Diversity of bacterial populations in abandoned Sitarjevec mine (Slovenia) with iron oxide/hydroxide formations

Abandoned Sitarjevec mine represents specific environment (completely dark, 10 degrees C, pH<3) with various ores (lead, mercury, zinc, iron, copper) and many different minerals (anglesite, baryte, cinnabar, pyrite, cerussite, quartz, siderite). The aim of the study was characterisation of iron oxide/hydroxide speleothem with extremely fast growth of up to 5 cm/year. Morphological and chemical characterisation of iron oxide/hydroxide speleothem showed that it has layered structure with different ferrihydrite minerals (goethite, feroxyhyte, lepidocrocite). SEM-EDS confirmed that surface layers are very porous while inner parts, bulk is compact. Bacterial populations in iron oxide/hydroxide speleothem and its environment were evaluated with analysis of 16S rRNA sequencing. Results revealed great diversity of bacteria including 19-34 phyla in different samples. Proteobacteria were dominant (60-95%) in all samples, but in sample of water at iron oxide/hydroxide speleothem the highest bacterial diversity was found with 670 different genera and 100 different species. Among them typical iron oxidizing bacteria such as Gallionella capsiferriformans, Sideroxydans lithotrophicus, Sphingomonas echinoides, Candidatus Nitrotoga Nitrotoga, Leptospirillum ferrooxidans, and Ferrovum myxofaciens were identified. These iron oxidizing bacteria were not found in other samples and along with structure of iron oxide/hydroxide speleothem these might indicate involvement of bacteria in growth of this speleothem. But specific bacterial interactions with minerals and environmental conditions in the mine need to be further analysed and evaluated.