electric bacteria feed on electrons

Posted comment on ´Meet the electric life forms that live on pure energy` by Catherine Brahic published in New Scientist no 2978 19th July 2014 p. 8


In her New Scientist article, Catherine Brahic describes a type of bacteria, known as ´electric bacteria` that feed on electrons from the rocks and metals they live on. Although two types of such bacteria are already known, the Shewanella and Geobacter, biologists have now found others in rocks and marine mud if they spike the surroundings with electric current.
Brahic describes in her article why these forms are biochemically unusual and quotes Kenneth Nealson of the University of Southern California, Los Angeles, who expresses surprise that we are not more familiar with this type of life form since the fundamental reactions in biochemistry are based on electron exchange and transfer. Nealson and colleagues can grow ´electric bacteria` directly on electrodes in the laboratory. They collect sediment from the seabed and in the laboratory insert electrodes into it. The natural voltage across the sediment is measured first and then a slightly different voltage is applied: higher, then there is an excess of electrons; lower, then the electrode accepts electrons from its surroundings. They found that bacteria living in the sediment either harvested the electrons from the higher voltage, or donated electrons on to the lower voltage electrode, hence generating a detectable electric current. Further examples of ´electric bacteria` are recorded: Jangir, University of Southern California found electron donors in a well in Death Valley, California; Bond and colleagues, University of Minnesota grew a type of bacteria that accepted electrons from an iron electrode; and Rowe, University of Southern California identified up to 8 different kinds of bacteria that harvest electrons.
Brahic ends her article describing the growing interest in ´electric bacteria` because of their potential applications. NASA is interested because they can survive on very little energy and so are an example of how some extra-terrestrial life may exist. Industries are looking at the creation of ´biomachines` that could perform tasks such as sewerage or groundwater cleaning, as too are others who are investigating into the fundamental questions about life for example the minimum energy demand required by life forms. Relating to the latter, Gorby at the Rensselaer Polytechnic Institute, New York believes that the bacteria should be grown between two electrodes with the bacteria accepting electrons from one electrode, using them in their biochemical actions and then donating them to the other electrode, making them a self-contained energy system that could maintain the bacteria indefinitely if all conditions were ideal, even if they do not grow or divide.


What makes this topic interesting is the biochemical difference between ´electric bacteria` and other cells and organisms regarding source and ´form` of electrons. Free electrons are not normally substrates in biochemistry since they are usually ´packaged` with molecules, metal ions, ATP etc. to make them electrically neutral. ´Electric bacteria` appear to feed on and take in pure electrons from their surroundings and in the case quoted above, electrodes are the source. Another group used magnetite as electron donor and acceptor. Byrne and colleagues at the University of Tübingen, Germany used tiny crystals of magnetite and grew mixed colonies of Geobacter and Rhodopseudomonas with the former accepting electrons and the latter donating.
Once harvested, the transfer of electrons between substrates/molecules etc. means energy transfer, with losing electrons linked to less energy and gaining, higher and this is a common biochemical mechanism. Electron gradients, proton pumps, electron-transport proteins etc. are all common methods of transferring electrons and hence, energy and this we assume is the same for the ´electric bacteria`.
So, what features of the ´electric bacteria` must exist so that free electrons are the energy source? We can assume that if this type of bacteria donates or accepts free electrons then it is likely that what stands it apart from other bacteria exists at the bacterial cell surface. And since free electrons are involved then not all aspects of the bacterial cell surface are electrically neutral. This can be seen with neuronal cells where there is a negative charge inside the cell and positive outside. The cell surface electron acceptors/donors molecules have to be assumed to be stable in this form and exist either completely through the membrane themselves, or link to the next part of the electron transfer cycle on the inner side/inner membrane or within the periplasmic space. Unless it is a unique structure, then it is likely that the electron acceptor or donor part of the molecule, at least at the site of direct contact, is a flavin, iron-sulphur cluster, quinone, heme or copper ion constituent. Binding or loss of the electrons leads to configuration changes of the surface molecules initiating the next step of the transfer process, which need not be unique to this type of bacteria. This next step also probably signals a return of the surface molecule to its electron-accepting/donating form and this may be simply a single stage (configuration change only due to loss or gain of electrons) or more complicated requiring other membrane bound constituents.
Only detailed investigation of the ´electric bacteria` membrane structure will elucidate the process and it is important research because of the possible interrelationship of electric bacteria to cellular organelles mitochondria or chloroplasts and to the proposed life fields/electric fields. The evolutionary tree has endosymbiotic bacteria as the basis of mitochondria and endosymbiotic photosynthetic active bacteria for chloroplasts. Both of these are heavily involved in cellular electron and energy transfer, and hence, electric bacteria could be the origin of these organelles. Electric bacteria may also provide a good system for investigating cellular life fields and since they already known to require or donate electrons directly then comparisons to other electrically active membranes might give new information about the mechanisms involved, eg the repair of bone cells.

Since we´re talking about the topic……..
…can we assume that gene sequencing of membrane proteins found in electric bacteria and in mitochondria and chloroplasts will provide evidence whether this type of bacteria is the evolutionary origin of these organelles?
….if magnetic fields are applied will we see changes in the electron transfer across the cell membrane? The use of luminescence may aid the investigation.

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