Final answer:
A single-celled organism found in a geyser most likely belongs to the domain Archaea, known for extremophiles that thrive in extreme conditions such as hot springs.
Step-by-step explanation:
If a single-celled organism is living in a geyser, it most likely belongs to the domain Archaea. Organisms in this domain are known as extremophiles because many of them thrive in extreme environments such as hot springs, geysers, and salt flats. The other domains of life include the Bacteria, which are also composed of single-celled prokaryotes but do not generally live in such extreme conditions, and the Eukarya, which consist of organisms with more complex cells that have a nucleus and include protists, fungi, plants, and animals. For instance, members of the phylum Korarchaeota, considered to be among the most primitive forms of life, have been found exclusively in hot environments like the Obsidian Pool at Yellowstone National Park and are classified within the Archaea domain. Unlike Eukarya and Bacteria, the Archaea and Bacteria domains consist entirely of small, single-celled prokaryotes without a nucleus, but Archaea specifically are known for their ability to inhabit environments that are inhospitable to most other forms of life.