Microbial Biofilms in the Proshchal’naya Karst Cave (Far East, Russia)

Effect of abiotic (composition of drip and fracture water) and biotic factors (composition and activity of microbial complexes) on biofilm formation in the Proshchal’naya karst cave (Far East, Russia) is reported. The effect of natural infiltration water on the structure of cultured microbial complexes and on the elemental composition of the moonmilk speleothem formed on the walls of the cave was studied. The highest concentrations of Fe and Mn were observed in drip water in spring. Organic compounds were shown to have an important effect on the rates of microbial processes at the water–rock interface, on the moonmilk elemental composition, and on the structure of communities involved in carbonate precipitation. The structure of moonmilk microbial communities was investigated using culture-based techniques and quantitative PCR analysis. Iron bacteria of the genera Rhodoferax and Geothrix, as well as Bacillus species, were identified in moonmilk samples. The microstructure and elemental composition of moonmilk samples from the Proshchal’naya cave were investigated by scanning electron microscopy coupled to X-ray microanalysis. SEM images of the moonmilk mass revealed diverse microstructures and high heterogeneity. Tubular microstructures contained more elements (C, O, Ca, Fe, Mn, Si, Al, and S) than claviform structures consisting of calcium carbonate (СаСО3). The moonmilk binding matrix consisted of thin nanofibers.