Inspection of domain Archaea shows that two sub-divisions exist the Euryarchaeota and the Crenarchaeota. The Archaea are chemotrophic although Halobacterium can use light to make ATP. Many others are chemolithotrophs with hydrogen gas being a widely used energy source. The Euryarchaeota includes Methanobacterium, Methanocaccus, Thermocaccus. The differential features among bacteria, Archaea and Eukarya are: 