Involved in heme d1 biosynthesis (PubMed:8982003). Catalyzes the decarboxylation of siroheme into didecarboxysiroheme (By similarity). 2 H(+) + siroheme = 12,18-didecarboxysiroheme + 2 CO2 Porphyrin-containing compound metabolism. Probably forms a complex composed of NirD, NirL, NirG and NirH. All proteins are required for the total conversion of siroheme to didecarboxysiroheme. The nirDLGH mutant lacks dissimilatory nitrite reductase (NIR) activity. The NIR activity is restored by adding purified heme d1. Belongs to the Ahb/Nir family.