Archaea and bacteria are prokaryotes that lack cell nuclei and membrane-bound organelles, while protists are eukaryotes that possess both of these features. Archaea and bacteria are always single celled, while a small number of protists are multicellular organisms.
Archaea and bacteria both store their DNA in rings called plasmids, and both are surrounded by cell walls. Protists store their DNA inside a defined cell nucleus. Some protist groups, such as algae, possess cell walls; other groups, such as the animal-like protozoa, do not have cell walls.
While all archaea and bacteria are single-celled organisms, some protists are multicellular organisms. Multicellular protists include kelp and other large brown algae species; all other species of protists are single celled. All three groups include varied species that obtain energy in diverse ways: through photosynthesis, through consuming chemicals and through consuming other organisms.
Most archaea consume chemicals, but some species can switch to photosynthesis when necessary. All species of algae obtain their energy via photosynthesis, but animal and fungus-like protists consume other organisms for food. Bacteria are represented in all three groups and have a more diverse range of habitats than archaea or protists. Archaea have the smallest habitat of the three groups, living in extreme environments that cannot support other life.