Presence of <i>Prochlorococcus</i> in the aphotic waters of the western Pacific Ocean

Prochlorococcus, the smallest but most abundant marine primary producer, plays an important role in carbon cycling of the global ocean. As a phototroph, Prochlorococcus is thought to be confined to the euphotic zone, with commonly observed maximum depths of ∼150–200 m. But here we show, using flow cytometry and cellular ribosomal content, for the first time the presence of abundant and active Prochlorococcus in the dark ocean ("deep Prochlorococcus" hereafter). Intensive studies at the Luzon strait in the western Pacific Ocean show that the deep Prochlorococcus populations are exported from the euphotic zone. Multiple physical processes including internal solitary waves could be responsible for the transportation. The unexpected abundance of the tiny phototrophs in the dark ocean reveals a novel mechanism for picoplankton carbon export other than the known mechanisms such as sinking of phytodetritus and aggregates or grazing-mediated transportation. Such direct transportation of picoplanktonic phototrophs from surface to deep waters is poorly understood, but could significantly contribute to both the biological pump (through particulate organic carbon) and the microbial carbon pump (through release of dissolved organic carbon from microbial processes) for carbon sequestration in the ocean.