Research Report www.microbialcell.com Airborne bacteria in show caves from Southern Spain Irene Dominguez-Moñino1, Valme Jurado1, Miguel Angel Rogerio-Candelera1, Bernardo Hermosin1 and Cesareo Saiz-Jimenez1,* 1 Instituto de Recursos Naturales y Agrobiologia, IRNAS-CSIC, 41012 Sevilla, Spain. * Corresponding Author: Cesareo Saiz-Jimenez, Instituto de Recursos Naturales y Agrobiologia, IRNAS-CSIC, 41012 Sevilla, Spain; E-mail: [email protected] ABSTRACT This work presents a study on the airborne bacteria recorded in doi: xxx three Andalusian show caves, subjected to different managements. The main Received originally: 11.06.2021; differences within the caves were the absence of lighting and phototrophic In revised form: 13.07.2021, Accepted 21.07.2021, biofilms in Cueva de Ardales, the periodic maintenance and low occurrence of Published 26.07.2021. phototrophic biofilms in Gruta de las Maravillas, and the abundance of pho- totrophic biofilms in speleothems and walls in Cueva del Tesoro. These factors conditioned the diversity of bacteria in the caves and therefore there are Keywords: aerobiology, airborne large differences among the CFU m-3, determined using a suction impact col- bacteria, phototrophic biofilms, lector, equipment widely used in aerobiological studies. The study of the bac- Bacillus, Arthrobacter, Micrococcus. terial diversity, inside and outside the caves, indicates that the air is mostly populated by bacteria thriving in the subterranean environment. In addition, the diversity seems to be related with the presence of abundant phototrophic biofilms, but not with the number of visitors received by each cave. INTRODUCTION ing, or massive visits. The introduction of this ecological The region of Andalusia, Southern Spain, has an important indicator was possible due to the well-characterized air- number of show caves and shelters, most of them contain- borne fungal patterns, but unfortunately, this cannot be ing Paleolithic art and with high cultural interest. Nerja, extended to bacteria. Indeed, bacteria do not present defi- Tesoro, Piletas, Ardales and Murcielagos, are some repre- nite patterns in aerobiological studies, thus avoiding fore- sentative caves with rock art, receiving different numbers casting of bacterial outbreaks in caves. As a consequence, of visitors and adopting distinct conservation and man- aerobiological studies should be adopted for each cave agement protocols [1]. Other caves are visited by its geo- individually in order to measure airborne bacteria and logical interest and the occurrence of characteristic speleo- adopt preventive conservation actions. The literature rec- thems, such as Gruta de las Maravillas [2]. ords several studies on bacteria in cave air [7, 10-16], but One of the management practices, not frequently reports different conclusions. adopted in cave conservation, is a periodic aerobiological The three show caves studied are located in Andalusia monitoring. In fact, the major threat for a show cave is (Southern Spain) and were Cueva del Tesoro, Cueva de derived from the presence and colonization of microorgan- Ardales and Gruta de las Maravillas. Cueva del Tesoro and isms in the air, water pools, sediments and speleothems. Cueva de Ardales contain valuable Paleolithic paintings and This is particularly important when bacteria and fungi engravings, which can be observed by the visitors, while threat the paintings. A few and well-known examples can Gruta de las Maravillas, without paintings, holds rare spe- be found in Altamira [3-5] and Lascaux caves [6-8]. leothems. Comprehensive data on these caves were re- To help in the cave management of airborne fungi, Por- ported in a previous work investigating the aerobiological ca et al. [9] launched an ecological indicator that provides behavior of fungi [17]. The data previously reported in- data on the amount of fungal spores and marks the limits clude number of visitors at the sampling time, geomor- of danger for the conservation of Paleolithic paintings. phology and microclimatological data (temperature, con- These authors categorized caves into five classes, from one centration of carbon dioxide and radon) inside the caves. without fungal problems to the worst case, a cave with Briefly, in the sampling year the visits ranged from 130,314 irreversible ecological disturbance. The two last classes peoples in Gruta de las Maravillas to 4,018 in Cueva de generally corresponded to caves experiencing fungal out- Ardales. Cueva del Tesoro received 28,257 visitors. Cave breaks, phototrophic biofilms resulting from artificial light- OPEN ACCESS | www.microbialcell.com 1 Microbial Cell | in press I. Dominguez- Moñino et al. (2021) Airborne bacteria in Spanish show caves FIGURE 1: Map and sampling locations in the caves. (A) Cueva de Ardales; (B) Cueva del Tesoro; (C) Gruta de las Maravillas. OPEN ACCESS | www.microbialcell.com 2 Microbial Cell | in press I. Dominguez- Moñino et al. (2021) Airborne bacteria in Spanish show caves lengths