Thermohygrometric Climate, Insects and Fungi in the Klosterneuburg Monastic Library

Abstract

The abundance of insect and fungal pests under a changing climate may threaten historic interiors, libraries and museums, with warmer, potentially more humid winters. This work examines local and indoor climate, insects and fungi in a historic library near Vienna. It reveals a mostly dry and cool environment for the storage of books, but few visitors to induce changes. Temperature and relative humidity have been monitored for 12 months (2021-07/2022-07), with 14 monitors positioned insect traps (blunder traps and some pheromone traps). Fungi in air, on surfaces and in settled dust were also sampled. Winter temperatures in library cupboards and behind shelves were slightly warmer (~1 °C) and more humid than in the library environment. Over the last decade there have been infestations of the biscuit beetles (Stegobium paniceum) but since treatment with sulfuryl difluoride, Anthrenus sp. have dominated. Silverfish are also present, but only in one corner. Fungal outbreaks have also been found, but over five years fungi in air samples have shifted from Penicillium commune and P. chrysogenum to Aspergillus sp. The stable environment at Klosterneuburg is suitable for books, yet insects and mould present suggests vigilance remains necessary, as some microenvironments (e.g., cupboards) can be at risk and there may be materials with high water content, hygroscopic or of nutritional value.

1. Introduction

Change in the type and abundance of insect and fungal pests can threaten historic interiors and museums [1,2,3,4,5]. The risks posed to libraries were classically described by William Blades in The Enemies of Books [6] from the 1880s as: fire, water, gas and heat, dust and neglect, ignorance and bigotry, bookworms, vermin, bookbinders, book collectors, servants, and children. Blades recognised that humidity is important in its interactions with books, especially in the promotion of mould growth, and additionally he worried about “paper-eating species” of insects.

Historic libraries in Central Europe are not a special focus of preventive conservation, as they are often located in monasteries with no conservation staff responsible for the collections. However, these libraries hold valuable books and archive materials, commonly made from paper, leather, parchment and wood. In contrast, museums typically have conservators aware of the needs of preventive conservation and part of their duty is the care for the museum objects. Collections are at constant risk of being damaged by many agents of deterioration [7]. However, stored under optimal conditions, paper can last for a long time, though in historic buildings it is particularly susceptible to infestation by insects [8,9,10,11,12,13,14], fungal damage [3,15] and variations in climate—it is especially sensitive to temperature and humidity fluctuation [16,17,18].

Books and archive materials are at risk of damage from a variety of insect species. The most important paper damaging pests are beetles (Coleoptera) [8,9,10,11,12,13,14]: the biscuit beetle (Stegobium paniceum), spider beetle, such as the Australian spider beetle (Ptinus tectus), the white-marked spider beetle (Ptinus fur) and the furniture beetle (Anobium punctatum). The larvae especially like to feed on the starch-based adhesives that were used in binding books. These larvae feed inside the books and cause damage by tunnelling through the material. Biscuit beetles can be particularly destructive because of their high rate of reproduction. Silverfish are also found in libraries across Europe: the common silverfish (Lepisma saccharinum), the grey silverfish (Ctenolepisma longicaudatum) [19], the ghost silverfish (C. calvum) [20] and the four-lined silverfish (C. quadriseriatum) [21] are all found in Austria. Silverfish feed primarily on cellulose materials and will damage paper, bindings, wallpaper, papier-mâché, and starch-based adhesives. Although many of these insects prefer high humidity environments, it is often temperature that favours their presence [2].

Fungi are another major museum pest affected by indoor climate [22,23]. The growth of fungi is essentially determined by the availability of water. The relative humidity at which fungal growth can occur depends on the temperature due to the relationship between absolute and relative humidity. A representation of limit-values for spore-germination and fungus-growth finds itself with Krus et al. [24]. From the isopleth system, it emerges that fungus-growth is barely possible at a temperature of 10 °C, even with 80% relative humidity, while with 20 °C and a humidity of just over 70%, already numerous—though not xerophilic—fungi can grow. For museums, therefore, a threshold value of 55% is assumed as the generic threshold value for fungi, at ~10 °C. Although relative humidity may decrease under warmer conditions [25,26], mould risk was predicted to increase in historic buildings in Southern England, because of increased relative humidity during future winters, which may also be warmer [27]. Hitherto, mould outbreaks in central European museums are often dominated by a very limited number of xerotolerant and xerophilic species within the genera Aspergillus/Eurotium and Penicillium [22], able to live at low water activity. However, with increasing temperatures, especially in climates where additionally relative humidity (RH) were to increase, it is likely that not only would fungi grow faster, but exhibit greater diversity.

