The only organisms with prokaryotic cells belong to the domains Bacteria and Archaea. These organisms are considered to be prokaryotes, which are characterized by being cells that do not possess a distinct nucleus.
Prokaryotic cells are missing many of the structures that define eukaryotic cells, and they are most often unicellular life forms. While they lack membrane bound organelles, they still conduct protein synthesis via ribosomes and double-stranded DNA. Prokaryotic organisms include cynobacterias and algae, which are categorized under the domain Bacteria. Archaea is a newly established domain that consists of the prokaryotic organisms that typically exist in inhospitable climates such as deep sea thermal vents.