Unusual differential erosion due to endolithic microorganisms on limestone (Martvili, Georgia).

The presence of a diffuse biodeterioration phenomena can be observed across large areas of the outersurface of the limestone walls of the Church of the Virgin in Martvili. A differential erosion phenomenonwas detected, suggesting a possible association with biological colonization. The erosion phenomenon ischaracterized by a circular discoloration leading progressively to the detachment of flakes of limestoneat its center. Since the aforementioned process has not been described in the literature until now, thepresent study was focused on the interpretation of its origin and evolution. Three sides of the Churchare affected by a variety of occurrences of circular differential erosion which display different stages ofthe process, whereas the western side is affected by epilithic growths across its entire surface. Surveysof the petrographic, mineralogical and physical features of the stone were made, and parameters suchas density and porosity were recorded. Samples were analyzed by optical microscopy and SEM analysisin order to detected the presence of microorganisms whose species were then identified by means ofmorphological and molecular examinations. The study of cross-sections stained using the PAS methodhas confirmed the causal connection between microorganisms and the deterioration phenomena, pro-viding information about their spread of penetration into the substrate and their substantial potentialfor causing damage. Dryness appears to be the main ecological condition favoring endolithic rather thanepilithic growth. The most common isolated microorganisms were cyanobacteria (Chroococcales) and,to a lesser extent, meristematic fungi. The characteristics of the stone, and especially some endogenousdiscontinuities related to the accumulation of sedimentary layers and to the action of stone carving tools,might explain why the same differential erosion phenomenon occurs in specific areas of the block stones.The development of the differential erosion phenomenon shows a variety of stages of biological colo-nization. The progressive evolution of the deterioration process has been observed in detail: (a) firstly,white circular discolorations appear on the stone, which darken over time eventually becoming almostblack; (b) circular perforations appear around the perimeter; (c) and finally a flake detaches from thecenter, leaving a deep depression in the stone.