Department of Geology and Geophysics, Yale University, New Haven, Connecticut 06520, United States.
Environ Sci Technol. 2011 Aug 1;45(15):6254-61. doi: 10.1021/es200456e. Epub 2011 Jul 6.
A key question to address in the development of oxygen isotope ratios in phosphate (δ(18)O(p)) as a tracer of biogeochemical cycling of phosphorus in ancient and modern environments is the nature of isotopic signatures associated with uptake and cycling of mineral-bound phosphate by microorganisms. Here, we present experimental results aimed at understanding the biotic and abiotic pathways of P cycling during biological uptake of phosphate sorbed to ferrihydrite and the selective uptake of sedimentary phosphate phases by Escherichia coli and Marinobacter aquaeolei. Results indicate that a significant fraction of ferrihydrite-bound phosphate is biologically available. The fraction of phosphate taken up by E. coli attained an equilibrium isotopic composition in a short time (<50 h) due to efficient O-isotope exchange (between O in PO(4) and O in water; that is, actual breaking and reforming of P-O bonds) (biotic pathway). The difference in isotopic composition between newly equilibrated aqueous and residual sorbed phosphate groups promoted the ion exchange (analogous to isotopic mixing) of intact phosphate ions (abiotic pathway) so that this difference gradually became negligible. In sediment containing different P phases, E. coli extracted loosely sorbed phosphate first, whereas M. aquaeolei preferred Fe-oxide-bound phosphate. The presence of bacteria always imprinted a biotic isotopic signature on the P phase that was taken up and cycled. For example, the δ(18)O(p) value of loosely sorbed phosphate shifted gradually toward equilibrium isotopic composition. The δ(18)O(p) value of Fe-oxide-bound phosphate, however, showed only slight changes initially but, when new Fe-oxides were formed, coprecipitated/occluded phosphate retained δ(18)O values of the aqueous phosphate at the time of formation of new Fe oxides. Concentrations and isotopic compositions of authigenic and detrital phosphates did not change, suggesting that these phosphate phases were not utilized by bacteria. These findings support burgeoning applications of δ(18)O(p) as a tracer of phosphorus cycling in sediments, soils, and aquatic environments and as an indicator of paleo- environmental conditions.
在开发磷酸盐氧同位素比值（δ(18)O(p)）作为古、今环境中磷生物地球化学循环示踪剂的过程中，一个关键问题是微生物对矿物结合态磷吸收和循环过程中与同位素特征相关的本质。在这里，我们提供了旨在理解生物吸收铁氢氧化物结合态磷酸盐和大肠杆菌和海栖盐单胞菌（Marinobacter aquaeolei）对沉积磷酸盐相选择性吸收过程中 P 循环的生物和非生物途径的实验结果。结果表明，大量铁氢氧化物结合态磷酸盐是生物可利用的。由于有效的 O 同位素交换（PO(4) 中的 O 与水中的 O 之间；即，P-O 键的实际断裂和形成），大肠杆菌吸收的磷酸盐部分在短时间（<50 h）内达到平衡同位素组成（生物途径）。新平衡水相和残留吸附磷相之间的同位素组成差异促进了完整磷酸盐离子的离子交换（类似于同位素混合）（非生物途径），因此这种差异逐渐变得可以忽略不计。在含有不同磷相的沉积物中，大肠杆菌首先提取松散吸附的磷酸盐，而海栖盐单胞菌则更喜欢铁氧化物结合态的磷酸盐。细菌的存在总是会在被吸收和循环的磷相中留下生物同位素特征。例如，松散吸附磷酸盐的 δ(18)O(p)值逐渐向平衡同位素组成转变。然而，铁氧化物结合态磷酸盐的 δ(18)O(p)值最初仅略有变化，但当形成新的铁氧化物时，共沉淀/包裹的磷会保留新铁氧化物形成时水相磷酸盐的 δ(18)O 值。自生和碎屑磷酸盐的浓度和同位素组成没有变化，表明这些磷酸盐相没有被细菌利用。这些发现支持了将 δ(18)O(p)作为沉积物、土壤和水生环境中磷循环示踪剂和古环境条件指示物的新兴应用。
