Gas vesicles are rigid hollow structures found in five phyla of the Bacteria and two groups of the Archaea, but mostly restricted to planktonic microorganisms, in which they provide buoyancy. By regulating their relative gas vesicle content, aquatic microbes are able to perform vertical migrations. The gas vesicle is impermeable to liquid water, but is highly permeable to gases and is normally filled with air. Two proteins have been shown to be present in the gas vesicle: GVPa, which makes the ribs that form the structure, and GVPc, which binds to the outside of the ribs and stiffens the structure against collapse.