The Buda Thermal Karst System (BTKS) is an extensive active hypogenic cave system
located beneath the residential area of the Hungarian capital. At the river Danube,
several thermal springs discharge forming spring caves. To reveal and compare the
morphological structure and prokaryotic diversity of reddish-brown biofilms developed
on the carbonate rock surfaces of the springs, scanning electron microscopy (SEM),
and molecular cloning were applied. Microbial networks formed by filamentous bacteria
and other cells with mineral crystals embedded in extracellular polymeric substances
were observed in the SEM images. Biofilms were dominated by prokaryotes belonging
to phyla Proteobacteria, Chloroflexi and Nitrospirae (Bacteria) and Thaumarchaeota
(Archaea) but their abundance showed differences according to the type of the host
rock, geographic distance, and different water exchange. In addition, representatives
of phyla Acidobacteria, Actinobacteria, Caldithrix, Cyanobacteria, Firmicutes Gemmatimonadetes,
and several candidate divisions of Bacteria as well as Crenarchaeota and Euryarchaeota
were detected in sample-dependent higher abundance. The results indicate that thermophilic,
anaerobic sulfur-, sulfate-, nitrate-, and iron(III)-reducing chemoorganotrophic as
well as sulfur-, ammonia-, and nitrite-oxidizing chemolithotrophic prokaryotes can
interact in the studied biofilms adapted to the unique and extreme circumstances (e.g.,
aphotic and nearly anoxic conditions, oligotrophy, and radionuclide accumulation)
in the thermal karst springs.
