Centimetric circular areas uncolonized by microbial biofilms (CUMBs) on marble surfaces and insights on a lichen-related origin

This study investigated the poorly known phenomenon of Centimetric circular areas Uncolonized by Microbial Biofilms (CUMBs) which is frequently observed on natural and heritage stone surfaces displaying widespread lithobiontic colonization. In order to unveil a possible relationship with past lichen colonization, analyses were carried out on the distribution, morphometry, physical and mineralogical properties, and microscopic features of CUMBs on the marble surfaces of a balustrade in the Garden of a Savoy residence in Torino (Italy; UNESCO-WHS 823bis) and in its original quarry site in the W-Alps. Image analyses of CUMBs displayed a distributional and dimensional compatibility with lichen thalli (re-)colonizing surfaces in their vicinity. Invasive analyses on quarry materials displayed similar microscopic modifications in marble layers beneath CUMBs and lichens, associated to a higher stabilization of the calcite {01-12} form, which is favoured by the presence of organic substances. These findings support the hypothesis of a lichen origin for some CUMBs, which may derive from the modification of physical stone properties and/or a long-lasting allelopathic effect affecting surface bioreceptivity.