The molecular interface of microbe-mineral electron transfer

我们人类通过呼吸（“呼吸”）氧气来获得生命所需的能量。这个过程包括使用从我们吃的食物中提取的电子将氧气转化为水，这个过程被称为氧气还原。自由能在这个过程中被释放出来，我们用它来制造ATP，这是生命的通用能量货币。我们对氧气的依赖使我们成为“专性需氧生物”，如果没有氧气，我们就会死亡。因此，我们只能生活在地球表面，在那里氧气是免费的。然而，地球上可居住环境的巨大优势并没有被人类利用，而是被包括细菌在内的微生物利用，它们可以在缺氧的“缺氧”环境中生存。令人难以置信的是，其中一些细菌可以生活在地球的地下深处，靠“呼吸岩石”生存。这是因为在地球的地下环境中，一些最丰富的呼吸基质是不溶性矿物质，尤其是铁矿物质。这些矿物质使一些土壤呈红色，它们也可以被看作是裸露悬崖上的红色接缝。事实上，“铁呼吸”是缺氧地区最普遍的呼吸过程之一，因此具有广泛的环境意义。例如，它直接影响若干生物地球化学循环的平衡，如氮、硫和碳循环，这反过来又可能影响强效温室气体的释放，如氧化亚氮。它还可能对石油工业有害，因为它会导致地下或海底输油管道的溶解。在某些方面，细菌呼吸矿物铁的方式类似于它们呼吸氧气的方式，利用电子来“减少”呼吸基质。因此，细菌细胞内代谢产生的电子被传递给铁，铁被带负电荷的电子从所谓的3+价态（FeIII）“还原”为2+价态（FeII）。然而，由于铁矿物是一种大的不溶性颗粒，它不能自由扩散到细菌细胞中。因此，如果细菌能够利用铁矿物作为呼吸电子受体，它必须具有一种分子机制，通过这种机制，它可以将细胞内细胞代谢产生的电子转移到细胞外矿物质。这个问题的部分解决方案在于特殊的“电子转移蛋白”，它们实际上位于细胞外部，可以将电子传递给细胞外不溶性矿物质。在所谓的“微生物-矿物界面”发生这种电子转移的机制尚不清楚。它代表了环境中丰富的细菌群的生物化学研究中的一个主要问题。回答这个问题将为研究细菌的能量过程提供新的见解。它还将具有重要的生物技术影响，因为有可能在生物修复过程中使用矿物氧化物呼吸细菌来清理被有毒有机污染物（如漏油）或放射性金属（如铀）污染的环境。在微生物燃料电池中，细菌可以用来产生电流，使用电极作为固体细胞外电子受体也在探索中。

We humans obtain the energy we need for life by respiring ('breathing') oxygen. This process involves using electrons extracted from the food we eat to convert oxygen to water in a process known as oxygen reduction. Free energy is released in this process and we use this to make ATP, which is the universal energy currency of life. Our dependency on oxygen makes us 'obligate aerobes', take away the oxygen and we die. Thus we are confined to living on the surface of planet Earth where oxygen is freely available. However, the vast propotion of Earth's habitable environments are not exploited by humans, but by micro-organisms, including bacteria, that can live in the absence of oxygen in 'anoxic' environments. Incredibly, some of these bacteria can live deep in the earth's subsurface and survive by 'breathing rocks'. This is because some of the most abundant respiratory substrates in the earth's subsurface environments are insoluble minerals, particularly minerals of iron. Such minerals give some soils a redish colour and they can also be seen as red seams in exposed cliffs. In fact 'iron respiration' is amongst the most widespread respiratory process in anoxic zones and so has wide environmental significance. For example, it directly impacts on the balance of several biogeochemical cycles, such as the nitrogen, sulphur and carbon cycles and this can in turn influence the release of potent greenhouse gases, such as nitrous oxide. It can also be detrimental to the oil industry through contributing to the dissolution of subsurface or submarine oil pipes.In some aspects the way bacteria respire mineral iron is similar to the way in which they respire oxygen, using electrons to 'reduce' the respiratory substrate. Thus, electrons generated by metabolism inside the bacterial cell are passed to the iron, which is 'reduced' from a so-called 3+ valent state (FeIII) to a 2+ valent state (FeII) by the negatively charged electron. However, because the iron mineral is a large insoluble particle it cannot freely diffuse into bacterial cells. Consequently, if a bacterium is to be able to utilise an iron mineral as a respiratory electron acceptor it must have a molecular mechanism by which it can transfer electrons generated by cellular metabolism inside the cell to an extracellular mineral. Part of the solution to the problem lies in special 'electron transfer proteins' that actually sit on the outside of the cell where they can pass electrons to extracellular insoluble minerals. The mechanism by which this electron transfer at the so called 'microbe-mineral interface' occurs is still not known. It represents a major question in the study of the biochemistry of an environmentally abundant group of bacteria. Answering it will provide new insights into bacterial energetic processes. It will also have important biotechnological impacts since there is potential for using mineral oxide respiring bacteria in bioremediation processes for the clean up of environments contaminated with toxic organic pollutants (e.g. oil leaks) or radioactive metals, such as Uranium (VI). There use in microbial fuel cells where the bacteria can be used to generate electric currents using electrodes as solid extracellular electron acceptors is also being explored.