Department of Microbiology and Molecular Genetics, Michigan State University, East Lansing, Michigan, USA.
Appl Environ Microbiol. 2013 Aug;79(15):4694-700. doi: 10.1128/AEM.01566-13. Epub 2013 May 31.
The microbial reduction of Fe(III) plays an important role in the geochemistry of hydrothermal systems, yet it is poorly understood at the mechanistic level. Here we show that the obligate Fe(III)-reducing archaeon Geoglobus ahangari uses a direct-contact mechanism for the reduction of Fe(III) oxides to magnetite at 85°C. Alleviating the need to directly contact the mineral with the addition of a chelator or the electron shuttle anthraquinone-2,6-disulfonate (AQDS) stimulated Fe(III) reduction. In contrast, entrapment of the oxides within alginate beads to prevent cell contact with the electron acceptor prevented Fe(III) reduction and cell growth unless AQDS was provided. Furthermore, filtered culture supernatant fluids had no effect on Fe(III) reduction, ruling out the secretion of an endogenous mediator too large to permeate the alginate beads. Consistent with a direct contact mechanism, electron micrographs showed cells in intimate association with the Fe(III) mineral particles, which once dissolved revealed abundant curled appendages. The cells also produced several heme-containing proteins. Some of them were detected among proteins sheared from the cell's outer surface and were required for the reduction of insoluble Fe(III) oxides but not for the reduction of the soluble electron acceptor Fe(III) citrate. The results thus support a mechanism in which the cells directly attach and transfer electrons to the Fe(III) oxides using redox-active proteins exposed on the cell surface. This strategy confers on G. ahangari a competitive advantage for accessing and reducing Fe(III) oxides under the extreme physical and chemical conditions of hot ecosystems.
微生物还原 Fe(III) 在热液系统的地球化学中起着重要作用，但在机制层面上还了解甚少。在这里，我们表明，专性 Fe(III)还原古菌 Geoglobus ahangari 在 85°C 下使用直接接触机制将 Fe(III)氧化物还原为磁铁矿。通过添加螯合剂或电子穿梭剂蒽醌-2,6-二磺酸钠 (AQDS) 来缓解与矿物直接接触的需要，从而刺激了 Fe(III)的还原。相比之下，将氧化物包埋在藻酸盐珠中以防止细胞与电子受体接触，除非提供 AQDS，否则会阻止 Fe(III)的还原和细胞生长。此外，过滤后的培养上清液对 Fe(III)还原没有影响，排除了分泌的内源性介体太大而无法穿透藻酸盐珠的可能性。与直接接触机制一致，电子显微镜照片显示细胞与 Fe(III)矿物颗粒密切相关，一旦溶解，就会显示出大量卷曲的附属物。这些细胞还产生了几种含有血红素的蛋白质。其中一些蛋白质在从细胞外表面剪切的蛋白质中被检测到，它们是还原不溶性 Fe(III)氧化物所必需的，但不是还原可溶性电子受体 Fe(III)柠檬酸盐所必需的。结果支持这样一种机制，即细胞使用暴露在细胞表面的氧化还原活性蛋白质直接附着并将电子转移到 Fe(III)氧化物上。这种策略使 G. ahangari 在热生态系统极端的物理和化学条件下具有获取和还原 Fe(III)氧化物的竞争优势。
