Bacteriorhodopsin is a light-driven H ion transporter found in some haloarchaea, most notably Halobacterium salinarum (formerly known as  syn. H. halobium). The proton-motive force generated by the protein is used by ATP synthase to generate adenosine triphosphate (ATP). By expressing bacteriorhodopsin, the archaea cells are able to synthesise ATP in the absence of a carbon source.