1. Introduction
Humankind cultural heritage objects, relics and sites can be undesirably altered and seriously damaged from the growth and metabolic activities of living organisms [
1,
2,
3]. These biodeterioration processes can occur at both indoor (e.g., museums) and outdoor environments (e.g., monuments), posing a serious risk for Worldwide historical sites, properties and objects. Materials such as paper, parchment, books, paintings, ceramics, frescoes, textiles, glass, vitrails, photographs, sculptures, funerary accessories, wooden materials and even stone monuments, can be colonized and deteriorated through microorganisms’ aesthetic, mechanical, acid and enzymatic vital actions and manipulations [
4]. Biodeterioration can thus be a result of the impact of various organisms (bacteria, cyanobacteria, microalgae, archaea, fungi and lichens [
4]), and requires that protective measures are constantly considered, developed and implemented. Depending on specific conditions and the substrate type, some of these microorganisms can also contribute to the protection of the materials, thus displaying a biodeteriorative/bioprotective dualistic effect/nature [
5]. For these reasons, molecular techniques, such as DNA sequencing, have been largely applied to investigate, understand and monitor biological colonization on art objects and cultural heritage monuments for more than two decades [
6,
7,
8]. Justifiably, the current focus resides in the application of Next-Generation-Sequencing (NGS) methodologies, since they have powerfully expanded the possibility to characterize microbial communities in a cheaper, quick and holistic manner [
9,
10,
11,
12,
13,
14,
15]. Due to the peculiar characteristics of this device, the last few years have seen the steady rise of the popular Oxford Nanopore
® MinION
™ sequencer in this research area, which has been applied in a variety of cultural heritage materials [
7,
8,
16]. MinION™ is a small, relatively inexpensive, mobile, long read DNA/RNA sequencer, with an enormous range of applications. The technology relies on nanopores (protein pores), acting as biosensors to detect negatively charged single-stranded DNA or RNA molecules driven through the nanopore along an ionic current, and decoding of electric variations with computational algorithms [
15].
In this review article, we aim to provide an update summary of previous studies using the MinION™ sequencer, while simultaneously providing holistic discussion of possible future directions, additional applications and associated impacts of their utilization in the field of cultural heritage materials biodeterioration (in order to stimulate further discussions).
2. Application of Oxford Nanopore® in Cultural Heritage BIODETERIORATION studies
So far, the Oxford Nanopore
® MinION
™ sequencer has been applied in a variety of cultural heritage materials including: stone monuments, granite chapels, salt-weathered buildings, petroglyph sites, oil paintings, drawings, textiles, waxes, bronze statues, waterlogged archeological wood pirogues, iron nails from a whale skeleton, documents and museum environments (
Table 1). Overall, the number of studies using a metabarcoding approach is predominant with the most sequenced genes being the ITS, 16S and 28S rDNA regions, and the most studied supports being related to stone substrates (
Figure 1). In addition, the most common keywords found include: biodeterioration, metagenomics, nanopore sequencing technology, building materials, 16s RNA ribosomal gene, microbiome, bacteria, MinION
™, microbiota and conservation (
Figure 2).
Grottoli and colleagues [
17] used a gene sequencing tailored approach (ITS, 16S and 18S rDNA regions) to apply the Oxford Nanopore
® MinION
™ sequencer in a study in the hypogeum of Basilica di San Nicola, in Carcere Church (Rome, Italy). The authors developed and tested a bioinformatics approach, named “AmpLIcon SequencIng Analysis” (ALISIA) to evaluate their results, and verified a feeble overlap between this approach and culture dependent methods.
Kisová and colleagues [
18] used a metabarcoding approach (ITS, 16S and 28S rDNA regions) to study the microbiomes of funeral accessories. The authors were able to characterize biodeteriorative microorganisms, including bacteria responsible for metal corrosion and bio-mineralization, and entomopathogenic and phytopathogenic fungi problematic in these textiles.
