"Ocean phytoplankton" refers to tiny plant-like organisms that live in the ocean. They are the foundation of the marine food web, producing oxygen and serving as food for many ocean creatures. Full definition
Le Quéré, C., Buitenhuis, E. T., Moriarty, R., Alvain, S., Aumont, O., Bopp, L., Chollet, S., Enright, C., Franklin, D. J., Geider, R. J., Harrison, S. P., Hirst, A., Larsen, S., Legendre, L., Platt, T., Prentice, I. C., Rivkin, R. B., Sathyendranath, S., Stephens, N., Vogt, M., Sailley, S., and Vallina, S. M.: Role of zooplankton dynamics for Southern Ocean phytoplankton biomass and global biogeochemical cycles, Biogeosciences Discuss., 12, 11935 - 11985, 2015. (embrace-project.eu)
A main indirect effect is the fertilization of ocean phytoplankton production by dust - mitigated input of iron to the ocean surface layer (6, 7). (pnas.org)
They are ancestors to terrestrial plants, which seem to have evolved from certain ocean phytoplankton hundreds of millions of years ago. (discovermagazine.com)