Following perhaps the largest DNA sequencing effort ever undertaken for ocean science, a multinational team of scientists now reports that plankton life in the seas is far more diverse than suspected.
The researchers, who spent more than three years sampling plankton on the research schooner Tara, have unveiled the first global analyses from the Tara Oceans consortium in five reports in the 22 May issue of Science.
Plankton include all the microscopic beings that drift on the upper layer of the oceans, including tiny crustaceans, squid, and mollusks. Previously, the ocean's numerous planktonic organisms have largely been uncharacterized.
"The results represent a one thousand-fold increase in data over previous attempts to characterize ocean microbial biodiversity," said a senior author on one of the papers, Peer Bork, during a teleconference for reporters on 19 May, "and yet, this is still the tip of the iceberg." Bork is a senior group leader in bioinformatics at the European Molecular Biology Laboratory (EMBL).
"Plankton are much more than just food for the whales," said Chris Bowler, a co-author on all five Science papers and research director at the French National Centre for Scientific Research (CNRS). "Although tiny, these organisms are a vital part of the Earth's life support system, providing half of the oxygen generated each year on Earth by photosynthesis and lying at the base of marine food chains on which all other life in the ocean depends."
One of the key findings from the research expedition is that temperature shapes which species are present in a given plankton ecosystem, a fact that could take on increasing importance in the face of climate change.
"We can see now at true planetary scale that increasing water temperature will have a huge impact on microbial life in the ocean," said Shinici Sunagawa, an EMBL staff scientist and a senior author on a second Tara paper. "Since the microbial community is a critical part of a food web, global warming will have direct impacts on organisms feeding on plankton…What exactly and how large this impact will be will require more data to understand."
The route of the Tara was chosen to sample plankton from most major ocean systems. [View larger version] | ©bepoles/Tara Expéditions
"The data generated here establish a baseline to evaluate the impact of climate changes on oceanic ecosystems going forward," Bowler added.
During expeditions from 2009 through 2013, the Tara Oceans scientists sampled viruses, bacteria, protists, and small animals in the upper ocean, ultimately collecting over 35,000 planktonic samples from 210 stations in all the major oceanic regions.
"Understanding such an invisible ecosystem requires advanced technology and concepts," explained Emlie Villar, a CNRS researcher and a senior author on one of the five research articles. "Until recently, studying species or a group of species required the difficult task of cultivating organisms before genome sequencing. In this study, we used cutting-edge genetic analysis to sequence DNA directly from organisms in seawater."
Sunagawa and colleagues then created a catalog of all the genes in the different plankton they sampled. It includes around 40 million genes, the vast majority of which are new to science.
The expedition also identified over 5,000 viral populations throughout the global upper oceans. Only 39 of these viral populations were similar to previously known viruses, said Matthew Sullivan, a senior author on one of the Tara papers and an associated professor of ecology and evolutionary biology at the University of Arizona. "Viruses impact important microbial processes in plankton — killing their cells, moving genes from one cell to another and reprogramming the cells to change their ecosystem output," said Sullivan.
Gipsi Lima-Mendez of the University of Leuven in Belgium, and a senior author on another study in the Tara package, and colleagues developed a map of more than 90,000 plankton species interactions. They discovered that most plankton interactions are parasitic.
These researchers applied novel computer programs to their map of species interactions to predict how the organisms would behave in different settings. "We could use the map to predict if extinction is likely to propagate through the plankton ecosystem, for example," Lima-Mendez said, "or to predict the presence of organisms living inside other ones, engaging in symbiosis."
A liter of ocean water holds:
1 to 100 million phytoplankton
1 to 10,000 zooplankton
1 to 10 billion bacteria
10 to 100 billion viruses
The scientists also studied how environmental factors — such as temperature, water pH, and nutrients — influence the microscopic organisms floating in the ocean. "We found that at depths still reached by sunlight, temperature was the main factor that influences the composition of bacteria and archaea communities," said Bork. "Different sets of organisms come together depending on the water temperature."
The researchers talked about how their datasets and results might be used going forward. For example, they may help researchers understand the full — and perhaps changing — potential for the plankton ecosystem to act as a sink to absorb carbon dioxide from the air.
"The next step," Sullivan said, "is for us to start linking these genetic diversity maps to myriad ecosystem measurements, including carbon flux, to try to make connections between particular organisms and their role in carbon flow."
The Tara data "represent some of the largest publicly available contiguous datasets available in genomics for any scientific project in the world," Bowler said. "This is going to be a tremendous resource for scientists and for people interested in studying ocean, climate, food webs and the evolution of life. We know life evolved in the oceans… but many of the organisms we studied are uncharacterized, little known to science, and we have a lot of work to do understand where these organisms fit in in our understanding of life."