Involved in heme d1 biosynthesis. Catalyzes the decarboxylation of siroheme into didecarboxysiroheme. Siroheme is probably decarboxylated to monodecarboxysiroheme, which is in turn decarboxylated to didecarboxysiroheme. 2 H(+) + siroheme = 12,18-didecarboxysiroheme + 2 CO2 Porphyrin-containing compound metabolism. H.thermophilus possesses the fused nirDL gene but lacks the genes encoding NirG and NirH. Belongs to the Ahb/Nir family.