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LONDON (CBS) - A team of about 15 scientists on a research sailboat has just completed a two-and-a-half year, 70,000-mile voyage looking at some of the world's smallest, but most important, creatures: plankton.

The microscopic creatures at the bottom of the food chain play an over-sized role in the global ecosystem.

"There are all kinds of microscopic life that do incredibly important functions for managing the planet, ensuring the well-being of the planet, generating the oxygen we breathe, removing carbon dioxide from the atmosphere and generally maintaining this earth in a state that is habitable for us human beings," explains Tara Oceans scientist Chris Bowler.

The Tara team collected samples of sea-water and found, they say, about 1.5 million species of plankton - twice the number previously known to exist.

They found something else too: minute shreds of plastic. In fact, they found more plastic than plankton - especially in the Antarctic.

"We thought that areas like the Antarctic were pristine, being isolated, far away from humanity -- the fact that we found plastic debris down there - in terms of tens of thousands of pieces - is very sad, because this will hang around for thousands of years," says Bowler.

The state of the world's oceans - and the tiniest creatures in it - may determine whether the planet can sustain life for the rest of its creatures, including us.

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