An investigation into biodeterioration caused by microbial communities colonising artworks in three Maltese palaeo-Christian catacombs

Abstract

The Maltese islands are rich in late Roman and Byzantine catacombs dating back to the third century A.D. These include pagan, Christian and Jewish burial sites, often decorated by wall paintings, ochre inscriptions, frescoes and reliefs. There are about twelve different subterranean burial sites in the village of Rabat alone. Environmental parameters i.e. a high relative humidity, frequent condensation phenomena, stable temperatures, low light intensities and long photoperiods, provide the ideal conditions for the occurrence of biofilms in Maltese catacomb environments. The main aims of this study were to describe, document and characterise a number of alterations present on wall paintings, ochre inscriptions and reliefs in Maltese Palaeo-Christian Catacombs and to investigate the biodeterioration potential of the microbes causing them using a variety of microscopy techniques. The different types of alterations present were described and catalogued in a visual glossary. Their distribution was evaluated and mapped. This seasonal documentation and monitoring was executed using non-invasive techniques. Predominant biofilms were collected using methods that are noninvasive to the substratum, through the use of adhesive tape strips. Only the superficial microbial film was sampled and not the underlying substratum. Microscopic observation of the most extensive biofilms revealed that the predominant groups of microorganisms present were cyanobacteria and microalgae, often associated to chemoorganotrophic bacterial populations. In cases where microorganisms with a high potential for biodeterioration were isolated from the archaeological surface, low-impact invasive microsamples (2x2 mm) were removed in order to investigate biodegradation processes and interactions of the microbial community with the underlying substratum.