Application of commercial biocides to lichens: Does a physiological recovery occur over time?

Biocidal products are widely used to devitalize lichen thalli on monumental surfaces before their mechanical removal, but there is still lack of information about the persistence of the toxic effects over time. This issue is of paramount importance since it can greatly influence the process of lichen recolonization. The aim of this study was checking for physiological recovery or residual vitality of lichens after exposure to two commercial biocidal products, Biotin T or Preventol RI80. Samples of the foliose lichen Xanthoria parietina were treated with solutions containing the two biocides at the highest concentration suggested by the producer (3% and 2% respectively). Selected physiological parameters were investigated as indicators of sample vitality: photosynthetic parameters (FV/FM and PIABS), the content of chlorophyll a, chlorophyll b, beta-carotene, ergosterol and soluble proteins, after 24 and 72 hours and, to check for recovery, after 20 and 90 days. Both biocidal treatments induced severe physiological alterations, causing impairment to both the lichen photobiont and mycobiont, with Preventol showing a faster effect. The substantial loss of vitality following treatments with Biotin T and Preventol persisted over time, and no physiological recovery was found after 90 days.