Factors driving epilithic algal colonization in show caves and new insights into combating biofilm development with UV-C treatments
Introduction
Caves are probably the most particular ecosystem, as they harbor life despite their lack of sunlight and reminding us of how life on earth may have first begun. While light-driven photosynthetic production by plants is the key source of energy in outdoor ecosystems, chemical-driven microbial production can support food webs in caves (Simon, 2012). However, the discovery of a given cave and its opening to the public give irremediable rise to a combination of structural changes that greatly affect its relatively stable microclimate (Groth et al., 1999). The installation of a lighting system to make cave formations visible to visiting tourists may, over time, lead to the appearance of greenish biofilms due to the growth of prototrophs such as algae and cyanobacteria, which are considered a major source of biodeterioration on rock surfaces (Albertano, 2012, Cutler et al., 2013). These colonists, or “r-selected species,” possess invasive, fast-growing and prolific propagule production and are later joined by more varied, larger and more slowly growing species, and eventually by species that, through the exclusion of their competitors, achieve complete dominance over an ecological climax (Aleya, 1991).
The Moidons Cave (Jura, France), discovered in 1966 and open to tourists for 6 months a year since 1989, may constitute a plausible example of this phenomenon. Each year, approximately 25,000 people visit the Moidons Cave to observe a wide variety of cave formations, including stalagmites, stalactites, fistulous shapes, draperies, columns, flowstones, rimstone dams, rimstone pools and nodular shapes such as “cave popcorn”. A successive incidence of epilithic algae has occurred as a result of this tourism, conforming to a common pattern of increasing biomass and composed mainly of the algae r-strategist Chlorella minutissima (Reynolds, 1997, Aleya et al., 2011), which has colonized parts of the cave's soils, walls and speleothems. Another example of a cave disturbance has been identified by Lefèvre (1974) in the world famous Lascaux Cave (Dordogne, France), where an increased biomass of the r-colonist algae Bracteacoccus sp. has been recorded and threatened the integrity of that cave's unique and renowned 17,000-year-old prehistoric paintings. Nugari et al. (2009) have also reported algal colonization (Apatococcus lobatus, Chlorella vulgaris, Chlorococcum sp., Muriella terrestris) on mural paintings in the Crypt of Original Sin (Matera, Italy).
Serious efforts must therefore be undertaken to identify the best tools for the protection and conservation of natural and cultural heritage objects with minimal environmental impact. Two methods are used to combat algal proliferation on cave formations in show caves. Physical and mechanical methods, such as brushing or high pressure vapor, have been shown to destroy the fragile crystal structure of speleothems and cannot kill biological materials that can easily spread throughout a cave (Mulec and Kosi, 2009). Chemical methods, such as algaecides, fungicides and a wide range of biocides, are commonly used, but have been heavily criticized because they can lead to the corrosion of carbonate formations by dissolving calcite (e.g., chlorine compounds, Faimon et al., 2003) and may seep into the surrounding treated area, contaminating groundwater by infiltration through karst network. Therefore, in an attempt to preserve speleothems, numerous authors have proposed improving illumination management by minimizing lighting time, using weaker-intensity lamps or light-emitting diodes (LEDs) (Grobbelaar, 2000, Mulec and Kosi, 2009), or at least by establishing drastic illumination management when opening new tourist sites to the public. However, such management is usually insufficient for removing algal proliferation in caves. Furthermore, environmentally friendly processes must be developed to clean areas contaminated by green biofilms at sites that have been open for several decades and have a high level of biological contamination.
In this study, we investigated the use of UV-C irradiation as a possible alternative to chemicals in caves because such irradiation does not generate pollution phenomena and the mineral matrices of physical supports (calcite, limestone, etc.) remain unaltered. UV-C irradiation is harmful to living organisms due to its short wavelength, which confers highly energetic photons and germicidal properties upon these organisms. Many studies have reported multiple negative effects of UV-C irradiation on plants (Büchert et al., 2011, Chairat et al., 2013, Costa et al., 2006, Najeeb et al., 2011), microalgae and cyanobacteria (Gao et al., 2009, Moharikar et al., 2006, Ou et al., 2012, Tao et al., 2010) in terms of both viability and metabolic activity. A strong bleaching effect has also been observed on chlorophyll during UV-C irradiation (Zvezdanović et al., 2009), making it useful in eliminating the greenish, dirty appearance taken on by cave formations. UV-C irradiation is widely used to disinfect surgical tools (Menetrez et al., 2010) and eliminate hydrocarbons prior to the conventional treatment of wastewater (Shirayama et al., 2001). However, as far as we know, no similar attempt has been made to investigate the effects of UV-C on cave-dwelling epilithic algae that form biofilms. To the best of our knowledge, the only available data on the topic is our previously published work (Borderie et al., 2011), in which we investigated the effects of different short UV-C doses, in laboratory conditions, on both the photosynthetic activity and viability of a mixture of algae strains isolated from three caves located in southwestern France.
In this paper, we first describe our in situ investigation in Moidons Cave that sought to identify and understand the factors driving epilithic algal colonization. Next, we examine the use of colorimetric measurements to assess the intensity of colonization on two selected green biofilms. Finally, we describe the procedure in which colorimetric monitoring combined with UV-C treatment was applied to four different biofilms to assess the efficiency of UV-C irradiation as a substitute for chemicals.
Section snippets
Description and characterization of the cave
Located in the Moidons National Forest (department of Jura, municipality of Molain, Fig. 1) on the Lons-le-Saunier plateau, 10 km from the town of Arbois, Moidons Cave was discovered in 1966 by two speleologists named Pierre Murat and Fred Meyer. The geographic coordinates of the original entrance are 05° 48′ 21.5″ E and 46° 50′ 20.7″ N (at an altitude of 475 m). Moidons National Forest is comprised of a large circular field of karren found on a bed of Jurassic limestone (Bajocian and Bathonian
Physico-chemical parameters
According to our measurements and those obtained from the CDSJ, the temperatures in Moidons Cave were highly stable over the course of a year (mean: 9.7 ± 1 °C). The air humidity rate was close to 90% in all parts of the cave. Weak air currents flowing in a south–north direction were measured from the original entrance (near LrB in the Main Chamber) to the North Gallery (0.1 m s− 1). No air current was detected in the South Gallery (0 m s− 1). The CO2 rates recorded by the CDSJ from July 2009 to July
Discussion
To date, no other research has been reported on colorimetric measurements in conjunction with the colonization of epilithic algae biofilms in cave environments. Our study is also the first published report on the effects of UV-C on multiple biofilms found inside a cave.
Conclusion
New avenues for understanding the processes of epilithic algal biofilm colonization in cave environments were obtained in this study by combining UV-C irradiation with colorimetric assessment. Our research provided interesting and encouraging results concerning the efficiency of UV-C treatment at combating algal proliferation. The analysis of hypothetical factors influencing algal growth, such as physico-chemical variables, functional cave dynamics and tourist activity, constituted a suitable
Acknowledgments
We would like to thank the owners of Moidons Cave, Ms Isabelle and Mr François Gauthier, who kindly gave us permission to access the cave and conduct all our field experiments. We also thank S. Colin (Comité Départemental de Spéléologie du Jura) for his CO2 data recordings in Moidons Cave and N. Crini, C. Amiot and C. Druart for their water analysis. Finally, financial support was kindly and thankfully provided by the Ministère de la Culture et de la Communication (50%), the Centre National de
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