In 1975 he became a junior research group leader at the Friedrich Miescher Laboratory in Tübingen.
[2][3] In 1969, Oesterhelt went to the University of California at San Francisco, where he joined the lab of Walther Stoeckenius to study the cell membrane of Halobacterium salinarum.
He proved that retinaldehyde was contained in a protein of the so-called "purple membrane" of Halobacterium.
[4] After returned to Germany, Oesterhelt showed that physiological function of bacteriorhodopsin is to pump protons out of the cell.
[5] Members of his department at the Max Planck Institute for Biochemistry researched the structure-function relationships of membrane proteins and other microbial rhodopsins such as halorhodopsin, which later became a molecular tool in optogenetics.