Final answer:
The organism described is an Archaea, a unicellular prokaryote that thrives in extreme environments and lacks a nucleus. They can either produce their own food or consume organic material.
Step-by-step explanation:
The unicellular organism that lives in extreme environments such as volcanic vents, deep ocean saltwater, or the Arctic, and does not have a nucleus is referred to as Archaea. As the description suggests, these organisms are prokaryotes, meaning they lack membrane-bound organelles including a nucleus. Archaea are known for their ability to survive in extreme conditions, and unlike typical bacteria and eukaryotic organisms such as protists, they thrive in harsh environments which make them unique among the domains of life.
Archaea can be autotrophic, producing their food through processes such as chemosynthesis, or they can be heterotrophic, consuming organic material for sustenance. Their remarkable adaptive capabilities and prokaryotic cell structure clearly differentiate them as a distinct domain separate from Bacteria and Eukarya.