Rosy Discolouration in an Alpine Chapel: Beyond Salt Dependence

Area of Science:

• Geosciences
• Microbiology
• Conservation Science

Background:

• Alpine stone heritage is vulnerable to deterioration from moisture and temperature fluctuations.
• Rosy discolouration on monuments is often linked to pink biofilms, particularly in higher elevations.
• The role of salt efflorescence in modulating these biofilms remains poorly understood.

Purpose of the Study:

• To investigate rosy discolouration on alpine stone heritage at St. Cyprian Chapel.
• To characterize the influence of salt efflorescence on biofilm composition and pigment production.
• To link substrate chemistry and mineralogy to biofilm characteristics.

Main Methods:

• Comparative analysis of two discoloured walls (north with salt efflorescence, south without).
• Metabarcoding for microbial community analysis.
• Chemical and mineralogical analyses (including Raman spectroscopy).

Main Results:

• Similar mineralogy but distinct surface chemistry (salt efflorescence) on the north wall.
• Comparable biofilm biomass, but differing microbial communities between walls.
• South wall hosted carotenoid biomarkers (Rubrobacter, Pontibacter) with low pigment expression; north wall showed stronger discolouration and detected bacterioruberin, with Chryseobacterium potentially contributing pigments.

Conclusions:

• Salt efflorescence is not essential for pink biofilm formation but enhances pigment production in salt-rich microhabitats.
• Microbial communities differ significantly with or without salt efflorescence.
• Findings offer new insights into alpine cultural heritage deterioration and biofilm drivers.