# Cleaning of Phototrophic Biofilms in a Show Cave: The Case of Tesoro Cave, Spain
## 1 Introduction  
Caves are heterogeneous karst systems composed of different compartments. As one progresses from the outside to inside the cave, it can be observed that the microbial populations colonizing the walls and ceilings are different, and separate into different zones. In consequence, there is a transition between adjacent ecological systems that involves a change in the structure and composition of the community and is limited by their spatial distribution. Phototrophic communities (cyanobacteria, algae, lichens, bryophytes and lower plants) are usually abundant in the area closest to the outside, generally illuminated by natural light, and disappear towards the deepest area, depending on the decrease in the light gradient. Cyanobacteria and algae are especially successful microorganisms in subterranean environments due to the high humidity and carbon dioxide concentration and the very low light intensity in which they thrive. Some authors call these biofilms lampenflora [1; 2; 3]. The lighting in show caves and other underground environments generates a series of problems, derived from the growth of phototrophic communities, which generally represents a seriousecological distortion and an aesthetic problem due to the presence of biofilms with green to black colorations colonizing the speleothems and walls. The problem of the appearance of green biofilms in caves with Paleolithic paintings is not new. A few cases have been documented in the scientific literature and the phototrophic communities threaten the Paleolithic paintings. Lascaux Cave in France had to be closed at the beginning of the 1960s due to the invasion of the alga _Braceacoccus minor_ as a result of the intense flow of visitors and hours of artificial light. The biofilms were removed with antibiotics and formaldehyde [4; 5]. Altamira Cave was closed in 2002 due to the occurrence of cyanobacteria and algae on the paintings [6; 7]. A third case was recently reported in El Castillo Cave, with the occurrence of biofilms in the Polychromes Panel [8]. In this cave, chlorophytes, specifically _Jenufa aeroterrestricta_, a new species described in 2015 [9], as well as _Cocomyxa_ sp. were found in the biofilms, but no cyanobacteria. In Altamira and El Castillo caves, with initial stages of phototrophic colonization, lighting was turned off. Other caves showed extensive green biofilms on speleothems, but far from the paintings, such as in Tito Bustillo Cave, Asturias, whose salactites and salagmites were abundantly colonized by phototrophic microorganisms and, in particular, the sediments showed an exuberant colonization by the calcifying cyanobacterium _Scytonema julianun_, common in caves, tombs and catacosmols [10; 11; 12]. [PERSON] et al. [13; 14] found _Jenufa_ sp. in Nerja Cave, with abundant phototrophic biofilms on the speleothems and walls. In areas with water percolation (occasional or habitual), chlorophytes (_Jenufa_ sp. and _Desmococcus endolithicus_) were commonly identified, while cyanobacteria and the red alga _Cyanidium_ sp. abounded in drier areas. The Andalusian caves, Gruta de las Maravillas, Murcielagos, Tesoro and Nerja, to mention a few caves, exhibit different grades of phototrophic biofilm development. All of them can be accessed and receive a varied number of visits, from a few thousand to hundreds of thousands of visitors annually. Among them, Tesoro and Nerja caves stand out due to the abundance of phototrophic biofilms. While Nerja Cave biofilms have been thoroughly studied [15], the composition of the biofilms from Tesoro Cave is relatively unknown. Tesoro Cave (Figure 1) is situated in Rincon de la Victoria, about 11 km from Malaga, Spain. The cave is located in a calcareous massif (Cantal Alto), formed on a thick bed of about 70 m of limestone. The cave is about 80 m above sea level. The Cantal hill is widely karstified with an abundance of small cavities and two main caves, Tesoro and Rincon de la Victoria, the first located in the central area of Cantal Alto massif, and the second more to the south.