Culture-based analysis of ‘Cave Silver’ biofilms on Rocks in the former Homestake mine in South Dakota, USA

Abstract

Tunnels in a warm, humid area of the 1478 m level of the Sanford Underground Research Facility (SURF), located in a former gold mine in South Dakota, USA, host irregular, thin whitish, iridescent biofilms, which appear superficially similar to ‘cave silver’ biofilms described from limestone and lava tube caves, despite the higher rock temperature (32°C) and differing rock surface (phyllite) present at SURF. In this study, we investigated the diversity of cultivable bacteria constituting the cave silver by using several media: CN agar, CN gellan gum and 0.1X R2A agar. The highest colony count (CFU/g of sample) was observed on 0.1X R2A medium. The bacterial strains were grouped into 39 distinct genotypes by randomly amplified polymorphic DNA (RAPD) analysis. In addition, the bacterial strains were further characterized based on their phenotypic and biochemical properties. 16S rRNA gene sequencing classified the cave silver isolates into three major bacterial phyla: Proteobacteria, Actinobacteria and Firmicutes. Isolates included some known genera; such as Taonella, Dongia, Mesorhizobium, Ralstonia, Pedomicrobium, Bauldia, Pseudolabrys, Reyrnella, Mizugakiibacter, Bradyrhizobium, Pseudomonas, Micrococcus, Sporichthya, Allokutzneria, Amycolatopsis, Pseudonocardia, and Paenibacillus. Several isolates; related to Taonella, Dongia and Variibacter; may represent undescribed genera.