Some archaebacteria are photosynthetic, meaning they make their own food; however, rather than use the pigment chlorophyll like green plants and algae, they employ a light-sensitive purple protein called bacteriorhodopsin. Other archaea live in places where no sunlight penetrates, such as deep-sea thermal vents. These bacteria rely on a process called chemosynthesis to make ATP.
Chemosynthesis uses a strategy similar to photosynthesis except for two key differences. First, the heat from a thermal vent can substitute for solar energy. Second, the water around a thermal vent is rich in hydrogen sulfide. Archaebacteria can split hydrogen sulfide into its atomic components, releasing elemental sulfur while pumping the protons across their membranes to generate an ionic gradient that drives ATP production. Other archaebacteria can use methane as an energy source as well as a carbon source to synthesize sugars and lipids.
Compared to run-of-the-mill eubacteria, archaebacteria are extremophiles, meaning they can tolerate harsh environments unsuitable for other life forms. Thermophiles can tolerate temperatures close to the boiling point of water, while halophiles can withstand high concentrations of salt, such as those found in the Great Salt Lake or the Dead Sea. Finally, certain acidophiles like Ferroplasma can withstand sulfuric acid concentrations as high as pH zero (the equivalent of battery acid).