On land, most photosynthesis is performed by multicellular life (plants), but in the ocean, nearly all photosynthesis is done by single-celled organisms. The most abundant photosynthetic organism on earth is a prokaryote.
Bacteria is a prokaryote. The image below of bacteria was captured using the digital biological microscope MW4-HD2 with phase contrast.
Bacteria captured under the microscope at 1000x magnification.
You can learn more about prokaryotes in the ocean here.