Spectra obtained with JWST MIRI-MRS of the low-mass Class 0 protostars B1-c (top) and Serpens SMM3 (bottom) illustrating the different molecular gas (red) and ice (blue) features that can be observed and analyzed. Different components of the protostellar system are enhanced in the panels. The B1-c spectrum (top) highlights the gas-phase molecular lines in this source. Atomic emission lines are present but not very strong. In contrast, both H₂ and atomic emission lines are very prominent for Serpens SMM3 (bottom). At the bottom of the silicate feature at 10 μm, close to the noise limit, the spectra have been binned by a factor of four to enhance the S/N. (See Figs. B.3 and B.11 of van Gelder et al. (2024a) for individual MIRI-MRS sub-bands.)