Fluid-like electrodes and Purple Phototrophic Bacteria: bridging the gap in wastewater biorefineries

Wastewater biorefineries aim to generate value-added products in an economically viable process while removing pollutants. In this scenario, Purple phototrophic bacteria (PPB), the most versatile microorganisms on earth, are highly effective for sustainable wastewater treatment and nutrient recovery as cell protein. One of the most innovative approaches for applying PPB in the wastewater sector is their capacity for interchanging electrons with electroconductive materials. In contrast with classical biofilm-based techniques, we have demonstrated that a fluid-like electrode can accept electrons from planktonically grown PPB. We anticipate that such findings will impact in wastewater electrobioremediating capacity of PPB. Moreover, controlling the electrochemical nature of the extracellular electron acceptor (fluid-like electrode) allows for fine-tuning the metabolism of a planktonic PPB-dominated community to enhance their biodegradation rate (2-fold) while growing on brewery wastewater. For this purpose, a twin set of microbial electrochemical fluidized bed reactors (ME-FBR) were operated in identical conditions, except for illumination conditions (dark vs. infrared), to promote the development of PPB. Illumina sequencing revealed that both infrared radiation and polarization led to changes in the microbial population while producing an electrical current of 7 A·m^-3. Indeed, the Geobacter genus was the electroactive bacteria outcompeting under dark conditions. In contrast, electroactive PPB genera like the Rhodopseudomonas and Rhodobacter outcompeted others under infrared illumination and electrostimulation. In this work, we have demonstrated how microbial selection can contribute to the sustainability of an electrobioremediation wastewater treatment by avoiding emissions of greenhouse gases such as methane. In addition, fluid-like bed bioreactors have shown their usefulness in recovering nutrients as PPB biomass, favoring planktonic growth and thus facilitating the recovery of a valuable product: the biomass of PPB.