Improving microbial electrosynthesis of polyhydroxybutyrate (PHB) from CO2 by Rhodopseudomonas palustris TIE-1 using an immobilized iron complex modified cathode

Microbial electrosynthesis (MES) is a promising bioelectrochemical approach to produce biochemicals. A previous study showed that Rhodopseudomonas palustris TIE-1 can directly use poised electrodes as electron donors for photoautotrophic growth at cathodic potentials that avoid electrolytic H2 production (photoelectroautotrophy). To make TIE-1 an effective biocatalyst for MES, we need to improve its electron uptake ability and growth under photoelectroautotrophic conditions. Because TIE-1 interacts with various forms of iron while using it as a source of electrons for photoautotrophy (photoferrotrophy), we tested the ability of iron-based redox mediators to enhance direct electron uptake. Our data show that soluble iron cannot act as a redox mediator for electron uptake by TIE-1 from a cathode poised at +100mV vs. Standard Hydrogen electrode. We then tested whether an immobilized iron-based redox mediator Prussian Blue (PB) can enhance electron uptake by TIE-1. Chronoamperometry indicates that cathodic current uptake by TIE-1 increased from 1.47 {+/-} 0.04 to 5.6 {+/-} 0.09 {micro}A/cm2 (3.8 times) and the production of the bioplastic, polyhydroxybutyrate (PHB) improved from 13.5 {+/-} 0.2 g/L to 18.8 {+/-} 0.5 g/L (1.4 times) on electrodes coated with PB. Overall, our data show that immobilized PB can increase direct electron uptake by TIE-1 and enhances PHB production.