The abandoned Sitarjevec mine represents a specific environment (complete darkness; 10 °C) with waters of pH 3 and it contains various ores (e.g. lead, mercury, zinc, iron, copper) and many different minerals (e.g. anglesite, baryte, cinnabar, pyrite, cerussite, quartz, siderite). The aim of this study was to characterize the iron oxide/hydroxide speleothems that show rapid growth of up to 5 cm/year. Their morphological and chemical characterization showed that they have a layered structure with different ferrihydrite minerals (i.e. goethite, feroxyhyte, lepidocrocite). Scanning electron microscopy–energy dispersive X-ray spectrometry confirmed that the surface layers were very porous, while for the inner parts, the bulk was compact. Bacterial populations in these speleothems and their environment were analyzed using 16S rRNA sequencing. These data revealed a great diversity of bacteria, which included 19–34 phyla across different samples. Proteobacteria were dominant in all of the samples (60–95%), although the highest bacterial diversity was seen for a water sample from the base of the speleothem, with 670 different genera and 100 different species detected. Among these there were typical iron-oxidizing bacteria, like Gallionella capsiferriformans, Sideroxydans lithotrophicus, Sphingomonas echinoides, Candidatus Nitrotoga Nitrotoga, Leptospirillum ferrooxidans, and Ferrovum myxofaciens. These iron-oxidizing bacteria were not present in other samples from the mine. Therefore, these bacteria might be involved in the structural growth of these speleothems. The specific bacterial interactions with the minerals under the environmental conditions in the mine need to be further analyzed and evaluated.

The abandoned Sitarjevec mine represents a specific environment (complete darkness; 10 °C) with waters of pH 3 and it contains various ores (e.g. lead, mercury, zinc, iron, copper) and many different minerals (e.g. anglesite, baryte, cinnabar, pyrite, cerussite, quartz, siderite). The aim of this study was to characterize the iron oxide/hydroxide speleothems that show rapid growth of up to 5 cm/year. Their morphological and chemical characterization showed that they have a layered structure with different ferrihydrite minerals (i.e. goethite, feroxyhyte, lepidocrocite). Scanning electron microscopy–energy dispersive X-ray spectrometry confirmed that the surface layers were very porous, while for the inner parts, the bulk was compact. Bacterial populations in these speleothems and their environment were analyzed using 16S rRNA sequencing. These data revealed a great diversity of bacteria, which included 19–34 phyla across different samples. Proteobacteria were dominant in all of the samples (60–95%), although the highest bacterial diversity was seen for a water sample from the base of the speleothem, with 670 different genera and 100 different species detected. Among these there were typical iron-oxidizing bacteria, like Gallionella capsiferriformans, Sideroxydans lithotrophicus, Sphingomonas echinoides, Candidatus Nitrotoga Nitrotoga, Leptospirillum ferrooxidans, and Ferrovum myxofaciens. These iron-oxidizing bacteria were not present in other samples from the mine. Therefore, these bacteria might be involved in the structural growth of these speleothems. The specific bacterial interactions with the minerals under the environmental conditions in the mine need to be further analyzed and evaluated.