Dependence of microbial magnetite formation on humic substance and ferrihydrite concentrations
Dependence of microbial magnetite formation on humic substance and ferrihydrite concentrations
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DOI：
10.1016/j.gca.2011.09.007
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发表时间：
2011-11
影响因子：
5
通讯作者：
Annette Piepenbrock;U. Dippon;K. Porsch;E. Appel;A. Kappler
中科院分区：
文献类型：
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作者：
Annette Piepenbrock;U. Dippon;K. Porsch;E. Appel;A. Kappler
微生物Fe（III）还原过程中的铁矿物（反式）形成具有环境相关性，因为它可以影响污染物如有毒金属离子或烃的命运。磁铁矿是微生物铁还原的重要生物矿化产物，并影响用于古气候重建的土壤磁性，并建议协助定位有机和无机污染物。然而，目前还不清楚不同浓度的Fe（III）矿物和腐殖物质（HS）如何影响微生物Fe（III）还原过程中磁铁矿的形成。因此，我们使用湿化学提取，磁化率测量和X-射线衍射分析，以系统地确定如何（i）不同的初始水铁矿（FH）的浓度和（ii）不同浓度的HS（即存在下，无论是吸附HS或吸附和溶解HS）影响磁铁矿的形成过程中FH还原希瓦氏菌oneidensis MR-1。在我们的实验中，磁铁矿的形成没有发生在FH浓度低于5 mM，即使发生快速铁还原。在较高的FH浓度的最小分数的Fe（II）的总铁的25-30%是必要的，以启动磁铁矿的形成。Fe（II）的分数，磁铁矿形成开始下降，随着FH浓度的增加，这可能是由于FH颗粒的聚集减少FH表面积在较高的FH浓度。215- 393 mg HS/g FH的HS浓度减缓（在部分FH表面被吸附的HS覆盖时）或甚至完全抑制（在完全FH表面被吸附的HS覆盖时）磁铁矿的形成，这是由于吸附的HS阻塞了表面位点。这些结果表明，要求的Fe（II）吸附，并随后与FH表面的相互作用，FH转化为磁铁矿。此外，我们发现微生物形成的磁铁矿被菌株MR-1进一步还原，导致在HEPES缓冲介质中形成溶解的Fe（II），即Fe 2+，或在碳酸氢盐缓冲介质中形成碳酸铁（II）（菱铁矿）。除了Fe（III）还原结束时形成的Fe（II）化合物的不同身份外，在FH还原过程中所用的两种缓冲体系中，微生物铁还原和磁铁矿形成的最大速率和程度没有差异。我们的研究结果表明，在铁还原过程中微生物磁铁矿的形成取决于地球化学条件，可以是次要的，在低FH浓度或抑制HS的FH表面的吸附。这种情况可能发生在铁矿物含量低或有机质含量高的土壤中。
Iron mineral (trans)formation during microbial Fe(III) reduction is of environmental relevance as it can influence the fate of pollutants such as toxic metal ions or hydrocarbons. Magnetite is an important biomineralization product of microbial iron reduction and influences soil magnetic properties that are used for paleoclimate reconstruction and were suggested to assist in the localization of organic and inorganic pollutants. However, it is not well understood how different concentrations of Fe(III) minerals and humic substances (HS) affect magnetite formation during microbial Fe(III) reduction. We therefore used wet-chemical extractions, magnetic susceptibility measurements and X-ray diffraction analyses to determine systematically how (i) different initial ferrihydrite (FH) concentrations and (ii) different concentrations of HS (i.e. the presence of either only adsorbed HS or adsorbed and dissolved HS) affect magnetite formation during FH reduction by Shewanella oneidensis MR-1. In our experiments magnetite formation did not occur at FH concentrations lower than 5mM, even though rapid iron reduction took place. At higher FH concentrations a minimum fraction of Fe(II) of 25–30% of the total iron present was necessary to initiate magnetite formation. The Fe(II) fraction at which magnetite formation started decreased with increasing FH concentration, which might be due to aggregation of the FH particles reducing the FH surface area at higher FH concentrations. HS concentrations of 215–393mg HS/g FH slowed down (at partial FH surface coverage with sorbed HS) or even completely inhibited (at complete FH surface coverage with sorbed HS) magnetite formation due to blocking of surface sites by adsorbed HS. These results indicate the requirement of Fe(II) adsorption to, and subsequent interaction with, the FH surface for the transformation of FH into magnetite. Additionally, we found that the microbially formed magnetite was further reduced by strain MR-1 leading to the formation of either dissolved Fe(II), i.e. Fe2+, in HEPES buffered medium or Fe(II) carbonate (siderite) in bicarbonate buffered medium. Besides the different identity of the Fe(II) compound formed at the end of Fe(III) reduction, there was no difference in the maximum rate and extent of microbial iron reduction and magnetite formation during FH reduction in the two buffer systems used. Our findings indicate that microbial magnetite formation during iron reduction depends on the geochemical conditions and can be of minor importance at low FH concentrations or be inhibited by adsorption of HS to the FH surface. Such scenarios could occur in soils with low iron mineral or high organic matter content.