Bacterial Calcium Carbonate Precipitation in Cave Environments: A Function of Calcium Homeostasis
To determine if microbial species play an active role in the development of calcium carbonate (CaCO 3 ) deposits (speleothems) in cave environments, we isolated 51 culturable bacteria from a coralloid speleothem and tested their ability to dissolve and precipitate CaCO 3 . The majority of these isolates could precipitate CaCO 3 minerals; scanning electron microscopy and X-ray diffractrometry demonstrated that aragonite, calcite and vaterite were produced in this process. Due to the inability of dead cells to precipitate these minerals, this suggested that calcification requires metabolic activity. Given growth of these species on calcium acetate, but the toxicity of Ca 2+ ions to bacteria, we created a loss-of-function gene knock-out in the Ca 2+ ion efflux protein ChaA. The loss of this protein inhibited growth on media containing calcium, suggesting that the need to remove Ca 2+ ions from the cell may drive calcification. With no carbonate in the media used in the calcification studies, we used stable isotope probing with C13O2 to determine whether atmospheric CO2 could be the source of these ions. The resultant crystals were significantly enriched in this heavy isotope, suggest- ing that extracellular CO2 does indeed contribute to the mineral structure. Thephysiological adaptation of removing toxicCa2+ ions by calcification, while useful in numerous environments, would be particularly beneficial to bacteria in Ca2+-rich cave environments. Such activitymay also create the initial crystal nucleation sites that contribute to the formation of secondary CaCO3 deposits within caves.