  Circumstellar disks are considered to be the birthplace of planets. Specific
structures like spiral arms, gaps, and cavities are characteristic indicators
of planet-disk interaction. Investigating these structures can provide insights
into the growth of protoplanets and the physical properties of the disk. We
investigate the feasibility of using molecular lines to trace planet-induced
structures in circumstellar disks. Based on 3D hydrodynamic simulations of
planet-disk interactions, we perform self-consistent temperature calculations
and produce N-LTE molecular line velocity-channel maps and spectra of these
disks using our new N-LTE line radiative transfer code Mol3D. Subsequently, we
simulate ALMA observations using the CASA simulator. We consider two nearly
face-on inclinations, 5 disk masses, 7 disk radii, and 2 different typical
pre-main-sequence host stars (T Tauri, Herbig Ae). We calculate up to 141
individual velocity-channel maps for five molecules/isotopoloques in a total of
32 rotational transitions to investigate the frequency dependence of the
structures indicated above. We find that the majority of protoplanetary disks
in our parameter space could be detected in the molecular lines considered.
However, unlike the continuum case, gap detection is not straightforward in
lines. For example, gaps are not seen in symmetric rings but are masked by the
pattern caused by the global (Keplerian) velocity field. We identify specific
regions in the velocity-channel maps that are characteristic of planet-induced
structures. Simulations of high angular resolution molecular line observations
demonstrate the potential of ALMA to provide complementary information about
the planet-disk interaction as compared to continuum observations. In
particular, the detection of planet-induced gaps is possible under certain
conditions.(abridged)
