DUBLIN—A scientific sailing trip around the world has shed new light on the vast biodiversity in the world's oceans. The expedition, called Tara Oceans, has yielded about 1.5 million different plankton taxa, based on an initial preliminary analysis of samples. Scientists will spend years analyzing the catch, Tara Oceans co-director Eric Karsenti said here today at the Euroscience Open Forum, a biennial science and policy meeting.
The Tara, a 36-meter-tall research schooner, returned to Lorient, France, on 31 March after a 938-day trip. Its mission was to help understand the evolution and ecology of plankton, roughly defined as anything that's small and floating in the ocean—including viruses, bacteria, archaea, protists, metazoans, and even fish larvae. Although plankton makes up the bulk of the oceans' biomass, its biogeography and the structure of its ecosystems are an "almost virgin field," Karsenti said.
The project, which brings together physical oceanographers, marine biologists, imaging specialists, molecular biologists, bioinformaticists and modelers, uses what it calls a "holistic" and "study it all" approach, analyzing many species at once using a variety of methods, many of them automated. The team took samples in 153 different spots around the world, from the Mediterranean Sea and the Red Sea to the Pacific Ocean and the Antarctic region.
Karsenti, a French molecular biologist, today presented the results of a first analysis of 27,000 samples collected at 35 spots, from the water surface to almost 1 kilometer deep. Half of the samples were taken for morphology studies; using filters with different mesh sizes, researchers separated the organisms according to size, from tiny viruses to zooplankton, and used automated imaging systems to classify them. The other half of the samples was used for molecular biology studies.
The analysis turned up a total of 1.5 million different taxa, Karsenti said. The team also carried out a so-called metagenomic analysis, in which the genomes from all organisms in a sample are sequenced collectively; the great majority of genes they found coded for proteins never seen before.
The survey revealed a general rule, Karsenti said: The smaller the organisms, the more abundant they are, both in numbers of taxa and individuals. Karsenti said the oceans accommodate "an almost entirely unknown viral diversity." There were other intriguing patterns as well. For instance, there are strong local correlations between different kingdoms: Archaea and bacteria prefer living together, as do bacteria and viruses, but archeae don't seem to get along at all with viruses and protists.
The diversity varied greatly from one location to the other. Local factors such as ocean streams, temperature, depth, and acidity determine the composition of plankton ecosystems, Karsenti said. He and his colleagues hope to find correlations between those circumstances and diversity, which might enable them to predict the impact of global warming and the resulting ocean acidification on marine ecosystems. While Karsenti is cautious about making predictions, he warns that many plankton species are very sensitive to temperature changes.
Karsenti finished his talk by showing several video clips of the project that highlighted the excitement and adventure of the trip, nature's beauty, and Tara's educational activities. He hopes they will help raise awareness about the importance of ocean science. In 2013, the ship will head to the Arctic Ocean for a new expedition.
*This article has been corrected on 27 July. The Tara returned to Lorient, France, after a 938-day trip, not a 362-day trip.