  The optical properties of bulk ZrSiS nodal-line semimetal are theoretically
studied within a many-body formalism. The G0W0 bands are similar to those
calculated within the density functional theory, except near the {\Gamma}
point; in particular, no significant differences are found around the Fermi
energy. On the other hand, the solution of the Bethe-Salpeter equation reveals
a significant excitonic activity, mostly as dark excitons which appear in a
wide energy range. Bright excitons, on the contrary, are less numerous, but
their location and intensity depend greatly on the polarization of the incident
electric field, as the absorption coefficient itself does. The binding energy
of these excitons correlate well with their spatial distribution functions. In
any case, a good agreement with available experimental data for
absorption-reflection is achieved. Finally, the possible activation of plasma
oscillations at low energies is discarded, because these are damped by
producing electron-hole pairs, more importantly for q along the {\Gamma}-M
path.