Šoltys and colleagues [
19] sequenced the ITS, 16S and 28S rDNA regions to study the biodeteriogenic microbiome of a XVIII Century wax seal colored with minium. The authors identified the presence of a complex microbiota dominated by fungi capable of conducting alterations to lipids, leads, and contributing to the formation of lead soaps and secondary biogenic minerals.
Piñar and colleagues [
20,
21] conducted two studies using a whole genome amplification (WGA) approach in various oil paintings and Leonardo da Vinci’s drawings. The authors were able to establish a fast and simple molecular WGA protocol for application in the area, while also highlighting microbiome variances in different composition and conservation status of distinct paintings [
20]. For the Leonardo da Vinci’s drawings [
21], they were able to observe the impacts of geographical region on the microbiome and obtain a microbiological “bio-archive”, and consequently a reference dataset for future monitoring efforts.
Planý and colleagues [
22] conducted a metabarcoding study of the 16S and 28S rDNA regions to understand iron nails corrosion from a whale skeleton displayed at the Natural History Museum in London, United Kingdom. Through this method, they were able to characterize the fungal and bacterial communities associated with the corrosion of the iron nails, while also proposing a fungal-mediated biodeterioration mechanism.
Brimblecombe and colleagues [
23] sequenced the ITS region to study the fungal surface contamination in the Klosterneuburg Monastic Library. The authors also compared these results with cultivated fungi retrieved from library dust at this site.
Delegou and colleagues [
24] applied 16S rDNA sequencing to characterize the microbiota of building materials of the holy Aedicule sepulcher in Jerusalem. They were also able to characterize and discuss the biodeteriorative impact of the major species found.
Pavlović and colleagues [
25] analyzed various genes (ITS and 16S rDNA region; and the nitrite reductase (nirK), sulfite reductase (dsr), the sox enzyme system for sulfur oxidation (soxB) and the ammonia monooxygenase (amoA) genes) to study a salt-contaminated twelfth century granite-built chapel. The authors contributed to the knowledge of the role of microorganisms in the presence of salt and damp stains, by founding a well-defined relationship between the microbiome and soluble salts, and consequently the biodeterioration phenomena affecting these chapels.
Through the analysis of the ITS, 16S and 28S rDNA regions, Pavlović and colleagues [
26] studied the microbiome of wax drippings from a candle on manuscripts. They found that wax did not make the paper more biodegradable, but provided niches for the accumulation of dust and eroded material, which in turn could act as nutritional hotspots for microbial proliferation.
Rabbachin and colleagues [
27] performed WGA sequencing in petroglyph sites of the Negev desert of Israel. The authors corroborated the connections of the microbial communities between rock varnish and stone inhabitants, and the microbiome contribution to the deterioration visualized.
Timoncini and colleagues [
28] conducted 16S rDNA sequencing to understand the role of microbial communities on bronze and marble statues patinas. They verified that differential bacterial communities occurred between marble and bronze statues and among different marble patinas. Additionally, the authors noted high microbial diversities in marble surfaces, low microorganism diversity in bronze statues, being able to expand the current knowledge regarding bacteria communities and various patinas.
Beccaccioli and colleagues [
29] targeted the ITS and 16S rDNA regions to understand the microbial communities contributing to the deterioration of waterlogged wooden fragments from pirogues. The authors also developed and tested a custom pipeline for their bioinformatics analysis. The results obtained allowed to identify high levels of many bacteria and ligninolytic fungi able to contribute to wood erosion of these artifacts.
Li and colleagues [
30] used a WGA approach (Illumina + Nanopore) in biofilms thriving in Leshan and Feilaifeng stone cultural heritage sites. They were able to understand the biofilms’ taxonomic profiles, while also gathering a strong knowledge of the microbial groups and gene families contributing to elemental nitrogen and sulfur metabolism enhancing stone biodeterioration. In addition, the authors were also able to shed light on biofilm resistome (see below).