A changing climate is likely to affect cultural heritage [23,28], but few studies look at pests such as insects. According to a summary in Brimblecombe and Lankester [3], changes in of few degrees of temperature have the potential to: (i) increase activity, (ii) increase the number of eggs and (iii) increase the number of reproduction cycles. Climate change is also expected to foster the invasion by new species [29]. Changes in outdoor climate, such as higher temperatures or an increase in the frequency of heavy rain events that might result in, even if short term, higher moisture content in parts of the building envelope. These might lead to novel indoor climates in future [23,30], which are likely to alter the risk from pests [4]. There has long been an interest in the climates of historic interiors [31], so there have been frequent measurements in museums and historic rooms, some at a high spatial resolution [32]. These reveal significant gradients in temperature and RH, though often there are just brief observations at such detail. Museums are likely to have more dynamic environments than historic libraries where the visitor flow can be small. There are a number of studies of the interior climate of historic libraries [33,34], and even a study of Viennese libraries, but it is largely from the perspective of the users [35]. Many of these institutions have to cope with both the protection of books and other items in their collections in addition to the comfort of their patrons, even though visitor flow may be less than in major museums. The importance of protecting books, given the vulnerability of paper to damage [16,17,18], has been recognised especially because monastic buildings can have highly variable climates, with RH being a notable risk [36].

Most of the past studies on climate and library pests have not specifically focussed on the impact of indoor biodiversity or damage by insects or fungi. Microclimate analysis in museums is often concerned with showcases and exhibition spaces, but for the living organisms the microclimates of the floor, in cracks and behind objects and furniture is likely to be more relevant, as places more frequented. Our research aims at gaining a better understanding of how climate affects insects and fungi, and is part of a larger research project, funded by the Austrian Academy of Sciences (Heritage_2020-043_Modeling-Museum [4]). The current study examines the record of climate, insects and fungi in the Klosterneuburg monastic library (Figure 1) to the north of Vienna, Austria. The library has been chosen for this study as it is not heavily used by the public, so it offers the opportunity to investigate a stable environment likely to be suitable for the storage of books. Nevertheless, the collection in Klosterneuburg experienced some biodeterioration problems 6–8 years ago, with mould evident on some books, and apparent insect infestations. The main room (Kuppelsaal) represents a single large space that has the potential to provide a homogeneous environment.

2. Method

2.1. Library

The library of Klosterneuburg Abbey has been preserved in continuity since the monastery was founded by regular Augustinian canons in 1133 [37]. It holds an extensive collection of mediaeval material: 1256 manuscripts, 836 incunabula, and about 1400 fragments, making it one of the largest mediaeval monastic libraries of the western world preserved in situ [38]. The book stock expanded to 300,000 volumes during the early modern period and grows by 2000 new books per year [39]. The library was configured in 1837 on the 2nd floor of the then newly erected parts of the Abbey, by architect Joseph Kornhäusel [40]. It adopted a dome in the centre of a baroque staircase, built earlier (1740), but in an unfinished state in the 1830s. Repurposing required the insertion of a steel girder ceiling into the initially open construction (Figure 1b) to create the main hall of the library, the circular Kuppelsaal.

In the 1880s, seven additional rooms on the second floor were adapted to the needs of the library. In 1943, the 1st floor was expanded for the books, and finally, in 2006, a mobile shelving system was installed in the cellar. The library’s four depository areas have different climatic conditions due to their diverse locations and architectural origins. Thus, this paper focuses on the oldest area of the library, the Kuppelsaal.

