Living Photovoltaics based on Recombinant Expression of MtrA Decaheme in Photosynthetic Bacteria

At the center of microbial bioelectricity applications lies the critical need to express foreign heme proteins that are capable of redirecting the electron flux of the cell’s metabolism. This study presents bioengineered Synechocystis sp. PCC 6803 cells capable of increased electrogenicity through the introduction of a genetic construct for cytochrome expression. We could demonstrate the functional expression of the periplasmic MtrA decaheme c-type cytochrome from Shewanella oneidensis, a dissimilatory metal-reducing exoelectrogen, inside Synechocystis. Protein expression was verified through western-blotting and immunostaining, and oxygen evolution, optical density, and absorption measurements confirm sustained cell activity and viability under the tested expression conditions. Furthermore, the bioengineered cells show enhanced mediated exoelectrogenicity, as confirmed through a colorimetric iron assay and electrochemical measurements. Compared to wildtype cells on graphite electrodes, the bioengineered cells show a 2-fold increase in light-dependent, extracellular electron transfer, achieving photocurrent densities of 4 μA/cm2 under white light illumination of ∼500 μmol m-2s-1. The increased capacitance obtained under illumination and suppressed photocurrents in the presence of the photosynthetic inhibitor, 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) suggest increased extraction of photosynthetically derived electrons from the recombinant cells. The improved bioelectricity transport across the outer membranes, as achieved through the heterologous heme expression inside cyanobacteria, presents new opportunities for re-wiring the metabolisms of light-harvesting microbes.