Despite providing half of all the oxygen produced on the planet, forming the base of many marine food webs, and even influencing the weather, the details of the planktonic life in our oceans have gone largely unknown and undocumented. 

It was to address this knowledge gap that the Tara Oceans group set out on a three-and-a-half-year global expedition, documenting the microscopic plankton that underpin the life in our oceans as they went. During this period they took an astonishing 35,000 samples from over 200 different locations worldwide. They’ve now published their first analysis of the data in a series of five papers in Science.

©Christian Sardet/CNRS/Tara Expeditions

©Noan LeBescot/CNRS/Tara Expeditions

Mostly invisible to the human eye, the results show that the blue waters of our planet are swirling with life. So far, they’ve found an estimated 150,000 eukaryotes - single-celled plants and animals. This is roughly ten times the number of plankton species currently described. In addition to that, they reckon that they’ve found around 35,000 types of bacteria and 5,000 viruses, most of which are probably new to science.

“In terms of eukaryotes, we sequenced nearly a billion genetic barcodes, and found that there is a greater variety of single cell eukaryotes in plankton than was thought,” says Colomban de Vargas who co-authored one of the papers, from CNRS. “They appear to be much more diverse than bacteria or animals, and most belong to little known groups.”

©C. Guiguand/Tara Expeditions

©M. Ormestad/Tara Expeditions

Not only were they cataloging the species; they were also documenting how they interact and where they reside. Whilst mostly focusing on the warmer upper layer of the ocean, they also took samples from the twilight zone 1,000 meters down. This was so that they could look at how environmental factors such as temperature, pH and nutrients affected the planktons' distribution.

“We  found  that,  at  depths  still  reached  by  sunlight,  temperature  was  the  main  factor  that  influences  the  composition  of prokaryotes (bacteria and archaea) communities,” says Peer Bork, another of the authors, from EMBL. “Different sets of organisms come together depending on the water temperature.”

©Christian Sardet/CNRS/Tara Expeditions

©Luis Gutierrez/UCD/Tara Expeditions

Incredibly, they were able to predict the temperature of the water with 97% accuracy simply by looking at the composition of plankton in the sample, which has implications for monitoring global warming. By examining the oceans as a whole and systematically sampling them across all domains of life, they hope that the data will form a baseline on a global scale, across which the impact of climate change can be measured.

“The finding that temperature shapes which species are present, for instance,  is  especially  relevant  in  the  context  of  climate change, but to some extent this is just the beginning,” says Chris Bowler, also from CNRS. “The resources we’ve generated will allow us and others to delve even deeper, and finally begin to really understand the workings of this invisible world.”