Geomicrobiology of biovermiculations from the Frasassi cave system, Italy

Sulfidic cave walls host abundant, rapidly-growing microbial communities that display a variety of morphologies previously described for vermiculations. Here we present molecular, microscopic, isotopic, and geochemical data describing the geomicrobiology of these biovermiculations from the Frasassi cave system, Italy. The biovermiculations are composed of densely packed prokaryotic and fungal cells in a mineral-organic matrix containing 5 to 25% organic carbon. The carbon and nitrogen isotope compositions of the biovermiculations δ¹³C = -35 to -43‰ and δ¹⁵ N = 4 to -27‰, respectively) indicate that within sulfidic zones, the organic matter originates from chemolithotrophic bacterial primary productivity. Based on 16S rRNA gene cloning (n=67), the biovermiculation community is extremely: diverse, including 48 representative phylotypes (>98% identity) from at least 15 major bacterial lineages. Important lineages include-the Betaproteobacteria (19.5% of clones), Gammaproteobacteria (18%), Acidobacteria (10.5%), Nitrospirae (7.5%), and Planctomypes (7.5%). The most abundant phylotype, comprising over 10% of the 16S rRNA gene sequences, groups in an unnamed clade within the Gammaproteobacteria. Based on phylogenetic analysis, we have identified potential sulfur- and nitrite-oxidizing bacteria, as well as both auto- and heterotrophic members of the biovermiculation community. Additionally, many of the clones are representatives of deeply branching bacterial lineages with no cultivated representatives. The geochemistry and microbial composition of the biovermiculations suggest that they play a role in acid production and carbonate dissolution, thereby contributing to cave formation.