![]() ![]() This will provide new insights for understanding biogeochemistry and the development of early Earth in addition to providing unique avenues for exploration and discovery in astrobiology. Thus, the study of dissimilatory metal reducing thermophiles provides a glimpse into some of Earth’s earliest forms of respiration. This suggests that EET evolved separately in Gram-positive thermophiles and Gram-negative mesophiles, and that EET in these bacterial types is a result of a convergent evolutionary process leading to homoplasy. ![]() On the contrary, EET in Gram-negative mesophilic bacteria is a relatively new phenomenon that is evolutionarily distinct from Gram-positive bacteria. Genetic evidence suggests that Gram-positive thermophilic bacteria capable of extracellular electron transfer (EET) are positioned close to the root of the Bacteria kingdom on the tree of life. Instead, these microorganisms performed respiration via dissimilatory metal reduction by transferring their electrons extracellularly to insoluble electron acceptors. These data suggest that dry sulfuric karst caves can harbor robust microbial communities under oligotrophic, endolithic, and troglophilic conditions.Īpproximately four billion years ago, the first microorganisms to thrive on earth were anaerobic chemoautotrophic thermophiles, a specific group of extremophiles that survive and operate at temperatures ∼50 – 125☌ and do not use molecular oxygen (O2) for respiration. 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. 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. ![]() 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. The relative abundance of Archaea represented 1.1 ± 0.9 % of all samples and 0.2 ± 0.04 % of samples were unassigned. 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 %. 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. 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. 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. We analyzed the microbial community of multicolored speleosol deposits found in Grand Canyon Caverns, a dry sulfuric karst cave in northwest Arizona, USA.
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