The Role of Fungi in Biodeterioration of Cultural Heritage: New Insights for Their Control

Fungi are nature’s major decomposers, and they play an essential role in biogeochemical cycles [...]


Introduction
Fungi are nature's major decomposers, and they play an essential role in biogeochemical cycles. Thanks to their wide enzyme repository, they can break down organic matter and hard-to-degrade biopolymers such as lignin, cellulose, and chitin. They also produce several inorganic and organic metabolites, organic acids, and chelating agents of fundamental importance in geological processes, leading to chemical transformations at local and global scales [1]. Though frequently overlooked, fungi are ubiquitous and can be found even in environments that are not so suitable for life due to their ability to metabolize xenobiotics [1]. Therefore, is not surprising that fungi are considered the most detrimental threat to both indoor and outdoor artefacts of historical and/or artistic value; their presence can lead to physical, chemical, and aesthetical damages [2,3].
Cultural heritage conservation refers to the measures, protocols, and methods taken to extend the life of artefacts, monuments, and sites. Biological deterioration of historical and artistic artefacts has gained significant attention in recent decades, evidencing how limited our knowledge of biodeteriogens is when we consider the wide range of heritage materials used and the environments in which they are kept. In conservation practices, precise and reliable identification of biodeteriogens is useful; however, further research is needed to assess their ecological requirements and metabolic profiles. Indeed, the environmental conditions favoring various taxonomic groups, and their limited lives, are essential in assessing the risk for artefacts and designing indirect (preventive) and direct (biocide treatments) control methods.
In this Special Issue, we present seven study cases and two reviews addressing the deterioration of different materials, such as stone, textiles, wood, and scientific instruments, exposed to outdoor, confined, or semi-confined environments.
From the UNESCO heritage sites of Cuma, Ercolano, Nola, Oplonti, and Pompei, Petraretti and colleagues [4] performed a deep sampling on frescoes, marble, mortars, plaster, and tuff to assess the biodiversity of the culturable fungal fraction. In addition to isolating fungal strains with detrimental potential, the authors suggest the importance of collecting detrimental fungi for future research on cultural heritage.
The study performed by Isola and colleagues focused on a selection of black meristematic fungal strains belonging to the Culture Collection of Fungi from Extreme Environments (CCFEE, Viterbo, Italy). All strains were isolated from marble monuments of the Bonaria Cemetery (Cagliari, Italy) and, using plate assays, the main ecological and metabolic traits and tolerance to traditional biocides were assessed [5]. The preservation of traditional Romanian clothes was the focus of the Ilies study group [6]. In detail, different essential oils were applied to different materials comprising coats (e.g., wool, cotton, leather) to control the detrimental fungal species. Other artefacts from museums include optical lenses studied by Ngo and co-workers [7]. This study evidenced the surface corrosion produced by fungi, here favored by the high humidity which is typical of North Vietnam's climate.
The study performed by Jurado and colleagues is of interest, instead, for the conservation of show caves [8]. In this study, the microclimatological and aerobiological monitoring gave insight into the seasonal dynamics of airborne fungi in the Nerja cave (Spain).
Barboux and colleagues [9] proposed an alternative instrumental method (FTIR) for detecting Serpula lacrymans; this method could be especially useful for protecting wooden structures from the dry rot fungi which are prevalent in indoor environments, especially in the holarctic regions. The study performed by Chlebicki and colleagues was devoted to testing the in vitro inhibitory effect exerted by bioactive ions exposed to galvanic systems with changing electrode distance [10].
This Special Issue also presents two reviews. The study performed by Zucconi and colleagues addressing wall paintings accounts for 60 years of fungal biodeterioration reports [11]. An exhaustive list of identified fungi was produced. However, the technical and methodological improvements that occurred within that period evidenced some limitations in the fungal identification (mainly morphological) and culturingprotocols, posing the basis for future improvements.
De Leo and colleague reviewed 109 papers published within the last 30 years to critically summarize the current knowledge on black fungi associated with the biodeterioration of stone monuments with a look at control methods and future perspectives [12].
This Special Issue offers an insight into the vast world of fungal deteriogens. More research is necessary in this field; however, with the incoming improvements in instrumental technologies, molecular diagnoses, and sustainable control methods, the research prospects in this field are exciting.