For years, most people thought of small organisms called plankton as food for whales and other large sea animals. But a four-year long study discovered that plankton are a major supplier of oxygen on our planet.

Plankton are a group of different ocean creatures, algae, bacteria and other organisms. They live close to the water's surface in oceans, seas and lakes. Plankton cannot be seen without a microscope. But they are the main source of food for larger animals, like whales.

Recently, 200 scientists from 45 countries took turns researching plankton on a two-mast schooner. The boat set sail in September 2009. It visited 210 areas. The scientists collected 35,000 samples of plankton. The researchers followed the lives of plankton, examined their genetic material and watched their movements.

A French non-profit group called Tara Expeditions launched the project, known as Tara Oceans. Chris Bowler was the project's scientific coordinator. He says the researchers found that plankton are very important to the health of our 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."

Mr. Bowler says almost half of the oxygen we breathe comes from very small algae called diatoms. These plants live in the part of the water where the sun reaches.

"That basically means every fifth time that you breathe, you're breathing oxygen -- which we can directly trace back to diatoms, so they're sort of as important as a tropical rain forest in terms of their global contribution."

Romain Troublé is the executive director of Tara Oceans. He believes the study shows that the ocean greatly affects the earth's climate.

"We believe that the ocean is also a main driver of the climate change, a main, main mitigator in the way that the ocean and the life in the ocean is storing carbon dioxide, storing heat. And this machinery works because the ocean is in good health."

Different groups of researchers studied different things. One group identified more than 40 million plankton genes formerly unknown to scientists.

The collected information will be used to identify which of the plankton have little or no trouble reacting to changing water conditions.

Results of the study appeared in the online scientific journal Science. The publication says many experts will be examining the study.

I'm Christopher Jones-Cruise.

VOA Science Correspondent George Putic reported this story from Washington. Christopher Jones-Cruise adapted it into Learning English. George Grow was the editor.

plankton – n. the very small creatures in an ocean, sea or lake

algae – n. simple plants that grow in or near water

sample – n. a small amount of something that gives you information about the thing it was taken from

generate – v. to produce (something) or cause (something) to be produced

maintain – v. to keep (something) in good condition by making repairs or correcting problems

habitable – adj. able to live in

trace back – idiom to learn about the development or progress of something by reasoning backward from an effect to a cause

in terms of – idiom regarding something; concerning something

contribution – n. something that is done to cause something to happen

driver – n. idiom cause

mitigate – v. to make (something) less severe, harmful or painful

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