Essential oils in hydrogel for microalgal biofilm removal: Application strategies for stone heritage preservation

This study faces the critical need to develop new eco-friendly biocides and effective application strategies to mitigate phototrophic biodeteriogens in cultural heritage sites. To address this challenge, a microalgal strain previously collected from the hypogeum of the Colosseum (Rome, Italy) was used to induce biofilm formation on Lecce stone specimens. The samples were treated with 0.5% and 1% of essential oils (EOs) from Thymus vulgaris L., Origanum vulgare L. and Cinnamomum verum Presl. The EOs were encapsulated in an alginate hydrogel support matrix (HG) and applied to the biofilms with different application times (24 h and 48 h). A mini-PAM portable fluorometer was used to determine the phototrophic activity up to 2 months after treatment. Scanning electron microscopy was used to investigate the effects of EOs on microalgal cell morphology within biofilms, while Fourier transform infrared spectroscopy analyzed changes in biomolecular distribution after treatment. The findings revealed that all EOs were effective when encapsulated in HG. However, by the end of the monitoring period, only cinnamon EO maintained photosynthetic inhibition, especially when applied at 0.5% for 48 h. Moreover, biofilm treated with cinnamon EO showed the most significant effects, particularly in disrupting cell membranes and reducing lipids signals, ultimately leading to cell lysis. This approach effectively inhibited the vitality of biofilm-forming phototrophs on stone surfaces, using low concentrations of EO for defined periods of time. • Essential oils (EOs) were tested on a microalgal biofilm isolated from Colosseum. • EOs were encapsulated in alginate hydrogel (HG) to enhance their efficacy. • Biocide concentration and exposure time were optimized. • Cinnamon EO was the most effective, causing extensive cell lysis. • PAM fluorometry, SEM and FTIR were used to assess biocidal effects on cells.