Bacterially mediated mineralisation processes lead to biodeterioration of artworks in Maltese catacombs

Mineral structures formed by bacterial and microalgal biofilms growing on the archaeological surface in Maltese hypogea were studied using Energy Dispersive X-Ray Spectroscopy (EDS) coupled to Environmental Scanning Electron Microscopy (ESEM), X-ray micro-diffraction (XRD) and X-ray fluorescence (XRF). These techniques have shown that mineral structures having different morphologies and chemical composition were associated with the microorganisms in the subaerophytic biofilm. Salt efflorescences and mineral deposits on the archaeological surface were often formed from gypsum (CaSO4∙ 2H2O), halite (NaCl) and calcite (CaCO3). Biogenic carbonates produced by microbial activities were a common occurrence. These assumed different forms, such as the production of mineral coats around cyanobacterial sheaths and the occurrence of calcite fibres with different morphologies on the surface of the biofilms. Moreover, vaterite (CaCO3) spherulites which appeared hollow in cross-section were observed. The presence of struvite was recorded from one catacomb site. These investigations have facilitated the study of the neoformation of metastable minerals by microbially mediated processes, which potentially contribute to a better understanding of the biodeterioration of artworks in Maltese palaeo-Christian catacombs.

Research highlights
► Mineral structures formed by subaerial biofilms growing in hypogea were examined.
► Efflorescences and mineral deposits were often formed from gypsum, halite, and calcite.
► Biogenic carbonates assumed different forms e.g. vaterite spherulite, calcite fibres.
► The formation of rare minerals e.g. struvite was mediated by bacteria.
► Understanding biomineralisation processes facilitates the conservation of artworks.