We observed that E. coli engineered with proteorhodopsin produced more ATP when exposed to light, due to the activation of the proton pump (see Proteorhdopsin). We wanted to see if this makes bacteria more MFC friendly (i.e. live happily in the anode chamber) and if that would produce more electricity.

Cells transformed with BBa_K1604010 (araC-pBAD + proteorhodopsin) and BBa_K731201 (negative control) were grown in M9 media and induced with arabinose (5 mM) and all-trans retinal (10 μM) for 4 hours in darkness. Preliminary tests showed that the optimal medium to be used was M9 medium supplemented with glucose, which gave a more stable signal (data not shown).

The bacterial cultures were split and then placed in the anodic chamber of a small Microbial Fuel Cell (borrowed from one of our instructor Martin Hanczyc) and exposed to the light of a blue LED. The experiment was repeated for 3 days, keeping the same experimental conditions.

Small Microbial Fuel Cell with bacteria expressing BBa_K731201 and the negative control in the light. Bacteria were placed in the anode covered with a layer of mineral oil to keep anaerobic conditions. The anode was exposed to blue light LED. Chemical mediators were added in the anode (Methylene blue, 100 μM) and in the cathode (Ferricyanide, 10 mM)

In the presence of a blue-light LED, the proteorhodopsin expressing strain showed in all three cases a better electrochemical response than the negative control (i.e. PR-expressing strain shows higher polarization and power curves), with a higher voltage and maximum power (P_max). In a biological scale this means that there is an increased transfer of electrons from the culture media to the electrode, and this is directly related to bacteria’s increased viability.

More electricity with proteorhodopsin! BBa_K1604010 and BBa_K731201 cells were grown and induced as described before. For each construct one MFC was placed in the light. The cells were connected to a data logging millimeter connected to an external variable resistor to register the voltage parameter of our system. Every hour the resistance was changed starting from 10MΩ to 1 KΩ Panel A: Polarization curve for BBa_K1604010 and BBa_K731201; for each data point the voltage was measured, while current and power were calculated with the Ohm law. Panel B: Power curve for BBa_K1604010 and BBa_K731201. The calculated power is plotted against the current to estimate the maximum power produced.

BBa_K1604010 polarization curve: light versus dark.The experiment was performed with the same experimental details described before. This time MFC with BBa_K1604010 was placed in the dark and one was exposed to the light of a blue LED.

When exposed to light BBa_K1604010 showed a remarkable response to the external load applied, as shown by the higher values of voltage and current in the light. However, it has to be pointed out that this behavior was not always consistent. A few times we also observed the reverse effect (more electricity in the dark). This data are in agreement with the functional characterization, in which it was shown that a few times there was a basal activation of the proton pump also in the dark.

All previous tests were operated by adding exogenous mediators to the anodic medium (i.e. Methylene blue, Neutral red). However this does not represent a valid method for future applications of the MFC. Related to our main project, we also characterized a mediatorless MFC by expressing Shewanella oneidensis electron export system in an engineered E.coli strain from Ajo-Franklin Lab in Berkley). We characterized this strain in the MFC because we wanted to use it later in our Solar pMFC prototype. It should be noted that the parts used here were not BioBricks.

E.coli Mtr electron transport system polarization and power curve.C43(DE3) cotransformed with a IPTG inducible plasmid carrying the cymAmtrCAB operon and a plasmid with ccmA-H under pTet constitutive promoter, were grown in LB and induced with IPTG (0.5 mM). The induced cells were placed in a MFC without mediators. The data were acquired as described earlier.

The increased electron flow we saw this time was mediated by the expression of Shewanella electron export complex. Such increase is not related to bacteria’s viability.

Proteorhodopsin can power the blue-light LED used by UniTN iGEM Trento 2013 to produce ethylene! We used small MFCs filled with proteorhodopsin-expressing bacteria (BBa_K1604010), connected in series, to light up a few electronic apparatus, including a calculator, a blue-light LED and a lab timer.

Watch this video to see our MFC in action: