The power of the sun

We observed that bacteria engineered with proteorhodopsin produced more ATP when exposed to light, due to the activation of the proton pump (more info on the proteorhodopsin section). 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. In total we had 4 MFCs, one for each construct in the dark and one exposed to the light of a blue LED. The experiment was repeated for 3 days, keeping the same experimental conditions.

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 (Pmax). 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.

When exposed to light BBa_K160410 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 and 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.

Our solar MFC in action

All previous tests were operated by adding exogenous mediators to the anodic medium (i.e. Methylen 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’s 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.

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.

Applications: 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.

Check out our results on this video!