After a nearly two-year “microbiome” mission around the world, scientists said Saturday they had collected thousands of samples of marine microorganisms to better understand plankton and ocean pollution.

The survey was carried out by the 33-year-old research schooner Tara, which returned to its homeport of Lorient on the west coast of France over the weekend.

From Chile to Africa, across the Amazon and Antarctica, almost 25,000 samples were collected along the 70,000 kilometer route.

“All of this data is being analyzed,” Romain Trouble, director of the Tara Ocean Foundation, said at a news conference.

“Within 18 months to two years, we will make the first discoveries of the mission,” he said.

At the base of the food chain, microorganisms are the “invisible people of the sea,” making up two-thirds of marine biomass, Trouble said.

“They capture atmospheric CO2 (carbon dioxide) and provide half of the oxygen we breathe.”

Trouble said the mission was trying to figure out how it all works.

“How do all these marine viruses, bacteria and microalgae manage to interact with each other to produce oxygen?”

“And how will that change tomorrow with climate change and pollution?”

The Tara team paid particular attention to the impact on the oceans of the Amazon River, which has a water flow rate of 200 million liters (53 million gallons) per second.

They wanted to test a theory that deforestation and the spread of agriculture have increased inputs of nitrite fertilizers, leading to an abundance of toxic algae along riverbanks and coasts, particularly in the Caribbean.

The 22-month odyssey also attempted to trace the sources of plastic pollution at estuaries to understand the distribution and types of material involved.

The mission was Tara’s 12th global voyage and included 42 research facilities around the world.

Next spring, Tara sets out to research chemical pollution off European shores.