In total, 1.5 million species of marine micro-organisms were recorded - significantly more than were previously believed to exist. They range from creatures that are 1cm long to tiny lifeforms that are measured in billionths of a metre. Studying their distribution and life cycle could provide important insights into the effect of climate change on the oceans.
The migration or extinction of microscopic species sensitive to sea temperature changes, or acidification, could cause the collapse of marine food chains, as well as the international fisheries that depend on them, scientists have warned.
Tara also encountered high levels of plastic pollution in the Antarctic. In the first research to quantify the scale of the problem in the Antarctic Ocean, scientists discovered that in a square kilometre of Antarctic sea, there could be as many as 50,000 plastic fragments.
Plastics that will remain in the sea for hundreds of thousands of years can get into the food chain, accumulating in the stomachs of fish, marine mammals and seabirds. They produce toxins - organic molecules such as phenols and phthalates - which can poison marine life and could, via fisheries, find their way into our food.
The Antarctic Ocean phytoplankton (plankton species which survive by photosynthesis, like plants) are vital for the regulation of CO2 in the atmosphere - as are all of the thousands of similar species recorded by the crew.
Dr Chris Bowler, scientific co-ordinator of the expedition, said the study provided a "snapshot and health check" of the world's oceans.
"Nobody has ever done this on the scale that we have before," he said.
"We will be analysing results in the lab for a number of years.
"The task now is to understand the physical and climatic constraints that have created these ecosystems.
"How much is pollution affecting them, how much is temperature change affecting them? If a species of plankton sensitive to temperature migrates, it could devastate that food chain and therefore local fisheries. We want to find out the role of each species within an ecosystem so that we can better predict what the oceans could be like in 50 or 100 years' time if we continue to change them."
The latest expedition set off from the Tara's home port of Lorient, France in June 2009. Over the next 36 months, a rotating crew of 14 sailed for several months at a time in cramped conditions, stopping to take samples to a depth of 2000m with specially designed nets.
On board, 3D imaging microscopes gave scientists an unprecedented insight into the complexity and beauty of marine life in miniature.
"We surveyed everything that is invisible, from one centimetre long, down to things measured in nanometres," said Dr Bowler. Among them were zooplankton - including the miniature crustaceans known as amphipods and the curious-looking larva, zoea - and their food: phytoplanktons, which produce their own food just like plants.
"These include creatures that are incredibly important for generating oxygen," said Dr Bowler. "Half of the oxygen on the planet is produced by oceans, so every second breath that you take is oxygen from these organisms. My favourites are the diatoms: little photosynthetic creatures in single cells which have a unique glass-like cell wall called a frustule. They essentially live inside glass houses. My lab is taking a close look at those. We thought there were 5000 species, but it turns out there are 25,000. Some of those are really different to the ones we already knew about."
The extraordinary images of plankton under the microscope inspired artists and writers who were invited on board the Tara to share the experience with scientists and crew.
Etienne Bourgois, son of Agnes Trouble, who persuaded her to invest in environmental causes, said his mother wanted the ship to be "scientific and cultural".
"It is important that we find ways for the message - of how human life depends on the oceans - to reach people," he said. "That message is: we have to change the way we live in this planet. The Earth will always be here, but it is a question for us - how are we going to live on this planet in the future?"
The Tara's findings were due to be presented at the Science Museum in London. Next spring, the schooner will return to the Arctic to conduct a survey of micro-organisms in the only seas not analysed by the latest expedition. Dr Bowler hopes to catalogue as much of the ocean's microscopic world as he can, before it begins to vanish.
The Tara legacy
Formerly owned by Sir Peter Blake and named Seamaster. The schooner was being used by the explorer when he was murdered by Amazon pirates in 2001.
The vessel is 36m long - a comparatively small ship to have undertaken such a scientific mission.
It is owned by the French fashion designer and environmental campaigner Agnes Trouble, who also provided half the funding for the expedition.
The ship was bought to continue Sir Peter's environmental legacy and its crew have become used to exposing themselves to danger.
On an earlier two-year expedition, the schooner locked itself in the Arctic sea ice to study climatic conditions in the polar regions.
The numbers
1.5m species of micro-organisms recorded
30,000 samples of sea water
70,000 nautical mile expedition
14 crew involved
- Independent
