Amorphous calcium carbonate (Acc) is a precursor of crystalline calcium carbonates that plays a key role in biomineralization and polymorph evolution. Here, we show that several bacterial strains isolated from a Hungarian cave produce Acc and their extracellular polymeric substance (epS) shields ACC from crystallization. The findings demonstrate that bacteria-produced ACC forms in water-rich environment at room temperature and is stable for at least half year, which is in contrast to laboratoryproduced ACC that needs to be stored in a desiccator and kept below 10 °C for avoiding crystallization. the Acc-shielding epS consists of lipids, proteins, carbohydrates and nucleic acids. in particular, we identified large amount of long-chain fatty acid components. We suggest that ACC could be enclosed in a micella-like formula within the EPS that inhibits water infiltration. As the bacterial cells lyse, the covering protective layer disintegrates, water penetrates and the unprotected Acc grains crystallize to calcite. our study indicates that bacteria are capable of producing Acc, and we estimate its quantity in comparison to calcite presumably varies up to 20% depending on the age of the colony. Since diverse bacterial communities colonize the surface of cave sediments in temperate zone, we presume that Acc is common in these caves and its occurrence is directly linked to bacterial activity and influences the geochemical signals recorded in speleothems. Amorphous calcium carbonate (ACC) is known as a precursor phase of crystalline CaCO 3 that plays a key role during calcium carbonate precipitation and biomineralization 1. It is the least stable CaCO 3 modification that rapidly transforms to crystalline calcium carbonate polymorphs. Laboratory-synthesized ACC crystallization can be delayed by keeping the physisorbed H 2 O below the critical level and storing the material in a desiccator and keeping it below 10 °C 2. Additives such as Mg 2+ , phosphate, and organic macromolecules can retard its crystallization 3-5. According to Purgstaller et al. 6 , the metastability of Mg-ACC is associated with the formation kinetics (pH and the Mg/Ca ratio) of the less soluble crystalline phase, i.e., the physico-chemical conditions of the environment. Biogenic activity can also modify the physico-chemical conditions, and thus can enhance the preservation of ACC. It has been reported from tissues of various eukaryotic organisms and several organic molecules have been associated with its occurrence. According to Aizenberg et al. 7 , the skeletal parts of calcareous sponge Clathrina and the spicules of ascidian Pyura pachydermatina contain ACC and its formation is associated with polysaccharides and proteins enriched in glutamic acid (and/or glutamine), serine and glycine. ACC was also described from the intraskeletal organic matrix of numerous scleractinian corals 8 , the spicules of the embryos of Strongylocentrotus purpuratus sea urchins 9 and the shell of Biomphalaria glabrata snail embryos 10. Amines, glycosylated proteins and phosp...

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Speleothems (dominated by cave-hosted carbonate deposits) are valuable archives of paleoclimate conditions. As such, they are potential targets of clumped isotope analyses that may yield quantified data about past temperature variations. Clumped isotope analyses of stalagmites, however, seldom provide useful temperature values due to various isotope fractionation processes. This study focuses on the determination of the microbially induced vital effect, i.e., the isotope fractionation processes related to bacterial carbonate production. A cave site with biologically mediated amorphous calcium carbonate precitation was selected as a natural laboratory. Calcite deposits were farmed under a UV lamp to prevent bacterial activity, as well as under control conditions. Microbiological analyses and morphological investigations using scanning electron microscopy showed that the UV lamp treatment effectively reduced the number of bacterial cells, and that bacterial carbonate production strongly influenced the carbonate’s morphology. Stable oxygen isotope analyses of calcite and drip waters, as well as clumped isotope measurements revealed that, although most of the studied carbonates formed close to oxygen isotope equilibrium, clumped isotope Δ47 values varied widely from equilibrium to strongly fractionated data. Site-specific kinetic fractionations played a dominant role in the distribution of Δ47 values, whereas bacterial carbonate production did not result in a detectable clumped isotope effect.