J.Bastion/ CNRS/ Oceanomics/ Tara Expéditions

For three years starting in 2009, the schooner Tara sailed the oceans collecting samples of water. For another three years, researchers analysed those samples. Now, the team has unveiled its findings -- a massive world of microscopic creatures dwelling in seas all around the globe. In a special issue of Science magazine released on May 22, an international, interdisciplinary team of 160 scientists published a series of papers detailing the findings of the Tara expedition.

This has provided an unprecedented look at the biodiversity of the ocean's micro-organisms -- from diatoms (single-celled algae) and eukaryotes (an organism that contains membrane-bound organelles, sub-units of cells that have a specific function) to the tiny larvae of fish and cephalopods.

"Beyond the cutting-edge science that was developed thanks to our collaborative work with the Tara Expéditions Foundation, this adventure is also about showing people all over the world how important the ocean is for our own well-being," said Tara Oceans director Eric Karsenti of the European Molecular Biology Laboratory (EMBL) and the French National Centre for Scientific Research (CNRS).

Christian Sardet, "Plankton -- Wonders of the Drifting World", University of Chicago Press 2015

Over the 1,140 days and 140,000 kilometres (87,000 miles) the Tara travelled, the expedition collected over 35,000 samples from 200 stations to depths of up to 1,000 metres (3,280 feet).

The genetic material was then analysed and compiled into a massive database -- the largest and most comprehensive DNA sequencing effort ever conducted for ocean science, using high-performance computing power provided by the EMBL.

The team's analyses revealed some 40 million genes, most of which are new to science -- indicating that the biodiversity of plankton is much broader than what had been previously known. The team revealed 150,000 genetic types; previously, only 11,000 species had been described.

"In terms of eukaryotes, we sequenced nearly a billion genetic bar codes and found that there is a greater variety of single-cell eukaryotes in plankton than was thought," said Colomban de Vargas of the CNRS, lead author on the paper "Eukaryotic plankton diversity in the sunlit ocean." "They appear to be much more diverse than bacteria or animals, and most belong to little-known groups."

Christian Sardet/CNRS/Tara Expéditions

Micro-organisms and viruses account for most of the biomass in the ocean, accounting for roughly 10,000 to 1 million cells for each millilitre of seawater. They form communities, with different sets of organisms coming together depending on the temperature of the water. This is impacted by influences such as sunlight and current, as demonstrated in the study "Global patterns and ecological drivers of ocean viral communities."

For instance, the Agulhas current in the southwestern Indian Ocean -- a current equivalent in length to 500 Amazon Rivers -- creates a phenomenon known as Agulhas rings. These are massive counter-clockwise swirls that are broken off from the current at the tip of South Africa and drift across the South Atlantic. They remain for years, and are large enough to be seen from space. These rings separate plankton communities.

"It's like plankton goes through a cold wash cycle at the tip of South Africa," said Daniele Iudicone from the Stazione Zoologica Anton Dohrn, a research institute in Naples, Italy. She's also co-author on the paper "Environmental characteristics of Agulhas rings affect interocean plankton transport."

"The current forms huge swirls that drastically mix and cool the plankton riding it, thus limiting the number of species that manage to cross."

The teams also found that parasitism is more important than previously thought for the functionality of the ecosystem. This will help scientists better understand the marine environment, which is still poorly known.

"When we mapped how planktonic organisms -- from viruses to small animal larvae -- interact with each other, we discovered that most of those interactions are parasitic, recycling nutrients back down the food chain," explained Jeroen Raes of the Vlaams Instituut voor Biotechnologie, University of Leuven and Vrije Universiteit Brussel, and co-author on the paper "Determinants of community structure in the global plankton interactome."

It is expected, given how important temperature is to planktonic communities, that climate change will have massive impacts on the oceanic ecosystem. The collected and analysed datasets will help evaluate this impact in the future.

"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," said Chris Bowler of the CNRS, co-author on the paper "Structure and function of the global ocean microbiome".

"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."