Methodological challenges for the investigation of the dual role of biofilms on outdoor heritage

Biofilm deterioration and biofilm protection should be considered as different aspects of the complex interactions between microbes and the surfaces of outdoor heritage (e.g. stones, bricks, mortar and plaster). Thus, it is urgent to verify and quantify to what extent the biofilm can protect from different weathering processes, to eventually determine the advisability of biofilm removal from the heritage surfaces. On one hand, it is necessary to more precisely describe the decaying processes caused by the microorganisms and to quantify the extent, severity, and rate at which the microorganisms are causing the decay. On the other hand, it is necessary to define methodologies to comprehensively study the bioprotection phenomena. So far, no decision-making tool is available to guide heritage professionals in deciding whether to remove or keep biofilms on heritage surfaces, and aesthetical alteration and discoloration is often the only criterion considered. In this work the different available approaches for the study of the dual role of biofilms on outdoor heritage have been critically reviewed. The open challenges and questions are also summarised. [Display omitted] •Microorganisms grow as biofilms on outdoor built heritage.•Bioprotection vs. biodeterioration is an open debate.•The study of the biofilm/substrate’ physico-chemical interactions is challenging.•Quantitative methods to determine biofilms' role on built heritage are needed.