Diversity of Endolithic Prokaryotes Living in Stone Monuments

Stone monuments (statues, fountains, tombs, memorials, buildings, etc.) are apparently indestructible artefacts that were built to last. Depending on the chemical composition and structure, stones may be friendly or hostile to colonization by eukaryotic and/or prokaryotic communities. Among various lithotypes, porous rocks (limestone, sandstone) have highest bioreceptivity. Abiotic factors have a strong impact on the deterioration rate of stone monuments. Biological activity of living colonizers may intensify the weathering of such substrates. Moreover, human activity could enhance the bioerosion by providing more nutrients to epilithic and endolithic microbial communities. Highly diverse lithic communities have been detected in stone monuments, both by culture-dependent and molecular approaches. Epilithic and endolithic prokaryotes are well represented by cyanobacteria that are among the pioneers of colonizing the stone surfaces in partnership with fungi and algae. Active boring algae and cyanobacteria are euendoliths par excellence. Chemoheterotrophic actinobacteria and gammaproteobacteria are dominant bacterial species found in biofilm as well as in endolithic niches of stone sampled from various types of monuments. To date, there is little information on endolithic archaea and chemolithoautotrophic bacteria. Further indepth investigations on the diversity and metabolic roles of lithic communities are needed with respect to the conservation strategies.