Right at home. Single-celled creatures of the deep settle in a small perforated tube (inset) set out by researchers.

New Depths of Diversity

Since the discovery of deep-sea hydrothermal vents in 1977, unearthing new and unusual vent species has become almost routine. But the discovery of whole new kingdoms--the largest of the groups biologists use to classify life--is a rare event. Now, researchers have uncovered such wild diversity among microbes that they say some of the organisms may deserve a kingdom of their own.

Deep-sea hydrothermal vents harbor a rich variety of life thousands of meters below the ocean's surface--in complete darkness, and under intense pressure. Since the discovery of the vents, scientists have studied the animals and bacteria that live there in some detail. But single-celled microorganisms called protists have been largely overlooked.

A team led by evolutionary biologist David Moreira of the National Research Center (CNRS) in Paris set out to look at the protists at hydrothermal vents on the Mid-Atlantic Ridge. Using a remotely operated vehicle named Victor, they sampled a core of sediment near an active vent. They extracted DNA from the sediment and searched for a specific region that encodes 18S ribosomal RNA. This region of DNA is often used to deduce evolutionary relationships because it changes slowly and at a predictable rate.

When the team constructed a phylogenetic tree based on the DNA sequence data, they found that the sediment contained at least two microbes that were genetically very different from known protists. In fact, these organisms are distinct enough to belong to their own kingdoms, the team suggests online this week in the Proceedings of the National Academy of Sciences. The researchers also found many new protist lineages within known groups called the alveolates and kinetoplastids.

The study “continues to expand our knowledge of the phylogenetic diversity of extant life on Earth,” says microbiologist Ed DeLong of the Monterey Bay Aquarium Research Institute in Moss Landing, California. There have been very few molecular surveys of vent protists, he says. “It's perhaps not all that surprising at this point to find these new lineages, but it is certainly enlightening.”

Related sites
Ecology, Systematics, and Evolution at the CNRS (Moreira's department; in French)
Ed DeLong's page
InterRidge--program that monitors Mid-Atlantic hydrothermal vents