The deepest parts of the world’s oceans hold many mysteries, one of which involves its tiniest residents. Tiny prokaryotes called Thaumarchaeota survive in the extreme environment of the deep oceans. While the metabolic functions of Thaumarchaeota that live in shallower depths are fairly well described, those that live in the deepest sea zone – hadal water – are less well known. Now, a new study provides insight into the metabolism and cellular adaptations of deep-sea Thaumarchaeota. Using metagenomics to evaluate seawater samples from the Mariana Trench, researchers found that Thaumarchaeota present in the hadal zone were distinct from those in shallower depths. Hadal zone Thaumarchaeota depended on genes involved in bioenergetics, including those horizontally transferred from other extremophiles, to survive, and their unique genetic makeup revealed the potential to import a wide range of organic compounds. Hadal Thaumarchaeota also used genes involved in ammonia oxidation and carbon fixation, suggesting that they are autotrophic. Although further studies with cultured Thaumarchaeota are still needed, the results suggest that these deep-sea prokaryotes have potentially novel adaptive mechanisms, giving them a key role in carbon and nitrogen cycling in the deepest oceans.