Halophiles are organisms that thrive in high salt concentrations. They are a type of extremophile organisms. The name comes from the Greek word for "salt-loving". While most halophiles are classified into the Archaea domain, there are also bacterial halophiles and some eukaryota, such as the alga Dunaliella salina or fungus Wallemia ichthyophaga. Some well-known species give off a red color from carotenoid compounds, notably bacteriorhodopsin. Halophiles can be found anywhere with a concentration of salt five times greater than the salt concentration of the ocean, such as the Great Salt Lake in Utah, Owens Lake in California, the Dead Sea, and in evaporation ponds.

Halophiles require what in high concentrations for growth? OK. Let's think. My stream of consciousness:
To answer the question, consider the following: Halophiles are organisms that thrive in high salt concentrations. The answer is salt.