Abstract
Fetida Cave is an active sulfuric acid cave influenced by seawater, showing abundant microbial communities that organize themselves under three main different morphologies: water filaments, vermiculations and moonmilk deposits. These biofilms/deposits have different cave distribution, pH, macro- and microelement and mineralogical composition, carbon and nitrogen content. In particular, water filaments and vermiculations had circumneutral and slightly acidic pH, respectively, both had abundant organic carbon and high microbial diversity. They were rich in macro- and microelements, deriving from mineral dissolution, and, in the case of water filaments, from seawater composition. Vermiculations had different color, partly associated with their mineralogy, and unusual minerals probably due to trapping capacities. Moonmilk was composed of gypsum, poor in organic matter, had an extremely low pH (0–1) and low microbial diversity. Based on 16S rRNA gene analysis, the microbial composition of the biofilms/deposits included autotrophic taxa associated with sulfur and nitrogen cycles and biomineralization processes. In particular, water filaments communities were characterized by bacterial taxa involved in sulfur oxidation and reduction in aquatic, aphotic, microaerophilic/anoxic environments (Campylobacterales, Thiotrichales, Arenicellales, Desulfobacterales, Desulforomonadales) and in chemolithotrophy in marine habitats (Oceanospirillales, Chromatiales). Their biodiversity was linked to the morphology of the water filaments and their collection site. Microbial communities within vermiculations were partly related to their color and showed high abundance of unclassified Betaproteobacteria and sulfur-oxidizing Hydrogenophilales (including Sulfuriferula), and Acidiferrobacterales (including Sulfurifustis), sulfur-reducing Desulfurellales, and ammonia-oxidizing Planctomycetes and Nitrospirae. The microbial community associated with gypsum moonmilk showed the strong dominance (>60%) of the archaeal genus Thermoplasma and lower abundance of chemolithotrophic Acidithiobacillus, metal-oxidizing Metallibacterium, Sulfobacillus, and Acidibacillus.This study describes the geomicrobiology of water filaments, vermiculations and gypsum moonmilk from Fetida Cave, providing insights into the microbial taxa that characterize each morphology and contribute to biogeochemical cycles and speleogenesis of this peculiar seawater-influenced sulfuric acid cave.
Highlights
Caves provide a unique portal into the deep subsurface habitat, which is typically characterized by relatively stable environmental conditions, absence of light and low nutrient supply
This study describes the geomicrobiology of water filaments, vermiculations and gypsum moonmilk from Fetida Cave, providing insights into the microbial taxa that characterize each morphology and contribute to biogeochemical cycles and speleogenesis of this peculiar seawater-influenced sulfuric acid cave
Each biodeposit is characterized by specific microbial taxa (Fig 14), which are selectively enriched in each of the microbial communities growing as water filaments, vermiculation or in gypsum moonmilk deposits
Summary
Caves provide a unique portal into the deep subsurface habitat, which is typically characterized by relatively stable environmental conditions, absence of light and low nutrient supply. As the influx of organic carbon by these mechanisms is generally low and sporadic, most caves are oligotrophic [5] To survive in these nutrient-poor environments, microorganisms typically organize themselves in collective structures, offering cooperation and mutualistic relationships and producing, as results of their interaction, biosignatures that can be observed within caves [5, 6]. In this context, underground environments have attracted wide attention because of the peculiar metabolic processes and microbial community structures featuring these oligotrophic ecosystems and because of the interesting mutual interactions established between microorganisms and minerals [4, 6, 7]. Several studies have demonstrated the strong influence of mineralogy and fluid composition on subsurface microbial diversity [3, 9]; in turn, the microbial activity has shown to have an impact on the mineral formations and cave speleogenesis [10,11]
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