Molecular gas mass, derived from the CO luminosity (in red if α_CO  =  0.8, and in blue if α_CO  =  4.0–4.36), as a function of L_IR for ALMA IDs 3 and 8 (filled stars) and galaxies in clusters (filled squares) or in proto-clusters (filled triangles). Gas masses refer to ISM masses derived from the mm continuum for all ALMA sources (green open stars with annotated IDs). The solid and dashed lines represent the expected relations for MS and SB galaxies, respectively, as derived from Eqs. (5) and (6), and assuming α_CO = 4.36 (for MS galaxies) or 0.8 (for SB galaxies, SFR >  3 × SFR_MS for our sample) (Sargent et al. 2014). The grey areas represent 1σ uncertainties in the theoretical parameters of the relations.