Microbial Fuel Cells Powered by Geobacter sulfurreducens Ihsan Tria Pramanda 10605103 BM 3204 Bacteriology

Introduction The development of processes to generate biofuels and bioenergy has been of special interest lately Microbial Fuel Cells (MFCs)  collects the electricity generated by microbes when they metabolize substrates Considered to be one of the most efficient energy sources: No burning is required to produce energy The only raw materials needed to power fuel cells are simple organic compounds or even waste materials from other reactions Ihsan Tria Pramanda - 10605103

Principles of the MFCs Ihsan Tria Pramanda - 10605103

Electron Transfer Mechanisms Ihsan Tria Pramanda - 10605103

Microbial Fuel Cells Ihsan Tria Pramanda - 10605103

Geobacter sulfurreducens Comma shaped gram-negative, anaerobic bacteria that are one of the predominant metal-reducing bacteria Generates electricity by oxidizing compounds and reducing the anode Has been shown to generate substantial amounts of energy due to: multiple mechanisms of transporting electrons to extracellular  pili or c-type cytochromes. Formation of biofilms on the anodes  all the cells are involved in electron transport to the anode Ihsan Tria Pramanda - 10605103

Simplified Microbial Anatomy Ihsan Tria Pramanda - 10605103

C-type Cytochromes Geobacter sulfurreducens has 111 different genes that code for c-type cytochromes  more than any other organism Most important ones that encode for c-type cytochromes are omcB, omcE, omcS and omcZ Ihsan Tria Pramanda - 10605103

Biofilms Biofilms formed around the surfaces which it uses as an electron acceptor  as thick as 50 µm The formation of biofilms is possible because of pili Gene involved  pilA Wild type cells that express the gene are able to form biofilms on almost any surface. Mutants that have a pilA deletion can adhere to different surfaces but are not able to either express pili or form thick biofilms Complemented pilA mutants (having a pilA gene reinserted) are once again able to express pili and form biofilms Ihsan Tria Pramanda - 10605103

Biofilm of Geobacter sulfurreducens Confocal scanning laser microscopy of G. sulfurreducens on anode surfaces. (A to C) Wild-type biofilms producing 1.4 mA (A), 2.2 mA (B), and 5.2 mA (C). (D and E) Biofilms of a pilin-deficient mutant (D) and the genetically complemented mutant strain (E) when current production was nearing maximum (ca. 1 mA and 3 mA, respectively). Live cells are green, while dead cells are red. Ihsan Tria Pramanda - 10605103

Why do Strains that Form Biofilms are Able to Produce More Energy? First Explanation  pili can act as microbial nanowires Pili do not have the chemical composition or functional groups that are required for this process Ihsan Tria Pramanda - 10605103

Why do Strains that Form Biofilms are Able to Produce More Energy? (2) Second Explanation  c-type cytochromes c-type cytochromes can interact with the same proteins from other cells and electrons can be passed from one cell to another. In cases where there are no electrons acceptors near the cell  c-type cytochromes can even act as electron sinks and store electrons until a source to which they can be transferred is available. Ihsan Tria Pramanda - 10605103

Gene Manipulation Engineered strains with higher expression of: pilA, OmcZ, OmcB, OmcE, and OmcS genes. More pili  allowing the formation of thicker biofilms and more nanowires for electrons to be transferred More c-type cytochromes  enabling the transfer of electrons to anode surfaces A modelling exercise predicted the effects of specific gene deletions in Geobacter sulfurreducens on the rate of respiration  Optknock strain design methodology Ihsan Tria Pramanda - 10605103

Strains selected in anaerobic environment Spontaneous Mutant Produced pili much more efficiently than the wild type strain  the pilA gene was over expressed. The expression of c-type cytochromes was equal to the control strain  genes like omcZ, omcS, omcB and omcE were not up regulated. This strain was also motile through the action of flagella  cells could move to the anode much more quickly before biofilms were formed. Directed most of its net electron flow to the anode rather than to cell synthesis Strains selected in anaerobic environment placed on a different MFC current production methods of different MFCs were compared One strain showed higher current yields than the initial Geobacter sulfurreducens strain Ihsan Tria Pramanda - 10605103

Limits and Applications At this moment, the biggest concern is trying to obtain higher current levels which could actually generate enough power to drive complex mechanisms Up to now, current levels are around 14 mA which means they could be used to power very simple components Because this technology is still relatively new, the actual current densities that could be generated are still unknown Ihsan Tria Pramanda - 10605103

Conclusions Geobacter sulfurreducens has a promising future in the field of MFCs because of the ability of this organism to transfer electrons to the anode through c-type cytochromes and pili allow it to generate relatively high levels of current. More research is required to determine how the microorganism could be engineered to create strains which are able to generate more current in MFCs. So that it will have higher electricity outputs and will eventually be available in the market. Ihsan Tria Pramanda - 10605103

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References Reguera, G., Nevin, K.P., Nicoll, J.S., Covalla S.F. Biofilm and Nanowire Production Leads to Increased Current in Geobacter sulfurreducens Fuel Cells. Appl Environ Microbiol 2006; 72:7345-7348. Salgado, Carlos Andres. Microbial fuel cells powered by Geobacter sulfurreducens. MMG 445 Basic Biotech. 2009 5:1 Yi, H., Nevin, K.P., Kim, B., Franks, A.E., Klimes, A., Tender, L.M. Lovley, D.R. Selection of a variant of Geobacter sulfurreducens with enhanced capacity for current production in microbial fuel cells. Biosens Bioelectron 2009; 24:3498-3503 Ihsan Tria Pramanda - 10605103