The correct answer is - phytoplankton.
The phytoplantkon is an organism that is at the bottom of the food chain, and it is the basis for the food chain in the marine environments. It is an organism that is an autotroph, which basically means that it manages to produce complex organic compounds from simple substances that are present in its surroundings, and very importantly, it uses mostly light or inorganic chemical reactions, thus the phytoplankton is a producer, and it is a main food source for large number of marine species.
