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.