EXAFS study on the cause of enrichment of heavy REEs on bacterial cell surfaces
EXAFS study on the cause of enrichment of heavy REEs on bacterial cell surfaces
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DOI：
10.1016/j.gca.2010.07.001
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发表时间：
2010-10-01
影响因子：
5
通讯作者：
Tanaka, Kazuya
中科院分区：
文献类型：
--
作者：
Takahashi, Yoshio;Yamamoto, Mika;Tanaka, Kazuya
稀土元素(REE)配分模式是一种独特的地球化学示踪剂，已对多种天然物质进行了测定。其中，细菌与水体之间的稀土分布模式在重稀土(HREE)部分表现出异常的富集性，这可以作为天然样品中细菌相关物质的特征。在这项研究中，利用扩展X射线吸收精细结构(EXAFS)结合稀土元素分布模式的变化，确定了导致HREE富集的革兰氏阳性菌(枯草芽孢杆菌)细胞表面的稀土结合部位。EXAFS数据表明，在较低的REE-细菌比([REE]/[Bac])下，HREE与多个磷酸盐中心(包括磷酸酯中心)形成配位数较大的络合物，而轻稀土和中等稀土与CN较低的磷酸盐中心形成络合物。随着[REE]/[Bac]的增加，所有稀土元素与羧酸盐配位的比例都增加。另一方面，随着[REE]/[Bac]的增加，细菌的REE分布模式中HREE的富集度并不明显。这一结果与EXAFS数据一致，因为标准物质中具有多个磷酸盐的表面络合物的REE模式随着稀土元素的增加而单调增加，而具有低CN和羧化中心的磷酸盐表面络合物在Sm和Eu附近达到最大值。根据这些结果，可以清楚地看出，稀土主要结合在细菌细胞表面的磷酸盐部位，然后结合到细菌细胞表面的羧酸盐部位。在pH依赖范围(3<；pH<；7)，EXAFS和REE模式数据都表明，REE-羧酸盐的比例随着pH的增加而增加。上述对枯草杆菌的研究结果也适用于革兰氏阴性杆菌--大肠杆菌，表明在大肠杆菌或其他革兰氏阴性菌的细胞壁中也存在类似的磷酸和羧酸位点。在我们的所有结果中，REE模式的变化与EXAFS指示的结合位置相关，表明REE模式本身反映了不同参数(pH和[REE]/[Bac])下REE在细菌表面的结合位置。比较了不同稀土元素在细菌表面吸附的平均键长，结果表明，由于稀土作为多个磷酸盐表面络合物的选择性结合，HREE(Er到Lu)的键长比从La和Dy之间的趋势推断的要短得多。我们的结果与HREE在细菌细胞表面的选择性浓缩是一致的，考虑到键长较短的化学物种更稳定。由此可见，HREE在细菌细胞表面的富集是由多个磷酸盐表面络合物的形成引起的。基于这些结果，我们认为具有这种磷酸盐中心的物质，如细菌和细菌相关物质，可以在自然系统中诱导异常的HREE富集。(C)2010爱思唯尔有限公司。保留所有权利。
Rare earth element (REE) pattern is a unique geochemical tracer and has been measured for various natural materials. Among these, the REE distribution pattern between bacteria and water exhibits anomalous enrichment in the heavy REE (HREE) part, which can act as a signature of bacteria-related materials in natural samples. In this study, the REE binding site on the cell surface of a Gram-positive bacterium (Bacillus subtilis) responsible for HREE enrichment has been identified using extended X-ray absorption fine structure (EXAFS) coupled with a study of the variation in REE distribution patterns. The EXAFS data showed that the HREEs form complexes with multiple phosphate site (including phosphoester site) with a larger coordination number (ON) at lower REE-bacteria ratios ([REE]/[bac]), while light and middle REEs form complexes to the phosphate site with a lower CN. The fraction coordinated to carboxylate increased for all REEs with increasing [REE]/ [bac] ratio. On the other hand, the enrichment of HREE in the REE distribution patterns of the bacteria was less marked with increasing [REE]/[bac] ratio. This result is consistent with the EXAFS data, because the REE pattern of surface complex with multiple phosphate in a reference material exhibits a monotonous increase for heavier REE, while phosphate surface complex with a low CN and a carboxylate site reach a maximum around Sm and Eu. Based on these results, it is clear that the REE are primarily bound to the phosphate site and subsequently to the carboxylate site on the bacterial cell surface.Regarding the pH dependence in the range (3 < pH < 7), both the EXAFS and REE pattern data indicate that the fraction of REE-carboxylate increased as the pH increases. The results above obtained for B. subtilis were also valid for Escherichia coli, a Gram-negative bacterium, showing that similar phosphate and carboxylate sites are also available in the cell walls of E. coli, or other Gram negative bacteria. In all our results, the variation in REE patterns correlated with the binding site indicated by EXAFS, showing that the REE pattern itself reflects the binding site of the REE at the bacterial surface for various parameters (pH and [REE]/[bac] ratio). Thus, the REE patterns can be used to estimate the binding sites for lower [REE]/[bac] ratios where spectroscopic techniques cannot be applied.The average bond length between the REE and oxygen was compared for various REE sorbed on bacteria, showing that the bond length for HREE (Er to Lu) was much shorter than those extrapolated from the trend between La and Dy, because of the selective binding of the HREE as the multiple phosphate surface complexes. Our results are consistent with the selective enrichment of the HREE at the bacterial cell surfaces, considering that chemical species with a shorter bond length are more stable. Thus, it is clear that the HREE enrichment at the bacterial cell surfaces is caused by the formation of the multiple phosphate surface complexes. Based on these results, it is suggested that materials having such phosphate sites such as bacteria and bacteria-related materials can induce anomalous HREE enrichment in natural systems. (C) 2010 Elsevier Ltd. All rights reserved.