The hall has two large north–south facing exterior windows, and an openning to the staircase, which in combination provides optimal ventilation. The central rotunda of the Kuppelsaal, and the niches of the hall are equipped with shelves made of spruce wood in the classicist Empire style (Figure 1c), which provide space for about 50,000 volumes. The room has no heating or air conditioning, so it is vulnerable to seasonal changes in temperature and humidity. The special situation above the thin steel girder ceiling over the main room, means temperatures there can drop to ~0 °C in winter.

An active biscuit beetle infestation and fungal activity were discovered on 2014-06-26 during a visit to the library (by PQ). This initiated regular insect pest monitoring, which led to further investigations of fungal growth, cleaning and treatment. This paper draws upon earlier work on the results at different stages of infestation in addition to more recent observations of pests. Insects within the library have been regularly trapped in the library several times a year since 2014, with fungi sampled in 2017 and then twice in the period 2021-07/2022-07, in line with the period of indoor climate monitoring.

2.2. Climate Monitoring

As part of the project Heritage_2020-043_Modeling-Museum, 14 monitors for temperature and relative humidity were placed out along with insect traps in the summer of 2021. The project chose MostraLog data loggers (Long Life for Art, Hauptstrasse 47, 79356 Eichstetten, Germany) to record indoor climate at 15 min intervals storing 35,000 manually downloaded thermohygrometic measurements from each site over the sampling period (2021-07/2022-07). The loggers have a temperature accuracy of ±0.5 °C (5 °C–45 °C) with a resolution of 0.1 °C and can determine relative humidity with an accuracy of ±2% RH (10–90% RH) at 0.1% resolution. Fourteen loggers were set out in sites within the library chosen to represent the range of indoor climates (Figure 2;

Table 1. Location (initial integer corresponds to position in Figure 2) of loggers in the Kuppelsaal of Klosterneuburg Library. Note: logger numbers designated “e” use an additional external sensor. SK012 and SK013 were placed in the library, but outside the Kuppelsaal.

Location	Site	Height/cm	Logger Number
1: shelf 5	in cupboard	107	SK001
2: shelf 5	shelf	228	SK002
3: shelf 76	in cupboard	104	SK003
4: shelf 76	shelf	184	SK004
5: shelf 51	top of shelf	490	SK005
5: shelf 51	behind shelf	390	SK005e
6: shelf 40	top of shelf	490	SK006
6: shelf 40	behind shelf	390	SK006e
7: shelf 30	top of shelf	490	SK007
7: shelf 30	behind shelf	390	SK007e
8: shelf 3	top of shelf	411	SK008
8: shelf 3	behind shelf	310	SK008e
9: shelf 6	top of shelf	411	SK009
9: shelf 6	behind shelf	310	SK009e
10: shelf 3	shelf	201	SK010
11: shelf 51	shelf	184	SK011
14 shelf 35	floor	0	SK014
15: shelf 3	floor	0	SK015
16: shelf 55	floor	0	SK016

). The circular Kuppelsaal under the cupola serves as a simple space that represents the key area for this study. Conversion of RH to absolute humidity (AH) used polynomial approximation [41]. Indoor climate was predicted for earlier years using a simple transfer function, which related indoor temperature to outdoor climate through simple correlations between indoor and outdoor conditions [27,30].

The sites were chosen to reflect the microclimates encountered by insects and fungi rather than to represent the air within the room, so were placed on shelves, behind the bookcases or on the floor as shown in Table 1. The table also lists the height of the loggers from 0 cm (on the ground next to an insect trap) to over ~4 m on some of the highest surfaces in the room.

Ambient meteorological observations (outdoor temperature and RH at 1 m height) were obtained from the Zentralanstalt für Meteorologie und Geodynamik (ZAMG) at their data hub [42] (https://data.hub.zamg.ac.at, accessed on 10 August 2022), where we retrieved hourly observations, from 2014-01-01/2022-07-31, for the station at Wien Hohe Warte, some 7 km from Klosterneuburg Library. Note: dates in this paper follow ISO 8601 (https://www.iso.org/iso-8601-date-and-time-format.html, accessed on 22 November 2022).

