Biofilms and lichens on stone monuments: do they damage or protect?

With this paper, I focus on a topic that, in my opinion, is worth consideration by scientific community involved in the conservation of cultural heritage. As far the role that biofilms and lichens play in the weathering of natural and artificial stones, an increasing number of researches account for a negligible effect and even for a protection. Thus, the axiomatic correlation among biofilms, lichens, and stone weathering is matter of controversy. These researches bring a novel perspective in a field where many studies showed that biofilms and lichens do damage stones. In this paper, I report some results that in my opinion add significant contributions and useful information to the subject. The microflora of outdoor stone monuments represents a complex ecosystem including bacteria, algae, fungi, lichens. Microorganisms colonize stonework whenever the conditions of moisture, light, temperature, and nutrition are favorable. When a biological colonization is evident, the conservator should verify at which extent it damages the materials and know the non-biogenic agents that take part in the degradation (Warscheid, 2003). Many causes have similar effects, act in synergy, or interact in quantitatively variable relations. Thus, the relevance of biological impact to the entire deterioration process should be evaluated very carefully. Detecting microorganisms on heritage objects does not automatically imply that they actually change the chemical composition or physical properties of the materials. A study of different kind of sandstone from Wyoming covered with lichens and old petroglyphs (Chiari and Cossio, 2002) demonstrated for example that lichens were not one of the key factors in the conservation of the petroglyphs, either in a negative (destruction of the outermost layer) or in a positive way (protection from rain, sun, etc.). Lichens filled the gaps between the grains, which were large enough to host them without exercising relevant pressure. Porosity was less toward the outside, since in the outer layer lichens occluded the pores. Counting the lichen thallus, the porosity proved to be the same as in the core of the rock. The deterioration of the sandstone depended mainly on the nature of the sandstone itself, in particular on the dimension of the quartz grains: the larger the grains, the greater the porosity, water absorption, fragility, and de-cohesion of the sandstone. The results may help decide whether to eliminate the lichens from the surface and to account the need for their removal to aesthetic or site management reasons rather than to chemical-physical reasons. Another stimulating study investigated the interaction over time among biofilms, lichens, and sandstone, and compared the behavior of siliceous rocks with that of carbonate ones in relation to biological growth (Hoppert et al., 2004). The authors hypothesized that lithobiontic organisms temporarily stabilize loosely to moderately cemented sandstones. When complex biofilms and lichens with moderate metabolic and reproductive rates colonize the rock in high densities, prevention of rapid decomposition of the stone is necessary. To enable undisturbed growth over several years or even decades, complex microbial communities and lichens protect the substratum from rapid decomposition by formation of a tight network of cells and extracellular polymers, which surround the mineral particles (e.g., quartz). Enwrapping the grains with a biogenic matrix temporarily stabilizes the surface and reduces weathering, which may allow the organisms to persist for years. On the contrary, microorganisms contrive a different pattern of growth in homogeneous carbonate rocks, actively boring cavities without regard of the pre-existing rock-fabric. However, a structural weakening of the substratum involving the risk of sudden desquamation and destruction of the endolithic environment create by microorganisms is uncommon. Thus, on carbonate substrata, endoliths with their relatively slow growth rates (compared to epiliths) have a chance for a sustainable life for long periods. The authors deduced that, on homogeneous carbonate substrata, maintenance of a stable population for tens of years accounts for a “sustainable” use of the rock substratum. They suggested it is a special feature of lichens that develop over decades. Understanding their strategies for colonization may be important for a variety of aspects concerning biogenic stone deterioration. A survey of four sandstone heritage structures in central Belfast exposed for around 100 years and colonized by green algae biofilms (Cutler et al., 2013) showed that algal patches were associated with less weathered surfaces (i.e., harder algal patches were associated with lower coefficient of variation of surface hardness). This might indicate that green algal cover had a broadly bio-protective role. Results from the comparative analysis of the water transport data on a