Filamentous Fungi and the Biodeterioration of Organic Cultural Heritage Materials: A Systematic Review of Mechanisms, Risks, and Preventive Conservation Strategies

Filamentous fungi are among the most significant biological agents responsible for the biodeterioration of organic cultural heritage materials preserved in archives, libraries, and museums. Cellulose-based substrates-such as paper, papyri, and plant-derived textiles-as well as protein-based materials, including parchment and leather, provide favourable conditions for fungal colonization due to their chemical composition and hygroscopic behaviour. Once activated, fungi contribute to deterioration through a combination of mechanical penetration and biochemical processes, including the secretion of hydrolytic enzymes, organic acids, and pigmented metabolites, which progressively compromise the structural integrity and visual appearance of heritage objects. This review aims to critically synthesize current knowledge on the mechanisms of fungal biodeterioration affecting organic heritage materials, with particular attention to material-specific vulnerabilities, indoor environmental drivers, and implications for preventive conservation. Recent advances in fungal ecology have highlighted the presence of xerophilic and extremotolerant taxa capable of persisting under conditions traditionally considered unfavourable for microbial growth, posing new challenges for conservation management. Rather than attributing biodeterioration directly to global climate change, this review explicitly emphasizes the role of indirect and building-mediated climate-related stressors-such as increased frequency of moisture intrusion events, infrastructure vulnerability, and microclimatic instability within buildings-in shaping fungal risk in indoor heritage environments. The integration of environmental monitoring, microbiological diagnostics, and predictive risk-assessment tools emerges as a key strategy for early detection and mitigation. By consolidating interdisciplinary evidence from microbiology, materials science, and heritage conservation, this work underscores the need to shift from reactive restoration toward anticipatory, risk-based preventive approaches to ensure the long-term preservation of organic cultural heritage materials.