2.3. Insect Monitoring with Traps

When the activity of biscuit beetles was discovered by chance in 2014, an insect pest monitoring programme was started to confirm active infestations of this dangerous library pest [11,14]. Sticky blunder traps (Catchmaster) were placed along the walls in the outer ring, the central part of the historic library (and adjacent rooms, data not presented here) and below and on the window sills. Additionally, a small number of pheromone traps for the webbing clothes moth (Finicon) was also added to monitor the activity of this common museum pest. The traps are replaced twice per year (spring and autumn) and examined additionally about 3 times during the warm months of the year. The collected insect pests are counted and identified to species level, while accidental visitors and other arthropods are only identified to group level, and also categorised as (a) insects from outside (accidental visitors), (b) predators or (c) humidity indicators. Figure 3a shows the location of the insect traps in the library.

2.4. Fungal Monitoring with Cultivation Samples and Metagenomics

In 2014, when the biscuit beetle infestation was discovered, fungal activity was also found. The need for treatment, cleaning and preventive measures, utilised an external evaluation and identification by one of the authors (KS) in 2017. This resulted in the first identification of fungal species present on the books and in this room of the historic library. A further spore count and analysis of the species was conducted from 11 surface samples (Hycon^® and YM, media pressed directly onto surfaces, incubation for 5–7 days at room temperature) and 17 air samples (MBV, device MAS-100 Eco^®, 100 l, incubation for 5–7 days at room temperature, on two different cultivation media—Malt Extract Agar (MEA) and Dichloran-Glycerol Agar (DG18), collected on 2017-11-08.

In early 2022, in parallel to the intensive indoor climate monitoring (2021-07/2022-07), a new investigation of the presence, diversity and activity of filamentous fungi began. As these two latest sampling campaigns (2022-01 and 2022-08) were not conducted to specifically analyse an active infestation, but rather as a broader, whole-room monitoring approach, a slightly modified set of methods was chosen. Three sampling methods were applied to detect and identify the genera/species present: (i) air sampling (MAS-100 Eco^®, 100 l), (ii) surface sampling using contact plates and (iii) further dust sampling using cotton swab samples for a cultivation-independent approach. As in the initial 2017 survey, MEA and DG18 were used for the cultivation plates. Where possible, sampling points were chosen in accordance with the positions of climate sensors and insect traps (Figure 3). Reference samples of the outdoor air were taken on each sampling day, using the same parameters as for the indoor air. These are important for later interpretation of results, to obtain additional information from the degree of similarity between the microbial community inside and outside environment. The viable and cultivable fungal species were assessed based on classical microbiological methods, so after incubation at room temperature for a minimum of 7 days, fungi present in the indoor air and on surfaces were analysed quantitatively and qualitatively, at least up to genus-level (identification literature used for samplings 2017 and 2022: [43,44,45,46,47,48]). The identification of species was attempted only for a selected number on the basis of their predominance in the cultures.

The non-cultivable fungi present on surfaces, were assessed by extracting DNA from the swab samples (MP Biomedicals, FastDNA™ Spin Kit for Soil DNA Extraction) and a metagenomic analysis conducted with the help of a third generation sequencing technique, using the Nanopore sequencing platform (Oxford Nanopore Technologies). This has previously been employed for cultural heritage collections [49]. We chose to apply the technology within this project, mainly because of its low cost and time-efficiency. We used the Nanopore PCR Barcoding Kit SQK-PBK004 Protocol with slight modifications for our experimental approach, concerning primer selection and sequence length of our target regions. Basecalling took place simultaneously during sequencing with the Device MinION Mk1C. For data analysis, the Fastq files were then uploaded into one of the pipelines available on the Nanopore community platform. The taxonomic classification was done through the cloud-based platform EPI2ME and the selected workflow “What is in my pot” (WIMP). Graphic output of the overall results was generated directly by the analysis pipeline. The broad identification of main fungal genera present (cultivable and non-cultivable), focussed on a well-researched and universal DNA barcoding region for fungi, the Internal Transcribed Spacer (ITS) regions 1&2 (PCR primers used: ITS1 (forward) 5′-TCCGTAGGTGAACCTGCGG, and ITS4 (reverse), 5′-TCCTCCGCTTATTGATATGC) [50,51].

