The ocean's invisible engine
The most abundant photosynthetic organism on Earth is a marine cyanobacterium named Prochlorococcus. Its existence was unknown until 1986, when it was identified by a team of scientists including Sallie "Penny" Chisholm and Robert J. Olson. The official discovery was published in 1988. The organism's tiny size, just 0.5 to 0.7 micrometers in diameter, allowed it to evade detection by all but the most sensitive modern instruments like flow cytometers. This bacterium is responsible for producing up to 20% of the oxygen in the planet's biosphere, which is more than terrestrial rainforests combined.
The global population of Prochlorococcus is immense, estimated at around three octillion (3 x 10^27) individuals. These cells are found throughout the world's oceans in a wide band between 40°N and 40°S latitudes. In the nutrient-poor open oceans where it dominates, a single milliliter of surface seawater can contain over 100,000 cells. It thrives in the sunlit upper layer of the ocean, known as the euphotic zone, down to depths of about 150 to 200 meters. Together with its relative, Synechococcus, it accounts for about half of all marine carbon fixation, forming the base of the ocean's food web.