  Planets form in protoplanetary disks and inherit their chemical composition.
It is therefore crucial to understand the disks molecular content. We aim to
characterize the distribution and abundance of molecules in the disk of DG Tau.
In the context of the ALMA chemical survey of Disk-Outflow sources in Taurus
(ALMA-DOT) we analyse ALMA observations of the disk of DG Tau in H2CO
3(1,2)-2(1,1), CS 5-4, and CN 2-1 at ~0.15", i.e. ~18 au at 121 pc. H2CO and CS
originate from a disk ring at the edge of the 1.3mm dust continuum, with CS
probing an outer disk region with respect to H2CO (peaking at ~70 and ~60 au,
respectively). CN originates from an outermost disk/envelope region peaking at
~80 au. H2CO is dominated by disk emission, while CS probes also two streams of
material possibly accreting onto the disk with a peak of emission where the
stream connects to the disk. The ring- and disk-height- averaged column
densities are ~2.4-8.6e13 cm-2 (H2CO), ~1.7-2.5e13 cm-2 (CS), and ~1.9-4.7e13
cm-2 (CN). Unsharp masking reveals a ring of enhanced dust emission at ~40 au,
i.e. just outside the CO snowline (~30 au). CS and H2CO emissions are
co-spatial suggesting that they are chemically linked. The observed rings of
molecular emission at the edge of the 1.3mm continuum may be due to dust
opacity effects and/or continnum over-subtraction in the inner disk; as well as
to increased UV penetration and/or temperature inversion at the edge of the
mm-dust which would cause an enhanced gas-phase formation and desorption of
these molecules. Moreover, H2CO and CS originate from outside the ring of
enhanced dust emission, which also coincides with a change of the linear
polarization at 0.87mm. This suggests that outside the CO snowline there could
be a change of the dust properties which would reflect in the increase of the
intensity (and change of polarization) of continuum, and of molecular emission.
