Most abundant species in the model presented in figure 4(a), (left panel) and the model presented in figure B.1(d), (right panel) for a disk age of 5 Myr. Molecules with a ‘J’ denote species that are frozen onto dust grains. The abundances are integrated between 50 AU and 70 AU (i.e. near the millimeter dust ring) and from the midplane to z/r = 0.43. It therefore includes the main reservoir of CO that contributes to the C¹⁸O flux. It is clear that when the abundances of carbon and oxygen are reduced, the main reservoir of carbon is shifted towards hydrocarbons that are frozen onto the dust.