Calcium Carbonate Precipitating Cultivable Bacteria from Different Speleothems of Karst Caves

The aim of the study was to reveal and compare the microbiota of different carbonate speleothems from two different epigenic karst regions (Baradla and Csodabogyos caves) in Hungary. Stalagmites, stalactites, soda straws, moonmilks, cave wall surfaces and dripping waters were sampled and plated on subsurface-environment imitating culture media containing low amount of nutrients, minerals and extract of cave sediments. Diverse oligo-heterotrophic bacterial community composition (192 different species in the phyla Actinobacteria, Proteobacteria, Firmicutes and Bacteroidetes) - with hitherto unknown cave bacteria - varied in the samples. The CaCO3 precipitating capacity of 126 bacterial strains from different taxonomical positions was tested on solid B4 medium. 75% of the strains precipitated crystals with various morphology in the colonies and metabolic tests showed that higher alkalizing activity led to higher rate of precipitation in the mucoid colonies. Scanning electron microscopy, high-resolution transmission electron microscopy and X-ray diffraction analysis indicated that the precipitates were dominated by calcite, and nanosize vaterite was identified for Paeniglutamicibacter kerguelensis (strain BaSD-225). Our study suggests that CaCO3 precipitated by proteolytic bacteria are widespread in the biofilms in karst caves and these bacteria can actively contribute to CaCO3 formation through alkalizing their microenvironment.