  Emission substructures in gas and dust are common in protoplanetary disks.
Such substructures can be linked to planet formation or planets themselves. We
explore the observed gas substructures in AS 209 using thermochemical modeling
with RAC2D and high-spatial resolution data from the Molecules with ALMA at
Planet-forming Scales(MAPS) program. The observations of C$^{18}$O J=2-1
emission exhibit a strong depression at 88 au overlapping with the positions of
multiple gaps in millimeter dust continuum emission. We find that the observed
CO column density is consistent with either gas surface-density perturbations
or chemical processing, while C$_2$H column density traces changes in the C/O
ratio rather than the H$_2$ gas surface density. However, the presence of a
massive planet (> 0.2 M$_{Jup}$) would be required to account for this level of
gas depression, which conflicts with constraints set by the dust emission and
the pressure profile measured by gas kinematics. Based on our models, we infer
that a local decrease of CO abundance is required to explain the observed
structure in CO, dominating over a possible gap-carving planet present and its
effect on the H$_2$ surface density. This paper is part of the MAPS special
issue of the Astrophysical Journal Supplement Series.
