Microbiome of Grand Canyon Caverns, a dry sulfuric karst cave in Arizona, supports diverse extremo-philic bacterial and archaeal communities

We analyzed the microbial community of multicolored speleosol deposits found in Grand Canyon Caverns, a dry sulfuric karst cave in northwest Arizona, USA. Underground cave and karst systems harbor a great range of microbial diversity; however, the inhabitants of dry sulfuric karst caves, including extremophiles, remain poorly understood. Understanding the microbial communities inhabiting cave and karst systems is essential to provide information on the multidirectional feedback between biology and geology, to elucidate the role of microbial biogeochemical processes on cave formation, and potentially aid in the development of biotechnology and pharmaceuticals. Based on the V4 region of the 16S rRNA gene, the microbial community was determined to consist of 2207 operational taxonomic units (OTUs) using species-level annotations, representing 55 phyla. The five most abundant Bacteria were Actinobacteria 51.3  35.4 %, Proteobacteria 12.6  9.5 %, Firmicutes 9.8  7.3 %, Bacteroidetes 8.3  5.9 %, and Cyanobacteria 7.1  7.3 %. The relative abundance of Archaea represented 1.1  0.9 % of all samples and 0.2  0.04 % of samples were unassigned. Elemental analysis found that the composition of the rock varied by sample and that calcium (6200  3494 ppm), iron (1141 ± 1066 ppm), magnesium (25  17 ppm), and phosphorous (37  33 ppm) were the most prevalent elements detected across all samples. Furthermore, carbon, hydrogen, and nitrogen were found to compose 4.7  4.9 %, 0.3  0.4 %, and 0.1  0.1 % of samples, respectively. Finally, Raman spectra compared to the RRUFF Project database using CrystalSleuth found that the mineral composition of the speleosol consisted of calcite, hematite, paraspurrite, quartz, and trattnerite. These data suggest that dry sulfuric karst caves can harbor robust microbial communities under oligotrophic, endolithic, and troglophilic conditions.