Electromicrobial strategies for a sustainable growth of purple phototrophic bacteria

Abstract

The impact of human activity on the environment has led the planet to a climate emergency situation. The concentration of greenhouse gases and the contamination of natural environments is having enormous consequences such as intense droughts, water scarcity, severe fires, rising sea levels, flooding, melting polar ice, catastrophic storms and declining biodiversity. This situation requires rethinking the model of consumption and production. The food sector is responsible for ca. 25% of current greenhouse gas (GHG) emissions. Consumption habits are changing and, according to the Boston Consulting Group (BCG), that alternative sources of protein will represent 11%-22% of protein consumption in 2035. Therefore, it is crucial to find new sustainable sources of protein for food. Purple phototrophic bacteria (PPB), the most versatile microorganisms on earth, have been proposed as a possible alternative source of protein. The main limitation is the feedstock used for cultivation, which represents a high percentage of the overall operational expenditure. Finding new affordable feedstocks will make the bioproduct economically viable. In this thesis, two alternative feedstocks by means of electrodes as protagonists have been studied. The first was electrical current through the use of an electrode for the cultivation of a PPB-dominated microbial consortium (Chapter 2). The second feedstock explored in this thesis was wastewater in combination with electrodes for controlling the metabolism of purple phototrophic bacteria (Chapter 3 and 4). This thesis report is organized into 5 chapters. In Chapter 1, we present an state of the art of electromicrobiology including the physiology of purple phototrophic bacteria and the interaction with electrodes. Then, the research results are contained in Chapters 2, 3 and 4. Finally, in Chapter 5, the experimental results are discussed and compared with the state of the art. In the same way that purple phototrophic bacteria use ferrous iron as an electron donor, these microbes can use carbon-based electrodes (Chapter 2). Carbon fixation by extracellular electron uptake allows electricity and carbon dioxide to be used as feedstock to cultivate purple phototrophic bacteria. The electrode, acting as a cathode (-0.6 V vs. Ag/AgCl), served to grow a PPB-dominated consortium. Both the electrochemical and the microbial population analysis point to the Rhodopseudomonas genus (PPB) as the main actor in the electron uptake, acting as a link between the electrode and the rest of the microbial community. Wastewater as feedstock is more than just a payable resource as many industries pay for its treatment. Indeed, wastewater treatment is eventually a service. In addition, using a residue such as wastewater to generate a bioproduct is environmentally positive. Although the wastewater has already been used as feedstock to cultivate purple phototrophic bacteria, the composition of the wastewater determines the metabolic microbial behaviour. The electrode, both as electron acceptor and donor, can help control microbial metabolism, allowing a fine control of purple phototrophic bacteria. Purple phototrophic bacteria cultured under anodic polarization showed a 2-fold enhancement in the brewery wastewater treatment (vs. non-polarized). On top of that, electrobioremediation of brewery wastewater using PPB showed ca. 3-fold higher yield than using non-photosynthetic culture (Chapter 3). Furthermore, polarization minimizes or completely prevented methanogenesis. The electroactive PPB genera Rhodopseudomonas and Rhodobacter outcompeted other genera during growth under polarization and illumination conditions. The electrode acting as a cathode, served as an extra source of electrons in the cultivation of purple phototrophic bacteria (Chapter 4). By providing extra electrons the electron sinks pathways were activated, specifically carbon fixation and consequently biomass production was maximized. Indeed, PPB biomass production was enhanced 3-fold to 7-fold in presence of cathodic polarization during brewery wastewater treatment. Finally, Chapter 5 discusses the experimental results of the thesis, putting their implications in context. In addition, some ideas about future work to bring this technology to the market are presented.