Biofuel cells select for microbial consortia that self-mediate electron transfer

1. Biofuel cells select for microbial consortia that self-mediate electron transfer K.Y. Lee

2. Contents 1. Introduction 2. Materials and Methods 3. Results 4. Discussion

3. Introduction • MFCs are a potential green energy technology. • bacteria do not directly transfer the electrons • these electrons are diverted to the anode • the electrons are conducted over a resistance or power user toward a cathode bacterial energy is directly converted to electrical energy • Shewanella putregaciens, Escherichiacoli Geobacter sulurreducens, Rhodoferax ferrireducens high coulombic efficiency = high electron transfer efficiency

4. Introduction • The crucial parameters for operational effectiveness bacterial metabolism overpotential PEM bacterial electron transfer efficiency of the cathode-oxygen electron transfer internal resistance

5. Materials and Methods • Glucose biofuel cells • anode : 240 ml • cathode : 240 ml, • graphite electrode • membrane : ultrex PEM • feeding rate : 1g glucose / l · day • sludge : 2g VSS / l (potato-processing company)

6. Materials and Methods • Metabolic changes • the influence of the biofuel cell on bacterial metabolism 100 ml biofuel cell contents (operated for 1 week at a loading rete of 1g glucose / liter day) • Chemical analysis • VFA : extraction in diethyl ether (GC) • glucose : IC • CO2, CH4 : GC • hydrogen : Microtox • Microscopy analysis • light microscopy : to verify bacterial growth on electrode surfaces and bacterial morphology (x 1000) • digital image analysis : MicroImage 4.0, Microsoft Excel XP • microscope : Zeiss Orthoplan epifluorescence microscope

7. Materials and Methods • Community analysis • total DNA was extracted from the sludge samples • 16S rRNA gene fragments were amplified with primers PRBA338fGC and P518r and analyzed by DGGE • Isolation procedure • bacteria was isolated from the mixed consortium by plating a serial dilution of the consortium from day 155 on nutrient agar • the plate were incubated under aerobic and anaerobic conditions for 2 and 5 • single colonies were purified by using at least two isolation steps • Biofuel cells with pure cultures • pure cultures of isolates KRP1, KRA1, KRA3 were prepared in nutrient broth after isolation from the agar plates • the bacteria were counted by plating on nutrient agar

8. Materials and Methods • Cyclic voltammetry • starting from – 450 mV and going up 900 mV and back • components were oxidized or reduced → current peaks appeared on the voltammogram • every components (that could be reversibly oxidized or reduced) had a peak both the upper and lower curves • one of the peaks disappeared → permanently oxidized or reduced • working electrode : 5㎠ graphite rod • counterelectrode : platinum wire • reference electrode : Ag / AgCl electrode • cyclic voltammogram : to calculate the quantity of redox-active species and the electrochemical activity → by determining the peak size for the voltammogram and standardizing it to energy (E = P x t)

9. serum flask biofuel cell 0.05% ± 0.09% 12% ± 3% Results • Enrichment of electrochemically active consortia • the power output of the biofuel cells gradually increased (during 71 day enrichment period) • the recovery of electrons from the carbon source : 4% → 81% • electron transfer rate : 0.65 W / ㎡ → 4.31 W / ㎡ • suspended bacteria : 4.31 W / ㎡ , 664 mV, 30.9 mA • attached bacteria : 3.63 W / ㎡ , 602 mV, 30.1 Ma • Change in electron acceptor • hydrogen production

10. Results • Community analysis • DGGE analysis • band patterns : the microbial consortium of the inoculum sludge evolved constantly during the enrichment procedure → 95% similarity (correlated with an increasing power output)

11. Results • Isolation of bacterial strains from the anode consortium • microbial consortium (after 155) : plated onto nutrient agar and incubated under both anaerobic and aerobic conditions • Electrochemical activity of the bacteria

12. Results • Characterization of the electrochemical activity

13. Results • Relationship between power output and electrochemical activity

14. Results • Verification of the importance of soluble redox mediators

15. Discussion • Power output : 0.65 W / ㎡ → 4.31 W / ㎡ • Growth of bacteria use the electrode or produce reduced metabolites (CH4, H2) • Anode potential = Cathode potential – overpotential using the electrode > using protons or CO2 • Overpotential • anode overpotential • cathode overpotential • internal resistance decrease with increasing electrode surface and addition of mediator component

16. Discussion • Community analysis

17. Discussion • Cyclic voltammetry • how the electrochemical activity of the bacteria is governed by the excreted metabolites • two mechanisms for extracellular electron transfer • production of extracellular electron shuttles • components associated with the bacterial cell wall • Outlook the relationship among bacterial density, mediator concentration, power output • Conclusion microbial communities evolve specifically, resulting in an optimized bioelectrocatalyzer

18. Any more Questions ?