Photosynthetic cave communities (“lampenflora”) proliferate in Carlsbad Cavern and other show caves worldwide due to artificial lighting. These biofilms mar the aesthetics and can degrade underlying cave surfaces. The National Park Service recently modernized the lighting in Carlsbad Cavern to an LED system that allows adjustment of color temperature and intensity. We hypothesized that lowering the color temperature would reduce photopigment development. We therefore assessed lampenflora responses to changes in lighting by monitoring photosynthetic communities over the course of a year. We measured photopigments using reflected light spectrophotometric observations and analyzed microbial community composition with 16S and 18S rRNA gene amplicon sequencing. Reflected light spectrophotometry revealed that photosynthetic biofilm development is affected by lighting intensity, color temperature, substrate type, and cleaning of the substrate. Gene sequencing showed that the most abundant phototrophs were Cyanobacteria and members of the algal phyla Chlorophyta and Ochrophyta. At the end of the study, visible growth of lampenflora was seen at all sites. At sites that had no established biofilm at the start of the study period, Cyanobacteria became abundant and outpaced an increase in eukaryotic algae. Microbial diversity also increased over time at these sites, suggesting a possible pattern of early colonization and succession. Bacterial community structure showed significant effects of all variables: color temperature, light intensity, substrate type, site, and previous cleaning of the substrate. These findings provide fundamental information that can inform management practices; they suggest that altering lighting conditions alone may be insufficient to prevent lampenflora growth. IMPORTANCE Artificial lighting in caves visited by tourists (“show caves”) can stimulate photosynthetic algae and cyanobacteria, called “lampenflora,” which are unsightly and damage speleothems and other cave surfaces. The most common mitigation strategy employs bleach, but altering intensities and wavelengths of light might be effective and less harsh. Carlsbad Cavern in New Mexico, a United States National Park and UNESCO World Heritage Site, has visible lampenflora, despite adjustment of LED lamps to decrease the energetic blue light. This study characterized the lampenflora communities and tested effects of color temperature, light intensity, rock or sediment texture, and time on lampenflora development. DNA amplicon sequence data show a variety of algae and cyanobacteria and also heterotrophic bacteria. This study reveals microbial dynamics during colonization of artificially lit surfaces and indicates that, while lowering color temperature may have an effect, management of lampenflora will likely require additional chemical or UV treatment.

Discoveries in the 1980s greatly expanded speleologists’ understanding of the role that hypogenic groundwater flow can play in developing caves at depth. Ascending groundwater charged with carbon dioxide and, especially, hydrogen sulfide can readily dissolve carbonate bedrock just below and above the water table. Sulfuric acid speleogenesis, in which anoxic, rising, sulfidic groundwater mixes with oxygenated cave atmosphere to form aggressive sulfuric acid (H2SO4) formed spectacular caves in Carlsbad Caverns National Park, USA. Cueva de Villa Luz in Mexico provides an aggressively active example of sulfuric acid speleogenesis processes, and the Frasassi Caves in Italy preserve the results of sulfuric acid speleogenesis in its upper levels while sulfidic groundwater currently enlarges cave passages in the lower levels. Many caves in east-central Nevada and western Utah (USA) are products of hypogenic speleogenesis and formed before the current topography fully developed. Wet climate during the late Neogene and Pleistocene brought extensive meteoric infiltration into the caves, and calcite speleothems (e.g., stalactites, stalagmites, shields) coat the walls and floors of the caves, concealing evidence of the earlier hypogenic stage. However, by studying the speleogenetic features in well-established sulfuric acid speleogenesis caves, evidence of hypogenic, probably sulfidic, speleogenesis in many Great Basin caves can be teased out. Compelling evidence of hypogenic speleogenesis in these caves include folia, mammillaries, bubble trails, cupolas, and metatyuyamunite. Sulfuric acid speleogenesis signs include hollow coralloid stalagmites, trays, gypsum crust, pseudoscallops, rills, and acid pool notches. Lehman Caves in Great Basin National Park is particularly informative because a low-permeability capstone protected about half of the cave from significant meteoric infiltration, preserving early speleogenetic features.

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