Ancient DNA Found Hidden Below Sea Floor

In the middle of the South Atlantic, theres a patch of sea almost devoid of life. There are no birds, few fish, not even much plankton. But researchers report that theyve found buried treasure under the empty waters: ancient DNA hidden in the muck of the sea floor, which lies 5000 meters below the waves.

The DNA, from tiny, one-celled sea creatures that lived up to 32,500 years ago, is the first to be recovered from the abyssal plains, the deep-sea bottoms that cover huge stretches of Earth. In a separate finding published this week, another research team reports teasing out plankton DNA thats up to 11,400 years old from the floor of the much shallower Black Sea. The researchers say that the ability to retrieve such old DNA from such large stretches of the planets surface could help reveal everything from ancient climate to the evolutionary ecology of the seas.

We have been able to show that the deep sea is the largest long-time archive of DNA, and a major window to study past biodiversity, writes Pedro Martinez Arbizu, a deep-sea biologist of the German Centre for Marine Biodiversity Research in Wilhelmshaven and an author of the paper on South Atlantic DNA in an e-mail.

The new studies are very exciting, says micropaleontologist Bridget Wade of the University of Leeds in the United Kingdom, who was not connected to the research. Until now, it wasnt clear how far back in time you could take these DNA studies. These records are telling you new information that wasnt found in the fossil record.

The South Atlantic team went looking for DNA in plugs of silt and clay coaxed out of the ocean floor hundreds of kilometers off the Brazilian coast. The researchers were after genetic material from two related groups of marine organisms, the foraminifera and the radiolarians. Both are single-celled, and both include many species with beautiful pearly shells that fossilize nicely, making them a favorite target of researchers studying the prehistoric oceans.

The researchers used special pieces of DNA specific to radiolarians and foraminifera to fish out DNA from those groups. Then they sequenced the DNA and compared the results to known foraminifera and radiolarian DNA sequences. Their analysis showed theyd found 169 foraminifera species and 21 radiolarian species, many of which were unknown. Whats more, many of the foraminifera species belonged to groups that dont form fossils, the researchers report online today in Biology Letters.

The work shows that its possible to trace all species, not just those that fossilize, says Jan Pawlowski, a foraminifera specialist and one of the papers authors, of the University of Geneva in Switzerland. The results give us a completely different view [that] may open new insights into whats happened in the past, he says. For example, he says, different species of these wee creatures prefer different water temperatures. So DNA from buried sediments could be used to track the abundance of different species over time, revealing changes in ocean temperature.

The second team looked at DNA buried in the floor of the Black Sea, which was once a giant lake but became connected to the Mediterranean Sea roughly 9000 years ago, though the date is debated. The researchers examined sediments from waters only 980 meters deep, which is much shallower than the abyssal plain. But the oldest Black Sea layers that were analyzed were similar to those at the South Atlantic site: The mud at the sea bottom had scant amounts of organic matter and had been exposed to oxygen, which, in theory, should have made it tough to scrape up any preserved DNA.

It didnt. New material had buried the older layers, cutting off their oxygen, and more recent Black Sea sediments werent exposed to oxygen at all. The result was a rich trove of ancient DNA from as many as 2700 species, including green algae, fungi, and dinoflagellates, a type of one-celled aquatic creature. The diverse collection allowed the scientists to track the fate of different species over time, as their DNA blinked in and out of the sediments.

One type of marine fungus, for example, first appeared in the sediments roughly 9600 years agoexactly when some forms of freshwater plankton and a freshwater mussel vanish, the team reports this week in the Proceedings of the National Academy of Sciences. That suggests that marine waters started to invade the lake roughly 600 years earlier than thought. The team also found DNA from a form of marine alga in 9300-year-old sediments, though the alga doesnt show up in the fossil record until 2500 years ago, says molecular paleoecologist Marco Coolen of the Woods Hole Oceanographic Institution in Massachusetts and an author of the Black Sea paper.

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Ancient DNA Found Hidden Below Sea Floor