, says Kallmeyer, would have led to an overestimation of the numbers. In contrast, much of Kallmeyer’s research has been in areas he refers to as “the deserts of the sea”, such as the The South Pacific Gyre, one of the most nutrient depleted regions in the ocean.

Geomicrobiologist Kai-Uwe Hinrichs is currently on his own expedition looking for life at a greater depth than any previous research team. He says Kallmeyer’s team "have done a better job of estimating the sheer volume of the marine sediment biomass". This is because, when they calculated how much carbon is present in the sediment microbes, they took into account the fact that the cells surviving here are so starved of energy that they rarely do anything for hundreds or even thousands of years. Whitman's estimate had presumed all microbes on Earth were similar to those in a laboratory culture, which Kallmeyer describes as “very happy cells that grow big and fat”.

But geomicrobiologist John Parkes of Cardiff University thinks that Kallmeyer's calculation may be on the low side due the difficulties associated with counting small numbers of cells encassed in mud. He also points out how microbes are continually proving that they can push the limits at which life can be discovered to further extremes. Kallmeyer, too, knows that his calculations are only as good as the data and assumptions he used to make them. He says he hopes that in twenty years he will see someone else improve on his work just as he has done with Whitman's. “Without his paper the whole area of deep biosphere research would not be where it is today.”

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