2.5. Statistical Analysis

Insects (individuals) and fungal counts (as colony-forming units, CFU) are necessarily represented as integers. Frequently insect traps are empty and there are small numbers of fungal spore counts, so care over the nonparametric nature of observations becomes important in statistical analysis, so the Mann–Whitney test for difference between two sets of data (http://www.vassarstats.net/ accessed on 13 December 2022) and the Kendall rank correlation coefficient (τ) were adopted where the number of samples was small (https://wessa.net/rwasp_kendall.wasp accessed on 13 December 2022), though with higher sample sizes, the more conventional t-test and Pearson regression coefficients were used.

3. Results

3.1. Library Climate

Daily means for temperature and relative humidity for the 16 sites in the Library (some beyond the Kuppelsaal) for the year 2021-07/2022-07 are shown in Figure 4. At sites in the library the summer temperatures reach 25.3 °C and in winter these fall to 6.7 °C, while RH ranges from 64% to 48%. The patterns are almost identical and a Pearson correlation matrix for the daily temperatures at the sites shows all correlation coefficients exceed 0.99. The profile of relative humidity indicates humid summers in the library, and although generally well correlated among the rooms it has more noise.

The great similarity between the temperatures measured at various sites in the library makes them hard to compare. However, this was done by taking the differences between the daily mean temperatures at various places compared with that from shelves nearby (Figure 5a). In warmer months the cupboard and the spaces behind shelves conditions were cooler, but in winter the temperatures were higher. Nevertheless, it should be noted that these differences are less than a degree. Although the differences were smaller, high shelves were slightly warmer (~0.3 °C) than low shelves in the summer, but almost the same temperature in winter. Absolute humidity appears to be higher in the cupboard (SK001) during early part of the record (Figure 5b) and reflects the higher values of RH (>65%) that can occasionally prevail for long periods (up to 30 days) in cupboards.

The daily temperature as a function of RH for a year in the library cupboard (SK001) is shown in Figure 6a. It typifies the measurements at sites throughout the library indicating a warm and modestly humid summer followed by a drier autumn and winter, with the winter cold. The temperature at Wien Hohe Warte is compared to the library interior in Figure 6b. The temperatures of the summer interior are not very different from those outside, though winter temperatures are warmer indoors. The temperatures and RH at Wien Hohe Warte over the years 2014/2022 are shown in Figure 6c. The estimated indoor temperature in the library over the period 2014/2022 as estimated from a simple transfer model is shown in Figure 6d (the estimates of RH trends for the library were less reliable, so are not shown). The figure suggests that there was no dramatic change in the interior temperatures over almost a decade, so seems unlikely to have affected the occurrence of library pests. Additionally hot and cold weather outdoors did not seem to have a strong influence on the indoor climate, as the interior appears well buffered. The median daily temperature from all the sites in the library was 15.6 °C and the lowest percentile (1%) was 6.6 °C and the uppermost 25.3 °C (99%) and the median daily RH was 54.2% and the lowest percentile was 46.1% and the uppermost 66.2%. These reflect seasonal change, but suggest an equable microclimate.

3.2. Insects Trapped

Figure 7 shows insects caught in the traps from 2014 to the first half of 2022. The data (see Supplementary Table S3) from all traps for each year were pooled, to show the total number of catches for the different pest species. There were large infestations of the biscuit beetle (Stegobium paniceum as discussed later in this paper), but their numbers were so large these are omitted from this figure. Anthrenus sp. adults (A. caucasius cf and A. verbasci) and larvae of the carpet beetle have the next highest numbers in all years, followed by different groups such as the webbing clothes moth (Tineola bisselliella), insects coming from the outside (labelled Ext and including flies, lacewings, mosquitoes, wasps, weevils, ants and beetles), spiders (predatory probably feeding on the insects of the floor), but none of these represent a special threat to books. The number of humidity indicators (e.g., silverfish, woodlice or booklice) were low across the nine-year sampling period, suggesting a relatively dry climate, aligning with the average of 55.3 ± 4.52% found from our 2021/2022 measurements. However, these species can take advantage of more amenable microclimates on the floor or in cracks. The white-marked spider beetle, mostly as Ptinus fur, and the book louse (Psocoptera) were found only in small numbers, along with small numbers of Plodia interpuctella in 2016 and 2017. Wood boring beetles, such as the furniture beetle Anobium punctatum were entirely absent.

