Slideshow: Marine Microbes Rule the Seas

Liz is a staff writer for Science.

Marine census-takers have spent the past decade tracking sea lions, sharks, and other large denizens of the sea. Now they have turned their sights on the ocean's tiniest creatures-the microbes, plankton, larvae, and other organisms that live in the water and sediment or even in and on other life forms. And the numbers are astounding.

Today, the Census of Marine Life, a network of researchers from 80 countries formed a decade ago to document the oceans' diversity, announced that marine microbes account for up to 90% of all ocean biomass and collectively weigh the equivalent of 240 billion African elephants. Six years ago, molecular studies had indicated the microbial marine world was more diverse than expected, but "we were blown away by what the data told us," recalls Mitch Sogin, a molecular biologist at the Marine Biological Laboratory in Woods Hole, Massachusetts, who co-led a microbial survey.

Four of the fourteen studies dealt with the hard-to-see organisms-microscopic creatures or ones that live in the mud. In two, samples of microbes or zooplankton were collected throughout the world by dozens of researchers on shipboard expeditions. A third covered deep-sea vents, and the fourth surveyed the mudflats on the sea floor, both depending on undersea vehicles.

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Slideshow: Marine Microbes Rule the Seas (Flash Slideshow)
Delicate.This amoeba's skeleton is a single crystal that dissolves when the cell dies.
Credit: Dr. Linda Amaral Zettler

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For the microbial work, Sogin used molecular methods to "count" the microbes-sequencing the DNA and seeing how many different versions of a gene existed in a sample. But he took advantage of so-called next-generation sequencing technology, which deciphers DNA much more quickly and cheaply than could be done 5 years ago, and he focused on looking at one family of genes, the ribosomal genes.

In this way, his team was able to characterize more kinds of microbes than previous molecular studies. Instead of getting out up to a few thousand sequences from each sample, "you could routinely pick out 20,000 even 100,000," says Sogin. "Within that, there's enormous diversity that wasn't appreciated before." That technology enabled the researchers to pull out 18 million DNA sequences from 1200 sites studied worldwide. There could be as many as a billion bacteria and archaea, another group of single-cell organisms like bacteria, he notes.

The most diverse samples came from flows emanating from a sea mount off the NorthwestPacificCoast-where 25,000 to 35,000 different kinds of microbes thrive per liter of seawater, Sogin reports. The researchers also saw that the microbial diversity increased the deeper into the water column they looked. In contrast, sometimes they found a microbe only in a single site-such as one that lived in association with a particular sponge.

The zooplankton survey, co-led by Ann Bucklin of the University of Connecticut, Groton, is expected to double to 14,000 the number of known zooplankton that live their entire lives in the water column. Her group used short DNA sequences known as DNA barcodes to help identify new species. Yet another survey used remotely operated deep-sea vehicles to tally organisms, many of them smaller than 1 millimeter, at the bottom of the deep ocean. These include loriciferans, the smallest known multicellular marine animals, whose 0.25-millimeter-long bodies have hind shells that look like a corset, hence the name "girdle wearers."

A few of these creatures are shown in the slideshow above.

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