Effects of microbial activity on the delta O-18 of dissolved inorganic phosphate and textural features of synthetic apatites

Laboratory growth experiments were conducted to investigate the oxygen isot ope effects associated with bacterial metabolism of phosphatic compounds co mmonly available in nature. The observed oxygen isotope fractionations sugg est complex patterns of exchange between dissolved inorganic phosphate (P-i ) and water, and significant circulation of P-i between intracellular and e xtracellular locations with extensive recycling of the dissolved P-i pool, even at high concentrations of dissolved P-i. Results of these experiments also support current models for bacterial utilization of phosphate. These r esults have important implications for the use of delta(18)O values of diss olved P-i to trace sources of P, and bear on integrity of original oxygen i sotope compositions of biogenic and sedimentary apatite minerals that have been subjected to processes of recrystallization and diagenesis. SEM images of laboratory synthesized apatite minerals show that similar tex tural features may be produced by microbially mediated and abiotic reaction s, and that spheroidal structures may be produced by processes of dissoluti on as well as precipitation. The interpretation of certain mineral structur es as microbial in origin solely on the basis of morphological and textural features may be misleading.