Autotrophs

Autotrophs are organisms that produce complex organic compounds (such as fat, carbohydrates or proteins) from simple substances that are found it the surroundings generally by using light from photosynthesis. These organisms make their own food by "fixing" carbon dioxide - basically they self-feed. Photoautotroph plants fix carbon using energy from the sun. Chemoautotrophs use chemical energy.

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.