Marine agar is a medium used to support the growth of numerous marine bacteria.
Originally formulation by ZoBell (1), its high saline levels are to simulate a close approximation of seawater which allows the marine bacteria to grow (2).
The sodium chloride and marine minerals provide the high salinity needed for marine bacteria to thrive and aims to simulate sea water.
The peptones provide the necessary nitrogen, amino acids and vitamins for growth. Agar is the solidifying agent.
(1) ZoBell, C.E. 1941. Studies on Marine Bacteria. I. The cultural requirements of heterotrophic aerobes. J. Mar. Res. 4, 42-75.
(2) Lyman, J. and Fleming, R. H. 1940. Composition of Seawater. J. Mar. Res. 3, 134-146