Conventional and innovative approaches to black fungi control for stone heritage preservation

Abstract

Black Meristematic Fungi (BMF) are characterized by a thick melanized cell wall and an isodiametric cellular expansion. BMF represent one of the most damaging groups of microorganisms causing the deterioration of outdoor exposed stone monuments mainly due to the formation of dark spots and patches leading to the darkening of their surface, cracking, and bio-pitting. BMF are among the most stress-resistant organisms on Earth, known for their remarkable ability to withstand solar radiation, desiccation, and extreme temperature fluctuations, which has led to their widespread distribution across the globe. These features make BMF very difficult to remove and restrict, representing a challenge for restorers. Despite the number of scientific works about BMF isolation and ecology, little is known about their response to antimicrobial treatments. Conventional biocides remain the most used treatment for the control of biodeterioration on stone artworks. In recent years, interest in alternative and safer antimicrobial treatments has risen in conservation strategies. The number of scientific works in which their efficacy against BMF is evaluated is, however, still low. The aim of this review is to assess existing studies regarding the response of BMF to both conventional and innovative treatments. This will encompass an in-depth examination of methodologies for the application and evaluation of treatments. Furthermore, we aim to highlight future research directions that will contribute to a more informed selection of effective anti-BMF interventions for stone preservation. We underscore the significance of pioneering, environmentally low-impact solutions.

Keywords: Black Meristematic Fungi; antimicrobial treatment; conventional and innovative biocides; ecological transition; rock inhabiting fungi; stone monuments biodeterioration; stress resistance.

FIGURE 1

Black Meristematic Fungi (BMF) on stone. (A) Phenotypic variety of colonies of four BMF strains isolated from the darkened marble of Florence Cathedral: M10, Vermiconia sp.; m16, Knufia sp.; M30, Coniosporium sp.; m21, Lithohypha sp.; (B) Meristematic cells of the newly identified fungus Saxispiralis lemnorum, isolated from deteriorated limestone, subdivided by septations in various directions (arrows). (C) Close‐up of the ‘Ratto delle Sabine’ marble sculpture in Florence, Italy, revealing colonization by Knufia petricola (syn. Sarcinomyces petricola) as darkened patches; (D) Scanning Electron Microscope image highlighting a K. petricola colony isolated from the ‘Ratto delle Sabine’ within a marble microcrack, arrow indicating the main growth area.