The molecular interface of microbe-mineral electron transfer

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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.