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Archaeans dine on a variety of substances for energy, including hydrogen gas, carbon dioxide and sulfur. One type of salt-loving archaean uses sunlight to make energy, but not the way plants do it. This archaean has a light-harvesting pigment in the membrane surrounding its cell. This pigment, called bacteriorhodopsin (back-tear-ee-oh-row-dop-sin), reacts with light and enables the cell to make ATP, an energy molecule.