Mechanisms of lead-containing pigment discoloration caused by Naumannella cuiyingiana AFT2T isolated from 1500 years tomb wall painting of China

Biodeterioration of ancient wall paintings is a ubiquitous phenomenon that threatens the long-term survival of invaluable cultural heritage. In this study, the Naumannella cuiyingiana AFT2T, isolated from the surface of tomb paintings dating back 1500 years, was observed to be capable of altering the appearing colors of Pb-containing pigments. By a combination of physiological and biochemical measurements and transcriptomic analysis of strain AFT2T under Pb amendment treatments, the discoloration mechanisms were investigated further in depth. Incubation experiments showed that the discoloration reaction resulted from an interaction between Pb-containing pigments and strain AFT2T cells. Subsequently, we found that strain AFT2T had ability to assimilate Pb ions from pigments by using EDX mapping. Finally, after comparing to the controls, it is speculated that the observed discoloration is the formation of cellular product PbS and a series of additional tests were conducted to verify this. In addition, controlling the chemical composition in the culture medium was utilized to confirm the source of S which might be related to the sulfur-containing amino acids of animal proteins in the medium. Transcriptomic analysis exhibited that Pb stress strongly affected the expression of some genes in strain AFT2T, such as metal ion transport and metabolism genes and genes related to the TlpA family protein disulfide reductases. Among them, the gene of the enzyme with disulfide-bond reducing properties and the gene encoding the divalent Pb ion uptake transporter are the key genes in the process of discoloration. A combination of biochemical reactions and transcriptomic analysis was utilized to reveal the key genes involved in the discoloration of Pb-containing pigments by strain AFT2T. These results provide not only new insights and a theoretical basis for the conservation of cultural relics but also a new means for the management of heavy metal pollution in the environments.