Classic library pests found at Klosterneuburg included infestations of S. paniceum and addition to smaller numbers of four species of silverfish (Zygentoma): the common silverfish L. saccharinum, the grey silverfish C. longicaudatum, the four-lined silverfish C. lineatum and the ghost silverfish C. calvum. Only the common silverfish is abundant, and the other species seem to be introduced with materials from time to time, but do not appear to have a stable or growing population. Silverfish are a threat as they will damage paper, bindings, wallpaper, papier-mâché and starch-based adhesives. Other museum pests (Anthrenus sp., Attagenus sp., T. bisselliella) feed on keratin for dead insects or material within dust, so do not damage books and are common to most buildings. They might come from surrounding rooms of the monastery (especially T. bisselliella, which is attracted by the pheromone traps) or live in the dead spaces behind the shelves or cracks in the floor.

3.3. Insect Distribution

A number of insects have been trapped in sufficient numbers to track their distribution. The cumulative numbers of S. paniceum (biscuit beetle), Anthrenus spp. (carpet beetles, as A. verbasci, A. caucasius cf and A. ssp. larvae) and silverfish L. saccharinum were caught in various parts of the library through to 2022, as shown in Figure 8a. We see that S. paniceum [52] is found throughout the library, although trapped predominantly on the window sills. Here, the daylight coming through the window attracts large numbers of insects, which are found both sills and in traps on the floor directly below. Silverfish are mostly found in the western corner of the room, close to a door that leads to another part of the library. Additional monitoring data (not presented here) suggests no elevated level of pests beyond the Kuppelsaal.

In 2014-06, almost 30 individuals of S. paniceum were found among traps X-2, X-4, X-5, X-8, X-9, X-11 and Ph1, all in the outer parts of the library. By 2014-08 more than 70 were found, which suggested there was an active infestation of this harmful insect. However, the temperature was not especially unusual at this time (Figure 6d). Biscuit beetles can be particularly destructive because of their high rate of reproduction and appetite for the starch glue in the book bindings. Hundreds of individuals can develop within a short period of time in a single book (in summer), severely damaging its binding. The insect is tolerant towards both temperature and humidity. The large number of S. paniceum in some places can easily be explained by location in the room. The beetles were frequently found on window sills or on traps directly below the sills, as this insect is strongly attracted by daylight. After the active infestation was confirmed, books with obvious beetle activity and severe damage were also found in different areas in the library.

As a treatment, Parasitoid wasps [53] were released in 2014-07 and 2014-12, and then five times in 2015, but this appeared unsuccessful as more than 500 biscuit beetles were found in 2016 (inset to Figure 8b). The library was then treated, in 2017, with the toxic gas sulfuryl difluoride as a further step to prevent more damage. No biscuit beetles were found after the treatment (except for a small number appearing to come from outside or the attic). Most insect or arthropod groups were found again after the treatment. This was not expected, as the toxic gas should have killed all arthrapods in the library.

Carpet beetles tend to be found regularly though, the distribution of the Anthrenus larvae and beetles did now follow a clear pattern as found with S. paniceum. The adult Anthrenus are also attracted by the light of the windows, but a large proportion of the Anthrenus caught are larvae and they are active on the floor, cracks and dead spaces, as they search for dead insects as a food source. This also means that insect traps with accumulated dead insects are attractive to this species.

Silverfish were only abundant in one area at the western extremity of the library. There is a weak correlation between the total numbers of carpet beetles and silverfish at the various trapping sites in the library (Kendall τ = 0.4, p₂~0.15). The trapping location of the silverfish might be explained by a different microclimate, as this is the southern side of the building, which is get most sun and light during the day. It might result in a slightly higher air temperature (up to 0.6 °C) along this section of the outer wall, although this warmer temperature means it is slightly drier (Figure 8c,d), an RH is low anyway. However, it is possible that insects find damper conditions at the stone floor or underneath the wooden shelves.

