## 1 Introduction  
The Mediterranean Basin countries are recognized to accommodate some of the world's most important cultural heritage sites, including stone monuments and lithic works of art (Figure 1). Due to their cultural, artistic, and religious meaning, the safeguarding of this precious cultural heritage is highly important. The wide availability of tertiary sedimentary rocks in Southern European countries, especially Spain, Portugal, and Italy, has made limestone the main source of creating stone monuments [1]. Limestone has undoubtedly been used since the Stone Age and even earlier, and the first records of its usage relate to the Egyptian Second Dynasty, which was about 5800 years ago, when it was used for the construction of the Giza pyramids [2]. Limestone is a sedimentary rock composed of two main calcium carbonate (CaCO\({}_{3}\)) minerals, calcite and argomite, assembled in trigonal and orthorhombic systems, respectively. It may also contain materials such as quartz, sand, chert, and clay. The bioreectivity of limestone is determined by a number of factors, including its porosity, pH, texture, roughness, chemical composition, density, and moisture [3]. There is a growing concern that microbial biofilms on cultural heritage buildings and monuments, especially on carboniferous stones such as limestone, impact their aesthetic and physical effects, since limestone is soluble in weak acid solutions and, therefore, particularly vulnerable to biocorrosion [1]. The locations from which the microbial samples discussed in this review originate range from the Giza pyramid complex [3], the archaeological site of the Herculaneum's House of the Bicentenary in Italy (1 CE) [4], the archaeological site of Cogan, one of the most remarkable Classic Maya (250-900 CE) centers in Mesoamerica [5], the ancient Chinese Buddhist culture sites from West Lake Cultural Landscape dating back to the Northern Song (960-1219 CE) and Qing dynistas (1636-1912 CE) [6], the 15 th century French Notre Dame Gothic Cathedral in Rheins [7], and some other selected examples, including prominent monuments, buildings, and landscapes of historical significance. Studied caves of CH significance from France [8; 9; 10] and Spain [11] are included as well. Carbonate rocks (limestone in particular) are easily biodeteriorated, yet it is the vulnerability of the substratum that made the limestone attractive for creating stone monuments and buildings, as it is soft and easy to carve [1]. Microorganisms, including bacteria, fungi, archaea, algae, and lichens, are major causes of damage to stone monuments and have, therefore, received significant attention from conservators, restorers, and conservation scientists. The microbial colonization of stones depends on environmental factors such as water availability, pH, exposure to wind and rain, nutrient sources, and petrologic parameters such as mineral composition, type of cement, and the porosity and permeability of the rock material [12]. Culture-dependent techniques have been adopted for many years for investigating microbial communities or for assessing specific patterns of degradation caused by certain microbial taxa. This strategy has the advantage of testing specific degradation problems, yet it fails us when a comprehensive view of the complexity of the microbial communities is needed, which is mainly due to the fact that approximately 99% of viable microorganisms cannot be cultured [13]. In this review, we refer to the deterioration of cultural heritage objects/items made of limestone (CHL) colonized by microbes that are identified and/or detected using molecular methods. Culture-dependent (CD) methods refer to all methods in which the identification of pure microbial cultures is based on selected target DNA regions, mostly obtained using Sanger sequencing, while culture-independent (CI) methods  
Figure 1: Biodeteriorated limestone outdoor monument of [PERSON], located at Dornava manson (Slovenia) from the middle of 18 th century.