Evaluation of plant-derived extracts for the control of terrestrial algal biofouling on building materials: In vitro and in situ studies

Microbial colonization by aeroterrestrial phototrophs is a major cause of biodeterioration of cultural heritage materials. This study evaluated the activity and mechanisms of selected plant-derived essential oils (EOs) against algal strains Jenufa aeroterrestrica, Stichococcus bacillaris, Coccomyxa galuniae and Aphanocapsa sp., isolated from stone substrates in the Archaeological Park of Baia's Thermal Baths (Italy). Essential oils from Mentha × piperita L., Achillea millefolium L., Matricaria chamomilla L., and Humulus lupulus L. were characterized by GC–MS and tested through growth inhibition, physiological assays, in situ applications, and untargeted metabolomics (UHPLC-MS). M. piperita L. EO showed the highest antimicrobial efficacy, while mature biofilms exhibited greater resistance to treatment. PAM fluorometry and chlorophyll a analyses revealed rapid photosynthetic impairment and early disruption of photosystem II after EO treatment. Metabolomic profiling showed alterations in redox balance, amino acid metabolism, and membrane-related pathways, consistent with oxidative stress and membrane destabilization. In situ tests confirmed effective biofilm reduction and substrate recovery highlighted by colorimetric analyses in the CIELAB space and fluorometry, supporting the potential of EOs as sustainable alternatives to conventional biocides in cultural heritage conservation.