3.4. Fungal Observations

Results of the sampling from 2017 show nine main species on the books on shelves and closed cupboards with Eurotium cf. halophilicum as the most abundant species. Detailed results and a list of all species are presented in the supplementary material (Table S1). We did not find an active fungal infestation in the open shelves or books. However, in the closed cupboards, a number of books were infested by active fungi growing on the bindings. This occurred mainly on linen bindings, where the organic materials (starch, animal glue, etc.) were especially suitable for fungal growth. No species of fungi requiring high humidity levels were detected. The dominant fungal species inside the closed cupboard are those adapted to a lower humidity (Eurotium cf. halophilicum). While the library is generally dry, the humidity level can rise above 60%. Comparing shelves (SK002 and SK004) with nearby cupboard interiors (SK001 and SK003), suggests that both shelves and cupboards are at a RH > 60% for about a quarter of the ten-minute measurement periods across the survey window 2021-07/2022-07. However, the cupboards were above RH 65% for longer (SK001 11.5% and SK003 16.3% of the time) and at times excursions above 65% were for as much as 30 consecutive days. The shelves exceed 65% infrequently (SK002 0.03% and SK004 2.3% of the time). Differences among more than 35,000 RH measurements from the cupboards and shelves were significant (p₂ < 0.0001) using a paired t-test.

Three fungal species of Aspergillus and three belonging to the genus Penicillium, among a total of 14 different species, were found in air samples collected in 2017. Detailed results and a list of all species are presented in the supplement material (Table S2). Two species were dominant in these samples: Penicillium commune and P. chrysogenum.

The results from the latest sampling campaigns 2022-01 and 2022-08 showed a slightly different picture, as presented in Figure 9 (especially in Figure 9b,d). Four main genera were identified on the plates: Aspergillus, Penicillium, Cladosporium, in lesser numbers also Alternaria and some species from the division of the Mucoromycota. Cultivation samples from the air suggested a significant increase (Mann–Whitney p₂~0.0003) in the number of viable cells in air (CFU m⁻³) in August compared to January (Figure 9b). As winter temperatures inside the room are relatively low, it is not surprising to see higher activity of organisms in summer. Unexpected, however, was a predominance of Aspergillus fumigatus in all summer air samples, followed by Cladosporium and Penicillium species, relatively few Alternaria colonies and small numbers of other species, compared to the January samples (Figure 9a–c). This composition in the cultivated samples was similar at all sites in the room (Figure 3b). It indicated a clear deviation from the outdoor reference samples collected on the same day, while in winter, there was no detectable dominance of any single species or genus on the plates, and the composition of species inside (Figure 9c) was similar to that of the outside air, with mostly Cladosporium, then Penicillium, Aspergillus, though few other species.

Table 2. Summary of species identified from air samples in 2017 and those from the sampling campaign in 2022. Dominant species are marked in bold.

Species of Fungi Identified in Air Samples
2017	2022
Alternaria sp. Aspergillus candidus Aspergillus ochraceus Aspergillus versicolor Cladosporium spp. Epicoccum sp. Exophiala sp. Fusarium sp. Oidodendron sp. Penicillium brevicompactum Penicillium commune Penicillium chrysogenum Paecilomyces sp. Wallemia sp.	Alternaria spp. Aspergillus fumigatus Aspergillus sect. nigri spp. Aspergillus spp. Aspergillus versicolor Cladosporium spp. Eurotium spp. Fusarium sp. Mucor sp. Paecilomyces sp. Penicillium chrysogenum Penicillium cf. rubrum Penicillium spp. Rhizopus sp. Syncephalastrum sp. Walllemia sp.

gives an overview of the species identified in the air samples of 2022, in comparison with those of the 2017 survey. An explanation for the high numbers of A. fumigatus spores in the August samples has not been found, as there is currently no active infestation visible in the library, but the situation will be monitored further. This fungus grows well at 37 °C and, as it is a potential human pathogen, the high numbers of spores may pose a health risk for personnel and visitors, especially immuno-compromised persons. The situation needs to be investigated more closely along with potential safety measures, such as restricted access to the room and wearing fine particle-filtering respirators (FFP3 masks), until the source has been found and treated.

In contrast with the air samples, the contact plates did not show such clear differences between the two seasons. However, the total CFU-numbers were larger and more variable among sampling points in winter (Figure 9d), although the difference between the medians is not significant (Mann–Whitney p₂~0.2) among the small number of samples (winter 7; summer 10).

