Efficient CO2 fixation by surface Prochlorococcus in the Atlantic Ocean

Nearly half of the Earth’s surface is covered by the ocean populated by the most abundant photosynthetic organisms on the planet— Prochlorococcus cyanobacteria. However, in the oligotrophic open ocean, the majority of their cells in the top half of the photic layer have levels of photosynthetic pigmentationbarely detectable by flow cytometry, suggesting low efficiency of CO2 fixation compared with other phytoplanktonliving in the same waters. To test the latter assumption, CO2 fixation rates of flow cytometrically sorted 14C-labelled phytoplanktoncells were directly compared in surface watersof the open Atlantic Ocean (30°S to 30°N). CO2 fixation rates of Prochlorococcusare at least 1.5–2.0 times higher than CO2 fixation rates of the smallest plastidic protistsand Synechococcus cyanobacteriawhen normalised to photosynthetic pigmentationassessed using cellular red autofluorescence. Therefore, our data indicate that in oligotrophic oceanic surface waters, pigment minimisation allows Prochlorococcuscells to harvest plentiful sunlight more effectively than other phytoplankton.