MICROBIAL COMMUNITIES AND SECONDARY MINERAL FEATURES IN AN ARTIFICIAL SULFIDIC CAVE STREAM

The Grotta Grande del Vento in the Frasassi Gorge (Marche region of central Italy; www.frasassi.com) contains one of the most popular show caves in Italy. The lower levels of the interconnected Frasassi cave complex are actively forming by sulfuric acid speleogenesis, where hydrogen sulfide-rich groundwaters are exposed to oxygen at or near the cave water table. These “sulfidic” passages, which are accessible by technical caving routes, contain isolated chemosynthetic ecosystems supported by sulfide-oxidizing microorganisms that speed up sulfide oxidation and acid production and create diverse biogenic secondary minerals above and below the water table. The show cave is in the upper levels, where each year, some 350,000 visitors tour impressively large cave passages that were formed during an ancient sulfidic phase. In 2012, scientists and staff created a new exhibit to display the unique fauna that inhabit the deep cave system and are otherwise inaccessible to tourists. The exhibit included an artificial spring with flowing sulfidic water. Over the subsequent years, microbial communities and mineral deposits formed in and above the artificial stream that resemble those found in the lower cave levels. In 2019, we sampled several of these features, including white microbial mats and sediments from the stream, subaerial “snottite” biofilms, secondary gypsum and elemental sulfur deposits, biovermiculations, and other microbial features. Small subunit rRNA gene libraries showed that white biofilms contained sulfur-cycling taxa such as Beggiatoa, Sulfurovum, and Desulfocapsa, and snottites and other acidic wall deposits were dominated by Acidithiobacillus spp. and acidophilic archaea. The same diversity trends were observed as in the natural cave communities, with the least diverse communities found in the acidic wall deposits and the most diverse communities in stream sediments and biovermiculations. We isolated strains of Acidithiobacillus from small snottite biofilms that formed on overhanging wall surfaces above the flowing stream, and are currently doing detailed genomic and phylogenetic analysis of two of these strains to evaluate whether they originated from the cave ecosystem or if they have a separate origin. This study offers a unique example of the capacity for an artificial ecosystem to mimic natural and hard to access sulfidic cave habitats.