Life on Earth depends on photosynthesis — the harvesting of light by organisms to draw CO2 out of the air, and to harvest nutrients from the soil and water. The planet would otherwise be barren. Phytoplankton, in salt or freshwater, do fully half of that photosynthesis, fixing roughly 35–50 billion tons of carbon each year, and all this despite weighing, in the collective, 1,000 times less than land plants. Phytoplankton are secretly efficient photosynthesizers.
Thirty years ago, biologists thought these phytoplankton ranged in size from one millimetre to ten micrometres in diameter, with the smallest being a tenth the width of a human hair. But advancing technology soon uncovered a surprise — the bulk of oceanic photosynthesis actually depends on organisms ten times smaller. The bacterium Prochlorococcus marinus is the smallest photosynthetic organism known, at just 0.5 to 0.8 micrometres across, and is perhaps the most plentiful species on Earth. In 2013, researchers estimated that there are some 1027 such cells in the oceans, mostly near the surface, and they account, in some ocean regions, for as much as half of the total chlorophyll.
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