Pretty in pink? New restoration treatments to mitigate salt crystallizations and pink discoloration in historic buildings and monitoring of their effectiveness through “omics” analysis

Climate change is one of the most serious threats our world is facing and predicted to amplify damage processes affecting our built cultural heritage. In this project, one factor directly associated to climate change, the increase of salt-crystallization cycles will be investigated. Salt weathering results from the combined action of salt transport and the in-pore crystallization under changing environmental conditions. The pressure exerted by the crystals on the pore surface is responsible for damage. Additionally, salt crystallizations mimic saline environments in buildings and offer an ecological niche for halophilic (salt-loving) microorganisms, most of them containing carotenoids pigments that produce an additional aesthetic damage consisting of a rosy discoloration phenomenon. The aim is to investigate to which extent the biodeterioration of stone and building materials is affected by salt crystallization cycles, a factor favoured by climate change, and how this factor affects microbial community successions, biological resilience and activity on stone materials.

To this end two historical buildings, displaying salt crystallization cycles, are chosen as natural models enabling the study of well-stablished microbial communities naturally exposed to this stress for hundreds of years. Conservator-restorers will monitor the microclimatic parameters and will undertake a sustainable desalination treatment on both buildings. They will apply new mineral poultices systems onto the walls, which offer promising advantages over cellulose poultices, as option of a long-term application and monitoring. Additionally, enrichment cultures and test specimens with different salinities will be set up to compare the community shifts caused by the artificial exposure to elevated salt concentrations. The community structure and function of the microbiota subjected to natural and simulated high salt concentrations will be investigated by omics analyses, using Next- (NGS) and Third-Generation Sequencing technologies (Illumina and Nanopore platforms, respectively). The analyses comprise amplicon 16S metagenomics, metatranscriptomics and full sequencing of the total DNA and RNA extracted from selected isolated strains.

The implementation of this project requires a coordinated interdisciplinary consortium of expertise spanning microbiology, molecular biology, bioinformatics and conservation-restoration. This initiative may form a solid basis for establishing standardized protocols for future and ongoing microbiological investigations on cultural heritage assets. The obtained results will expand our knowledge about the mechanisms of establishment and succession used by microbial communities to thrive, survive and be metabolically active in the extreme environments caused by elevated salt concentrations, which mimic our future climatic perspectives. This knowledge will help to decide upon appropriate conservation treatments for the future.