Metagenomic data gave us a first impression of the diversity of fungal material present in the library dust, as summarised in Figure 10. A major part of identified fungal taxa belongs to the order Eurotiales, containing some of the main genera also found in our cultivation assays from 2017 and 2022 (i.e., Aspergillus/Eurotium, Penicillium, Paecilomyces, etc.), which include the xerophilic and xerotolerant species relevant for the indoor environment and historic collections in particular [15,22,54].

It must be noted that these are results using the Oxford Nanopore Technologies-provided basic bioinformatic pipeline (WIMP). It is suitable for analysis of complex environmental samples, but the resolution of the fungal taxonomy is not very accurate. In-depth analysis, requires further development of precise bioinformatic pipelines using more representative, curated reference databases for our target organisms.

4. Discussion

The indoor climates of historic libraries are often regarded as humid and damp, characteristic of old buildings without climate control. However, the library of Klosterneuburg has a relatively stable and dry climate, although the temperature changes with the seasons. The situation is likely to be the result of: (i) low visitor numbers, (ii) the height of the room (~10 m above the ground of the historic monastery), with an air space (rooms and hallways) underneath, (iii) the southern exposure and (iv) only two small and two large double glazed windows. Although outdoor climate influences the climate indoors, with slow changes throughout the year (Figure 6), conditions are probably suitable for the preservation of books, even though the temperature can be as much as 25 °C in summer. The highest level of RH is just below the growth threshold for many fungal species [24].

The insect pests found in the library are typical of Austrian museum fauna. The source of the infestation of S. paniceum remains uncertain, but it is likely that books were infested prior to 2014. S. paniceum is a common pest in Vienna and Europe more generally. Its larvae can often be found in relation to human activity in domestic kitchens, where it is associated with stored products. The insect was not introduced to Klosterneuburg with infested books, as no books are added to the collection in the Kuppelsaal; furthermore, books do not leave the library. The three-year treatment at the library with parasitoid wasps failed, perhaps because they were killed by remaining traces of biocides in the dust within the library. Alternatively, wasps may not have been able to deposit eggs next to the live larvae in the bookbindings. The leather is difficult to penetrate, so only the outer binding would be accessible. After the sulfuryl dichloride treatment in the summer of 2017 very few biscuit beetles were found in the years 2018/2022, and the small number found seem to be part of accidental catches from outside. Windows are opened from time to time for ventilation, but are kept mostly closed. Insect screens were installed in 2021 to prevent further infestation. Sulfuryl dichloride is not effective against fungi, especially the spores remaining unharmed. Cleaning activities in the library were started in 2015, and ethanol was used to remove dust and fungi from all shelves and books. Since then, the library has been in a much better state of conservation.

Despite a low RH [55], fungi were detected in the air and in dust both in 2017 (by author KS) and 2022 (by author KD). However, active manifestation of mould was found only in the interior of closed cupboards, suggesting that it is in these unventilated compartments RH and possibly water content of materials is high and protracted enough for the organisms to grow. Frequent monitoring of the fungi, combined with regular ventilation of these closed spaces is a likely way to prevent this very localised fungal activity. The metagenomic data generated in our combined monitoring approach provides further insight into the fungal community (cultivable and non-cultivable) present inside the library and forms the basis for ongoing research.

5. Conclusions

The general climate of the library seems suitable and not in need of new controls, as few days each year have humidity levels likely to foster fungal growth. It is stable and changes only slowly through the seasons. However, even with these seemingly good conditions suitable for preventive conservation, the library has suffered from infestations of both insects and fungi. This reminds us that even when the climate is good there is still potential for damage, especially in some specific locations such as the cupboards at Klosterneuburg. The insects and fungi found in the library are typical of the museum environment and tolerant of dry conditions and additionally may be able to take advantage of materials with high water content or which are hygroscopic, and additionally some library materials may have high nutritional value, e.g., traditional glues used by bookbinders, containing animal collagens or starch. Future work may need to look more carefully at the library materials most at risk and consider aspects of their water content and composition that makes them susceptible to attack by pests and the development of infestations. Our study reminds us that librarians, especially in a changing climate, should continue to be observant and cognizant of small changes that might suggest the collection is threatened even in environments that seem appropriate for preventive conservation.