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The elemental composition of the gas and dust in a protoplanetary diskinfluences the compositions of the planets that form in it. We use theMolecules with ALMA at Planet-forming Scales (MAPS) data to constrain theelemental composition of the gas at the locations of potentially formingplanets. The elemental abundances are inferred by comparing source-specificgas-grain thermochemical models, with variable C/O ratios and small-grainabundances, from the DALI code with CO and C2H column densities derived fromthe high-resolution observations of the disks of AS 209, HD 163296, and MWC480. Elevated C/O ratios (~2.0), even within the CO ice line, are necessary tomatch the inferred C2H column densities, over most of the pebble disk. Combinedwith constraints on the CO abundances in these systems, this implies that boththe O/H and C/H ratios in the gas are substellar by a factor of 4-10, with theO/H depleted by a factor of 20-50, resulting in the high C/O ratios. Thisnecessitates that even within the CO ice line, most of the volatile carbon andoxygen is still trapped on grains in the midplane. Planets accreting gas in thegaps of the AS 209, HD 163296, and MWC 480 disks will thus acquire very littlecarbon and oxygen after reaching the pebble isolation mass. In the absence ofatmosphere-enriching events, these planets would thus have a stronglysubstellar O/H and C/H and superstellar C/O atmospheric composition.

Molecules with ALMA at Planet-forming Scales (MAPS). VII. Sub-stellar O/H and C/H and super-stellar C/O in planet feeding gas