Potential Biosignatures in Goethite Deposits in Mavros Vrachos Quarry Hypogene Cave

We report the first detailed geological description of the hypogene Mavros Vrachos Quarry cave close to Sidirokastro in N. Greece. This study includes experimental results obtained using an arsenal of chemical, biological, and scattering techniques on a crust deposit that covers the cave's interior. The major element involved in this study is iron. The crust deposit consists mainly (about 90 wt%) of goethite crystals. The typical crystallite size of goethite is between 15 and 21 nm. The goethite crystallites form a network of filaments in the shape of needlelike macrostructures arranged in spherical aggregates, resembling biologically induced structures. The crust deposit microflora is studied in detail using DNA sequencing. Proteobacteria and Firmicutes followed by Acidobacteria and Bacteriodetes are the most represented bacteria in the cave. Although the DNA sequencing of the crust deposit microflora is fully recorded, a large group of unclassified operational taxonomic units prevent us from fully identifying the biodiversity in the hypogene cave. Distinct places in goethite show secondary phases that might be related with distinct events in the life cycle of microbes. We claim that this study is a precursor relevant to the metabolic signature of microbes in a hypogene cave.