The microorganisms responsible for purple spots on a 13th century scroll have been identified in a study in Scientific Reports. Ancient parchments are commonly attacked by microbes, resulting in purple spots and the detachment of the superficial layer of the document, which affects the readability. The authors hope that their findings may help in the restoration and conservation of ancient parchments.
The five-metre long parchment, A.A. Arm. I-XVIII 3328, written in 1244 AD for a canonization inquiry, tells the story of a young soldier called Laurentius Loricatus who killed a man by accident. To make amends for his crime, he retired to a cave near Subiaco, Italy, for the next 34 years. However, the document is covered in purple spots, with damage to the collagen structure significantly affecting its readability. The damage is most likely to have occurred prior to the scroll being moved to the Vatican Secret Archive at the end of the 18th century, where it is now kept under controlled environmental conditions.
By performing a genetic analysis of the microbial communities colonizing the scroll, Luciana Migliore and colleagues found that Gammaproteobacteria were present in the purple spots but absent in the undamaged areas of the parchment. The authors suggest that deterioration of the parchment occurred during a process of microbial succession, in which Halobacteria - responsible for the rhodopsins that produce the purple spots - were replaced by Gammaproteobacteria, only leaving the purple stains behind. The authors suggest that further studies could help identify the exact sequence of microbes that produced the rhodopsins responsible for the spots and may reveal new approaches to aid in the restoration of documents damaged in this manner.