Involved in heme d1 biosynthesis. Catalyzes the decarboxylation of siroheme into didecarboxysiroheme (PubMed:21969545). Siroheme is probably decarboxylated to monodecarboxysiroheme, which is in turn decarboxylated to didecarboxysiroheme (PubMed:21969545). 2 H(+) + siroheme = 12,18-didecarboxysiroheme + 2 CO2 Porphyrin-containing compound metabolism. Forms a complex composed of NirDL, NirG and NirH. All proteins are required for the total conversion of siroheme to didecarboxysiroheme. Belongs to the Ahb/Nir family.