Nature and origin of the violet stains on the walls of a Roman tomb.

The Circular Mausoleum tomb (Roman Necropolis of Carmona, Spain) dates back from the first century AD and is characterized by a dense microbial (phototrophic) colonization on the walls and ceiling. However, some walls exhibited an important number of violet stains of unknown origin. The microbial communities of these violet stains are mainly composed of cyanobacteria, streptomycetes and fungi. A strain of Streptomyces parvus, isolated from the walls, produces a violet pigment in culture media. High performance liquid chromatography-mass spectrometry of the culture extracts obtained from this Streptomyces revealed the presence of a few granaticins, pigments with a benzoisochromanequinone structure. When metabolically active in the tomb, S. parvus synthesizes the pigments that diffuse into the mortar. During rain and/or wetting periods, the pigments are solubilized by alkaline waters and elute from the starting position to the surrounding mortar, enlarging the pigmented area and thus contributing to this exceptional biodeterioration phenomenon.