Unlike eukaryotes, archaea and bacteria are often considered devoid of intracellular vesicles and other compartments. However, decades of advances in light and electron microscopy and in bioinformatics have unambiguously established the prevalence, complexity, and diversity of intracellular organelles 1, 2, 3, 4, 5 and cytoskeletal elements 6, 7, 8, 9•, 10, 11 in archaea and bacteria. All cells, regardless of their size or structural complexity, must regulate cellular processes within their cytoplasm, both in time and space, for proliferation and survival. In bacteria, which account for most of the studies on the matter, this is mediated by organelles either bound by a lipid bilayer, such as magnetosomes and thylakoids; a single lipid layer, such as lipid bodies; a protein shell, such as carboxysomes and iron nanocompartments; or that are defined by liquid–liquid phase separation, such as nucleolus-like compartments. These compartments can significantly enhance the efficiency of biochemical reactions by concentrating metabolites or increasing the membrane surface area for membrane-associated reactions. They can act as storage compartments or contain toxic byproducts formed during other biochemical processes, thus protecting the rest of the cytoplasm against chemical stress. Other organelles even allow cells to orient themselves relative to the Earth’s magnetic field, or to regulate their buoyancy to move up and down the water column. Beyond proper organelles, archaea and bacteria use various mechanisms to regulate the intracellular positioning of large macromolecules and molecular machines.

In this review, we discuss intracellular organelles in archaea and other strategies used to organize the archaeal cytoplasm. Beyond the mere storage of polyhydroxyalkanoate or polyhydroxybutyrate bioplastics in the form of cytoplasmic granules in haloarchaea [12], archaea possess protein-bound nanocompartments, mechanisms for the intracellular positioning of molecular machines, as well as elaborate membrane systems to physically separate biochemical reactions. While some of these organelles are shared between archaea and bacteria, other mechanisms for compartmentalization are unique to archaea. We do not discuss here extracellular vesicles, the archaeal cytoskeleton, or the mechanisms for cell division in archaea, all of which were recently covered in excellent review articles 13, 14, 15.