Nevin Kelly P, Lovley Derek R
Department of Microbiology, University of Massachusetts, Amherst, Massachusetts 01003, USA.
Appl Environ Microbiol. 2002 May;68(5):2294-9. doi: 10.1128/AEM.68.5.2294-2299.2002.
Mechanisms for Fe(III) oxide reduction were investigated in Geothrix fermentans, a dissimilatory Fe(III)-reducing microorganism found within the Fe(III) reduction zone of subsurface environments. Culture filtrates of G. fermentans stimulated the reduction of poorly crystalline Fe(III) oxide by washed cell suspensions, suggesting that G. fermentans released one or more extracellular compounds that promoted Fe(III) oxide reduction. In order to determine if G. fermentans released electron-shuttling compounds, poorly crystalline Fe(III) oxide was incorporated into microporous alginate beads, which prevented contact between G. fermentans and the Fe(III) oxide. G. fermentans reduced the Fe(III) within the beads, suggesting that one of the compounds that G. fermentans releases is an electron-shuttling compound that can transfer electrons from the cell to Fe(III) oxide that is not in contact with the organism. Analysis of culture filtrates by thin-layer chromatography suggested that the electron shuttle has characteristics similar to those of a water-soluble quinone. Analysis of filtrates by ion chromatography demonstrated that there was as much as 250 microM dissolved Fe(III) in cultures of G. fermentans growing with Fe(III) oxide as the electron acceptor, suggesting that G. fermentans released one or more compounds capable of chelating and solubilizing Fe(III). Solubilizing Fe(III) is another strategy for alleviating the need for contact between cells and Fe(III) oxide for Fe(III) reduction. This is the first demonstration of a microorganism that, in defined medium without added electron shuttles or chelators, can reduce Fe(III) derived from Fe(III) oxide without directly contacting the Fe(III) oxide. These results are in marked contrast to those with Geobacter metallireducens, which does not produce electron shuttles or Fe(III) chelators. These results demonstrate that phylogenetically distinct Fe(III)-reducing microorganisms may use significantly different strategies for Fe(III) reduction. Thus, it is important to know which Fe(III)-reducing microorganisms predominate in a given environment in order to understand the mechanisms for Fe(III) reduction in the environment of interest.
在地下环境的铁（III）还原区发现的异化铁（III）还原微生物——地杆菌属发酵菌中，研究了氧化铁（III）的还原机制。地杆菌属发酵菌的培养滤液刺激了洗涤后的细胞悬液对结晶性差的氧化铁（III）的还原，这表明地杆菌属发酵菌释放了一种或多种促进氧化铁（III）还原的细胞外化合物。为了确定地杆菌属发酵菌是否释放电子穿梭化合物，将结晶性差的氧化铁（III）掺入微孔藻酸盐珠中，这阻止了地杆菌属发酵菌与氧化铁（III）之间的接触。地杆菌属发酵菌还原了珠内的铁（III），这表明地杆菌属发酵菌释放的化合物之一是一种电子穿梭化合物，它可以将电子从细胞转移到与生物体不接触的氧化铁（III）上。通过薄层色谱法分析培养滤液表明，电子穿梭体具有与水溶性醌相似的特性。通过离子色谱法分析滤液表明，在地杆菌属发酵菌以氧化铁（III）作为电子受体生长的培养物中，溶解的铁（III）高达250微摩尔，这表明地杆菌属发酵菌释放了一种或多种能够螯合和溶解铁（III）的化合物。溶解铁（III）是另一种减轻细胞与氧化铁（III）接触以进行铁（III）还原的策略。这是首次证明在没有添加电子穿梭体或螯合剂的特定培养基中，一种微生物可以还原源自氧化铁（III）的铁（III）而无需直接接触氧化铁（III）。这些结果与金属还原地杆菌的结果形成了显著对比，金属还原地杆菌不产生电子穿梭体或铁（III）螯合剂。这些结果表明，系统发育上不同的铁（III）还原微生物在铁（III）还原方面可能使用显著不同的策略。因此，为了了解感兴趣环境中铁（III）还原的机制，了解在给定环境中占主导地位的铁（III）还原微生物是很重要的。
