Bioinduced barium precipitation in St . Callixtus and Domitilla catacombs
Biofilms are widely distributed in different areas of the Roman catacombs of St. Callixtus and Domitilla due to the abundance of mineral elements in the volcanic rock, high humidity and artificial lighting. Cyanobacteria are responsible for biofilm formation to which heterotrophic bacteria and fungi are associated. Barite (barium sulphate) was detected as secondary mineralization in altered volcanic rocks from both catacombs. Its spatial disposition suggests an origin related with the observed bacterial mats. Witherite (barium carbonate) precipitated in cultures of bacteria isolated from the two catacombs. This suggests a role of bacteria in the biogeochemical cycle of barium in terrestrial environments and, particularly, in the mobilization of barium from the volcanic rocks and its further sulphate precipitation.