Mixotrophic basis of Atlantic oligotrophic ecosystems

Oligotrophic subtropicalgyres are the largest oceanic ecosystems, covering >40% of the Earth's surface. Unicellular cyanobacteria and the smallest algae ( plastidic protists) dominate CO2 fixation in these ecosystems, competing for dissolved inorganic nutrients. Here we present direct evidencefrom the surface mixed layerof the subtropicalgyres and adjacent equatorial and temperate regions of the Atlantic Ocean, collected on three Atlantic Meridional Transect cruises on consecutive years, that bacterioplanktonare fed on by plastidicand aplastidic protistsat comparable rates. Rates of bacterivory were similar in the light and dark. Furthermore, because of their higher abundance, it is the plastidic protists, rather than the aplastidic forms, that control bacterivory in these waters. These findings change our basic understanding of food webfunction in the open ocean, because plastidic protistsshould now be considered as the main bacterivoresas well as the main CO2 fixers in the oligotrophic gyres.