# Biodeterioration of Salon de Reinos, Museo Nacional del Prado, Madrid, Spain
## 1 Introduction  
The earliest known examples of wall paintings are found in Crete and in Egyptian tombs. The Ertuscan necroolpis of Tarquinia, included in the UNESCO World Heritage list, contains a large number of decorated tombs, the oldest of which date from the 7 th century BC. Examples of extraordinary Roman wall paintings are found in the magnificent _Villa dei Misteri_ (1 st century BC) in Pompeil. The late medieval period and the Renaissance are examples of the noticeable use of wall paintings, particularly in Italy, where most churches display this type of decoration. Wall paintings can be affected by various problems--environmental pollution, excessive humidity, water seepage from the ceilings, and poor quality of materials and construction methods--all contributing to the deterioration of the murals. In wall paintings, different phenomena can be observed; the most common include cracks, detachment of the painted surface, efflorescence formation, color changes and growth of films of microbial origin. In the initial stages of colonization, the growth of microorganisms in a wall painting causes only aesthetic damage, as little or no alteration of the painted surface occurs. Subsequently, the cells and hyphae penetrate the paint layer, and the chemical attack results in pitting, peeling, cracking and loss of paint. To this damage is added that caused by microbial metabolites, which alter the original color and sometimes produce red, green or black pigmentations [1; 2; 3; 4; 5]. Deteriorated wall paintings contain a variety of hygroscopic salts, including carbonates, chlorides, nitrates and sulfates, which form efflorescences on their surfaces. This means that, in many cases, efflorescence deposits in murals mimic the conditions foundin extreme habitats, favoring the growth of halophilic microorganisms. Due to its deteriorating effect, saline efflorescence in wall paintings is a common problem and can create micro-niches with a salt concentration high enough to allow the growth of halophilic _Archaea_, as demonstrated by [PERSON] et al. [6] in two deteriorated wall paintings subjected to different environmental and climatic conditions: the Chapel of Herberstein Castle (Austria) and the Tomb of Servilia in the Necropolis of Carmona (Spain). Of particular importance is the colonization and growth of bacteria in wall paintings. [PERSON] et al. [7] studied the deterioration of the wall paintings in the church of St. Martin in Germany. In these paintings, the authors isolated numerous species of _Arthobacter_, _Bacillus_ and related genera (_Paenibacillus_, _Graclibacillus_, _Salibacillus_), producers of spores, in addition to _Staphylococcus_ and _Nocardioides_. However, the most frequent and conspicuous biodeterioration phenomenon is the colonization of wall paintings by fungi, since they generally produce black or green colored patinas. In modern wall paintings from La Rabida Monastery, Spain, the most abundant fungus was _Cladosporium phaerospermum_, followed by _Engyodontium album_, recently transferred to the novel genus _Parenygodontium album_[8] and _Apergillus versicolor_. Less frequent were _Acremonium charticola_, _Alternaria alternata_, _Botrytis cinerea_, _Cunningham echinulata_, _Phoma glomerta_, etc. [9]. In addition, the mite _Tyrophagus palmarum_ was found in the paintings, feeding on bacteria and fungi. [PERSON] and [PERSON] [10] investigated the biodeterioration of wall paintings at the Pafnuti-Borovskii Monastery, Russia, finding that _Parenygodontium album_ comprised 90% of the fungi isolated from the deteriorated stone, while _Cladosporium phaerospermum_ accounted for 65% of the total isolates from the painted surfaces. [PERSON] and [PERSON] [11] studied various wall paintings in Germany and found _Acremonium_ spp., _Cladosporium_ spp., _Verticillium_ spp., _A synergillus sydowii_, _Aureobasidium pullulans_, _Parenygodontium album_, _Scopulariosis brevicalus_, _Beaureeria_ sp. and _Chrysosporium_ sp. The walls commonly included springtails, mites, flies and spiders.