Which domain is characterized by organisms that often live in extreme environments?

Archaea is the domain characterized by organisms that thrive in extreme environments, which can include extreme temperatures, salinity, acidity, or pressure. These extremophiles are well-adapted to their harsh habitats, such as hot springs, deep-sea hydrothermal vents, salt lakes, and even anaerobic environments.

The unique properties of Archaea's cell membranes, which often contain ether bonds and unique lipid compositions, contribute to their ability to withstand such conditions. In contrast, while some Bacteria can also inhabit challenging environments, the extensive range of extremophiles associated with Archaea is more pronounced.

Eukarya encompasses a broader range of lifeforms, including plants, animals, and fungi, typically found in more moderate environments. Protozoa, a subgroup within the Eukarya, primarily inhabit aquatic environments and soil, but most do not specifically thrive in extreme conditions like many Archaea do. Thus, the distinction lays in the unique adaptations and environments specifically associated with Archaea.