The thermal waters of Gellért Hill discharge area of the Buda Thermal Karst System
(Hungary) are characterized by high (up to 1000 Bq/L) 222Rn-activity due to the radium-accumulating
biogeochemical layers. Samples were taken from these ferruginous and calcareous layers
developed on spring cave walls and water surface. Accumulation of potentially toxic
metals (e.g. As, Hg, Pb, Sn, Sr, Zn) in the dense extracellular polymeric substance
containing bacterial cells and remains was detected by inductively coupled plasma
mass spectrometry. The comparison of bacterial phylogenetic diversity of the biofilm
samples was performed by high throughput next generation sequencing (NGS). The analysis
showed similar sets of mainly unidentified taxa of phyla Chloroflexi, Nitrospirae,
Proteobacteria, Planctomycetes; however, large differences were found in their abundance.
Cultivation-based method complemented with irradiation assay was performed using 5,
10 and 15 kGy doses of gamma-rays from a 60Co-source to reveal the extreme radiation-resistant
bacteria. The phyla Actinobacteria, Firmicutes, Proteobacteria (classes Alpha- Beta-
and Gammaproteobacteria), Bacteriodetes and Deinococcus-Thermus were represented among
the 452 bacterial strains. The applied irradiation treatments promoted the isolation
of 100 different species, involving candidate novel species, as well. The vast majority
of the isolates belonged to bacterial taxa previously unknown as radiation-resistant
microorganisms. Members of the genera Paracoccus, Marmoricola, Dermacoccus and Kytococcus
were identified from the 15 kGy dose irradiated samples. The close relatives of several
known radiation-tolerant bacteria were also detected from the biofilm samples, alongside
with bacteria capable of detoxification by metal accumulation, adsorption and precipitation
in the form of calcium-carbonate which possibly maintain the viability of the habitat.
The results suggest the establishment of a unique, extremophilic microbiota in the
studied hydrothermal spring caves.