Substrate type and light intensity determine lampenflora concentration on paleontological remains in show caves

Abstract

Artificial lighting in show caves is responsible for the growth of nuisance photosynthetic organisms, the so-called lampenflora, causing aesthetic, chemical and physical damage to cave cultural heritage, including paleontological resources in situ. This study focuses on the role of substrate in determining the concentration of lampenflora on paleontological findings in show caves, using the bone deposit "Cimitero degli Orsi" in the Toirano show cave (NW-Italy) as a testing ground. Specifically, we investigated whether the concentration of three distinct photosynthetic microorganisms-cyanobacteria, diatoms, and green algae-varies on different substrates, i.e., bones, rock, and soil, also keeping into account the role of light intensity. Our findings revealed that, among the tested organisms, diatoms exhibited higher concentration on bones compared to other substrates and it was even higher at increasing light intensity. On the other hand, cyanobacteria increased their concentration at increasing light without a clear preference for a specific substrate, while the presence of green algae was higher on rock and soil substrates rather than bones. When modelling the concentration of photosynthetic microorganisms within the bone deposit under different scenarios of light intensity reduction, we predicted a general decrease of all groups, that was stronger in cyanobacteria and green algae and weaker in diatoms on bone substrates. These results provide valuable insights on the colonization of nuisance photosynthetic microorganisms on bone substrates exposed to artificial lighting, with management implications for the conservation of paleontological findings in show caves.

Keywords: Bone deposit; Cave heritage; Photosynthetic microorganisms; Sustainable tourism; Tourist caves.

Figure 1. Overview of the bone deposit ‘Cimitero degli Orsi’ (A–C), details of paleontological remains (D–E), and lampenflora measurements on fossil remains (F).

Red arrows indicate areas of aesthetic damage caused by photosynthetic microorganisms.

Figure 2. Greyscale orthophoto of the ‘Cimitero degli Orsi’ with representation of sampling points and of bone deposits (yellow areas).

Figure 3. (A) Boxplots representing the concentration of the three examined photosynthetic groups on the three different substrates obtained with the full dataset; (B–D) regression lines with confidence intervals representing the relation between the log-transformed light intensity and the concentration of cyanobacteria (B) and diatoms (C) and the probability of presence of green algae (D) obtained with the full dataset.

Figure 4. (A) Boxplots representing concentration of the three examined photosynthetic groups on the three different substrates obtained with the restricted dataset; (B–D) regression lines with confidence intervals representing the relation between the log-transformed light intensity and the concentration of cyanobacteria (B) and diatoms (C) and the probability of presence of green algae (D) obtained with the restricted dataset.

Figure 5. Density of cyanobacteria (A, D, G), diatoms (B, E, H) and green algae (C, F, I) on the ‘Cimitero degli Orsi’ expressed as chl-a concentration (µg chl-a/cm²) under different scenarios of light intensity: (A–C) present light intensity scenario; (D–F) 30% light intensity reduction; (G–I) 50% light intensity reduction.

Scenarios of light intensity were obtained by reducing the values of light intensity of 30% and 50% in the light raster, while predictions of the three groups were obtained by applying the coefficients of the statistical models to the three rasters of light intensity.