The molecular interface of microbe-mineral electron transfer

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.

Electron exchange between microbes and transition metal containing minerals in the earth's subsurface drives the carbon cycle in many environments. It influences many other biogeochemical cycles and is a major shaper in rock weathering. It is also being harnessed in a range of biotechnologies including bioremediation, microbial fuel cells and microbial electro-catalysis. Anaerobic bacterial mineral respiration requires the transport of electrons across the outer membrane to solid-phase electron acceptors outside of the cell. For the mineral respiring bacteria Shewanella this is achieved through a porin-cytochrome complex and outer membrane cytochromes localised to the surface of the cell. The outer membrane cytochromes of Shewanella belong to one of four main clades MtrF, MtrC, OmcA and UndA. Despite an awareness of the importance and potential application of microbe-mineral electron transfer the individual role of each of these cytochromes in the transfer of electrons at the microbe-mineral interface is not well understood. This proposal will seek to elucidate the roles of the outer membrane cytochromes in the adsorption to solid-phase minerals and the mechanism by which the proteins interact with, and transer electrons to, these minerals. The work will include establishing: the functionality of potential mineral binding site on the protein surface; the influence of individual heme redox potentials in supporting the ability of the cell to respire upon minerals; and determining the nature of the interactions between outer membrane cytochromes on the surface of the cell.

Academic Impact.
This research aims to interrogate the properties of cell surface proteins on bacteria that terminate biological electrical wires between microbes and minerals. Understanding the molecular nature of the physical interaction between these proteins and minerals could ultimately enhance our ability to exploit these processes. The properties of these extracellular redox reactions will be of immediate importance to microbiologists and biochemists as they uncover the mechanisms by which bacteria survive in anoxic conditions.

Environmental impact.
Geochemists and biogeochemists will be able to use this research to identify how bacteria affect the content of minerals, such as iron and manganese oxides in the environment. Due to the importance of this system in driving the subsurface carbon cycle, it will also influence other elemental cycles, such as the nitrogen and sulfur cycles, and as such will be of long term importance to environmental scientists who seek to understand how bacteria can effect global elemental cycles.

Industrial Biotechnology impact.
In a microbial fuel cell bacteria extract electrons from organic substrates and pass them to electrodes such that an electrical current is produced. Shewanella that express these outer-membrane complexes are capable of passing electrons to the electrode without the need for artificial electron donors. This is also offers the tempting possibility of supplying electrons to power different reactions within the bacteria, and researchers in different countries are currently looking at ways to adapt electrodes to optimise this process. We are working with scientists in the multi-billion international oil provision company Schlumberger (cambridge research laboratory - BBSRC Industrial CASE Award) to explore the potential of our work in bioelectrocatalysis. Our molecular studies could improve our understanding bacterial-electrode interactions for optimisation of such applications. In broader terms the research is also of importance to the oil industry as metal-reducing since bacterial contribute to the dissolution of subsurface or submarine oil pipes.

Environmental impact.
Shewanella metabolism could have several important environmental 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, for example oil leaks, or radioactive metals, such as Uranium (VI). Mineral respiring bacteria have also been explored for their potential in bioremediation using Fe(III) as electron acceptors.

Societal impact.
Ground waters are critical to a functioning and healthy societies across the globe. Our fundamental work on a molecular understanding of major subsurface microbiological process will contribute gloablly to the the holistic understanding of biogeochemical functioning of the groundwater-surface water interaction zone and how this influences contaminant fate and transport, carbon and nitrogen fluxes, and climate change through the impact on the release of greenhouse gases.

International impact.
The work has international impact through our collaborations with researchers in the Advanced Water Management Centre, Univeristy of Queensland, Austrlalia where Richardson holds an Honorary Chair and our collaboration with the Pacific Northwest National Laboratory, Richland, USA that has evolved out of a US Department of Energy Biogeochemical Grand Challenge, which leads to Richardson making at least yearly presentations to US Department of Energy scientists and adminsitrators in venues that include Washington DC.