Nir and colleagues [
31] applied the Oxford Nanopore
® MinION
™ sequencer to gather knowledge on the genomic characteristics of cyanobacteria colonizing the Negev petroglyphs. The authors were able to generate a metagenome from
Trichocoleus desertorum strain NBK24 to understand genes crucial for survival at these extreme environments, and to contribute to elemental exchanges leading to biodeterioration.
Pavlović and colleagues [
32] conducted a metabarcoding analysis of 16S and 28S rDNA regions to study the microbiome of books, folklore prints, and archive documents. The authors noted that complex microbial communities were responsible to stain two types of paper analyzed.
Rabbachin and colleagues [
33] applied metagenomic sequencing (WGA) to study natural patinas in petroglyphs in the Austrian Alps. They found differences on stone microbiomes with and without visible biofilm/patinas and concluded that their removal could enhance and continue new cycles of colonization and biodeterioration.
Through the metabarcoding analysis of the 16S rDNA region, Tichy and colleagues [
34] were able to understand the relationships between the microbiome and pinkish patinas in salt-weathered buildings. The authors found that the microbial communities were influenced by salt concentration and chemical composition. Moreover, this study also highlighted the impact of presence/absence of K
+ ions on the bacteria and archaea communities and consequently biodeterioration at these sites.
Bastholm and colleagues [
35] sequenced the Calmodulin gene through metabarcoding approaches to identify xerophilic
Aspergillus growth in Danish Museum repositories. Comparisons with Illumina amplicon sequencing and cultivation-dependent methods were also performed, in an effort to understand if
Aspergillus section
Restricti species had become a novel contaminant nationally distributed.
Haedar and colleagues [
36] conducted analysis of the 16S rDNA region to understand the bacterial communities on degraded Prehistoric rock paintings in Maros-Pangkep Global Geopark. They found multiple genera known to be involved in carbonate precipitation and rock weathering contributing to the problems noted at this site.
Overall and taking into account these works, it is clear that the Oxford Nanopore
® sequencer is increasingly becoming a critical component of microbial cultural heritage biodeterioration and conservation studies [
7,
8,
16,
37].
Figure 2.
VOSviewer (
https://www.vosviewer.com/) co-occurrence network displaying the top 10 keywords (appearing at least two times) from works regarding the application of the Oxford Nanopore
® sequencer in the cultural heritage biodeterioration area.
Figure 2.
VOSviewer (
https://www.vosviewer.com/) co-occurrence network displaying the top 10 keywords (appearing at least two times) from works regarding the application of the Oxford Nanopore
® sequencer in the cultural heritage biodeterioration area.
4. Challenges in the Oxford Nanopore® Application in Biodeterioration Studies
While the Oxford Nanopore
® MinION™ sequencer presents numerous advantages for studying microbial communities in cultural heritage materials, several limitations must also be acknowledged. Among them it should be highlighted that: the occurrence of sequencing errors; high cost and difficulties to attain reagent storage in low-resource or remote settings; and the need for strong internet connections for analysis, can hamper new applications of the method [
69]. Moreover, the complexity of bioinformatics pipelines required for data analysis can also present further challenges for researchers. Additionally, while the MinION™ sequencer offers portability and in-field analysis capabilities, sample preparation protocols may need optimization to ensure reproducibility and reliable results in non-laboratory settings. On the other hand, and to a lower degree, difficulties to conduct routinary procedures
in situ, such as low DNA recovery from environmental samples, also needs to be noted [
71]. Nonetheless, the system is one of the fastest developing NGS platforms today and their limitation are systematically being improved. For instance, the new flow cell R10.4.1 allows quality scores of Q20+, very high accuracy levels (often over 99%) and even allows Short Fragmented Mode (SFM) applications [
60,
85,
86,
87,
88] which, in itself, can offer another additional range of possibilities. Additionally, protocols for improved DNA extraction and offline bioinformatic analysis are also currently being developed and tested (e.g., [
89,
90]). Overall, addressing these limitations will understandably be crucial to effectively ensure the utility of nanopore sequencing [
85] in cultural heritage biodeterioration studies and in advancing our understanding of microbial interactions and degradation processes affecting cultural heritage artifacts.