Oxygen isotope, micro-textural and molecular evidence for the role of microorganisms in formation of hydroxylapatite in limestone caves, South Korea

We present oxygen isotope, micro-textural, and molecular evidence of microbial activity in the formation of hydroxylapatite (HAP) in three limestone caves (Gosu, Sungryu, and Ssang caves) in South Korea. HAP typically forms as crusts (0.1 to 0.5 mm thick) coating carbonates of speleothems and host rock surfaces, on and near bat habitats. Micro-textures within HAP crusts indicate that a metastable apatite precursor (AP) is initially precipitated on and near the surfaces of sulfur-bearing microbial filaments and then transforms to HAP. Analysis of DNA extracted from the HAP crusts confirms that sulfur-oxidizing bacteria are present in some of the HAP samples. The δ18O values of phosphate (δ18OP) in HAP precipitated in the caves range from 14.6 to 15.6‰ and are close to isotopic equilibrium with the weighted mean annual δ18O value of rain water (= cave water) at the mean annual air temperature (= measured cave temperature). The difference in oxygen isotopic composition between speleothem carbonate (δ18OC) and phosphate (δ18OP) in adjacent apatitic crusts is similar to that of co-existing carbonate and phosphate in modern biogenic apatite. These results suggest that phosphate, likely derived from bat excretions, was metabolized by microorganisms and has undergone extensive oxygen isotope exchange with cave drip water by intense biological turnover of phosphate, and then precipitated as HAP in near-equilibrium with water and carbonate in the cave ecosystem. Results from these studies of δ18OP values of HAP crusts in limestone caves demonstrate the utility of δ18OP as an environmental temperature proxy and signature of microbiological processes.