Mineralization of calcium carbonate by cave bacteria

• Twelve bacterial isolates were generated from a UNESCO lava tube cave in Jeju Island, South Korea. • Isolates were obtained from enrichments selecting for microbially mediated CaCO 3 precipitation processes • Calcite production was linked to pH shifts driven by bacterial activities involving ureolysis and protein degradation. • Vaterite formation during the growth of cave bacteria was likely associated with mineral nucleation processes within biofilm material. • Findings provide insights into microorganisms and biomineralization mechanisms relevant to cave carbonate deposition and its application. This study investigated the growth of carbonate precipitating microorganisms from stalactite droplet samples and stalagmite biofilm samples from Yongcheon Cave, a lava tube on Jeju Island, South Korea. Microbial enrichments conducted at 15 °C and 25 °C included non-selective and selective media for ureolytic and non-ureolytic growth relevant to carbonate biomineralization. Twelve isolates belonging to the families Pseudomonadaceae, Bacillaceae, Moraxellaceae, Xanthomonadaceae, and Comamonadaceae were generated, and representative cultures of each family were investigated for their potential to biomineralize calcium carbonate (CaCO 3 ). Relationships between microbial growth, changes in medium pH, and dissolved calcium content, among other analyses, showed that representative cave bacteria were generally capable of driving changes consistent with microbially-mediated CaCO 3 precipitation. The highest CaCO 3 production potential in our tests was observed in a bacterium of the family Comamonadaceae isolated as an oxalotroph, which removed 85 % of the calcium dissolved in the urea-containing medium concurrent with the formation of crystalline calcite. Based on these and other results, we hypothesize that our cave isolates produce calcite via microbially-driven pH changes and vaterite through mineral nucleation within biofilm material. These findings provide valuable insights into the microorganisms and biomineralization mechanisms associated with speleothem formation in Korean lava tubes, as well as into optimization strategies for the microbial production of CaCO 3 in pure cultures and in biosynthetic communities.