were from about 1,500 to 2,130 m. Temperatures riod, has an enormous influence on the dispersal of out- inside the caves ranged between 14.8°C and 17.4°C [17]. door airborne bacteria and their increase in the cave air. The main differences in the caves were the absence of These stagnation and ventilation periods are common in lighting and phototrophic biofilms in Cueva de Ardales, the caves. periodic maintenance and scarce occurrence of photo- In Cueva del Tesoro 130 different bacterial species were trophic biofilms in Gruta de las Maravillas, and the abun- isolated from cave air and 34 outside, from which 17 were dance of phototrophic biofilms in the speleothems and also recorded inside. walls, in all the rooms and galleries, of Cueva del Tesoro. M. luteus was abundant in all the seasons, and Micro- Cleaning and removal of biofilms were carried out in 2015, coccus endophyticus in spring and summer, but some other after this study [18]. bacteria attained important abundances, such as Knoellia In this paper we present an aerobiological study of locipacati in spring and summer, and Streptomyces kurssa- three Andalusian show caves subjected to different man- novii in autumn and winter. M. luteus was also abundant agements. The caves cover examples from western to outdoor in spring, autumn and winter, whereas in summer eastern Andalusia and are separated by more than 300 km. Brevibacterium aurantiacum, Glutamicibacter spp. and Samplings were carried out along four seasons and the Pseudarthrobacter psychrotolerans comprised about 63% colony-forming units per cubic meter (CFU m-3) in different of the abundance (Supplementary Tables S5-S8). galleries and rooms were assessed (Figure 1). Gruta de las Maravillas exhibited a high abundance of bacteria in a few rooms. The greater CFU m-3 were found in RESULTS autumn (Sala de las Conchas and Exit Passage), although There are large differences among the CFU m-3 in rooms other rooms also displayed important CFU m-3 (Sala del and galleries, along the four seasons, in the three caves, as Volcan, Sala del Lago 2). In most rooms, the CFU m-3 was depicted in Figure 2 and Supplementary Tables S1-S12. The twice the number of bacteria outdoor. In winter, the abun- cave less frequented by visitors (Cueva de Ardales) pre- dance of airborne bacteria inside decreased with respect to sented the lowest amount of bacteria in the air, with a autumn between 7.5 and 1.5 times, but outdoor the bacte- maximum of 200 CFU m-3 counted at the end of the cave ria increased 3 times. In the stagnation period (spring and stair in spring, which was in agreement with the high quan- summer) the amount of bacteria in the air varied consider- tity of bacteria outdoor (320 CFU m-3). Summer was the ably. The lower abundances were obtained in summer; the season with a lower amount of bacteria inside and outside maximum reached 130 CFU m-3 in Sala del Lago 2, while the cave. Autumn and winter were the seasons with high outside the amount was 690 CFU m-3. Slightly higher rates and comparable airborne bacteria, both deepest in the were recorded in spring (100-250 CFU m-3), except for Sala cave and outside (180-110 CFU m-3), as corresponded to a del Lago 2, with 710 CFU m-3, although outdoor the rate ventilation period. In this period, the air outside is colder was the lowest (210 CFU m-3) of all the seasonal samplings. and denser with respect to inside and enters into the cave 94 species of bacteria were retrieved inside Gruta de las transporting airborne bacteria. Maravillas and 48 outside, but only 18 of these were also 41 different species were recorded inside this cave and isolated inside the cave. 40 outside. Only eight of the bacteria recorded outside Similar to Cueva del Tesoro, M. luteus and Micrococcus were also found inside the cave. endophyticus were also the most abundant bacteria in Gru- The most abundant bacteria identified inside Cueva de ta de las Maravillas. Other relevant bacteria, including Ardales were Pseudarthrobacter spp. and Micrococcus lu- members of ten genera (Aerococcus, Bacillus, Brevundimo- teus in spring; Bacillus altitudinis, Peribacillus simplex and nas, Staphylococcus, Kocuria, Corynebacterium, Empedo- Arthrobacter methylotrophus in summer; M. luteus in au- bacter, Microbacterium, Agrococcus, Glutamicibacter) tumn; M. luteus and Arthrobacter citreus in winter. Out- reached abundances above 20% in different rooms and door, the most abundant bacteria were Pseudarthrobacter seasons. Outside the cave a high diversity of bacteria was spp. in spring, and summer, Arthrobacter spp. in winter, recorded of which M. luteus stood out (Supplementary and M. luteus in autumn (Supplementary Tables S1-S4). Tables S9-S12). In Cueva del Tesoro the higher CFU m-3 of bacteria were obtained in the air of Sala de la Virgen and Sala del Volcan DISCUSSION (spring) and Galeria hacia Lagos and Galeria Breuil (sum- The wide difference in bacterial diversity inside and outside mer), whereas the lower one was recorded outdoor.