Comparison of biocides, allelopathic substances and UV-C as treatments for biofilm proliferation on heritage monuments

Abstract UV-C and biocide treatments are frequently used to treat historical monuments contaminated by autotrophic biofilms. In this study, the authors compare for the first time the efficacy of these treatments against microorganisms such as cyanobacteria and algae proliferating in the Vicherey church (Vicherey, Vosges Department, France). To identify the most environmentally friendly and also efficient method, an allelopathic treatment was also tested. Colorimetric and physiological measurements of treated biofilms were thus monitored for 6 months. Fungi growing on necrotic matter from treated biofilms were sampled and sequenced. With biocides, results showed incomplete eradication of biofilms, even after two treatments. Biofilm color loss was delayed in comparison to UV-C treatment, which appeared more efficient after just one treatment. Moreover, quantum yield (Fv/Fm) decreased immediately after UV-C treatment, indicating inhibition of algae and cyanobacteria photosynthesis. However, two species of fungi colonized the cyanobacteria biofilms treated with UV-C. Allelopathic treatment was not efficient on biofilm and showed no deleterious effect on photosynthesis. The present findings demonstrate that the UV-C method, coupled with a cleaning phase for necrotic organic matter, may be considered environmentally friendly and the best alternative to chemicals.