ABSTRACT
Microorganisms inhabit Roman catacombs where play a role in deterioration processes of lithic substrata and influence the biogeochemical cycles. These microorganisms form biofilms where cyanobacteria and heterotrophic bacteria are associated with a few green algae, diatoms, and mosses. Biofilms are harmful for the catacombs mural paintings because they are related to constructive and destructive processes in hypogean environments. The catacombs of St. Callixtus and Domitilla are among the greatest and most important of Rome. Physical and chemical characterization of lithic substrata revealed that the host rock of catacombs consisted of different episodes of volcanoclastic fall deposits from Colli Albani eruptive succession disposed in thick layers. The tunnels constituting both Domitilla and St. Callixtus catacombs were dug in fairly consolidated layers while hardened deposits were used as support elements. Spheroidal bodies, similar to found beneath the catacomb biofilms have been synthesized in laboratory cultures.
