Importance of hydrogen-mediated mechanisms for microbial electrosynthesis: regulation at the molecular level
- Autores: Elisabet Perona Vico
- Directores de la Tesis: Lluís Bañeras (dir. tes.), Sebastià Puig Broch (codir. tes.)
- Lectura: En la Universitat de Girona ( España ) en 2022
- Idioma: español
- Tribunal Calificador de la Tesis: Diana Machado de Sousa (presid.), Eileen Yu Sneeden (secret.), Annemiek Ter Heijne (voc.)
- Programa de doctorado: Programa de Doctorado en Ciencia y Tecnología del Agua por la Universidad de Girona
- Materias:
- Enlaces
- Tesis en acceso abierto en: TDX
- Resumen
Microbial electrosynthesis (MES) is engineered to use electric power and carbon dioxide (CO2) as the only energy and carbon sources in reductive bioelectrochemical processes for biosynthesis. This technology is conducted in bioelectrochemical systems (BES) and takes advantage of electroactive microorganisms. In MES, hydrogen (H2) has been highlighted as the key intermediate element involved in a whole range of microbial metabolisms for the reduction of CO2. A proper understanding of the role of H2, its production, and availability in MES might reinforce the overall productivity and applicability of the technology.
Basic processes in MES rely on the transformation of electric power (electrons) into chemical energy in a process called electron transfer. The study of extracellular electron transfer (EET) mechanisms can help to understand the integration of microbes and electrode materials in an operative tandem, and to elucidate the participation of intermediate molecules, such as H2, as opposed to direct electron transfer events for electrosynthesis. An electromethanogenic reactor conducting the reduction of CO2 to methane (CH4) was used to study putative genes taking part in EET. We aimed at determining short-time changes in the gene expression levels of [NiFe]-hydrogenases (Eha, Ehb, and Mvh), heterodisulfide reductase (Hdr), coenzyme F420-reducing [NiFe]-hydrogenase (Frh), and hydrogenase maturation protein (HypD) according to the electron flow (closed and open electric circuits). Microbial community composition analysis through both DNA and cDNA signatures revealed that electromethanogenesis was conducted by Methanobacterium sp. being the main archaeon present in the system. According to RT-PCR data, suspected mechanisms in electron transfer events were not regulated at the transcriptional level when exposing Methanobacterium sp. cells to short-time open/closed electric circuits.
Since H2 is the most relevant electron donor among microbial metabolisms, cathodic potentials generally used for CO2 recycling are characterised to ensure no H2 limitation. However, some microorganisms could serve as potentially interesting sustainable H2 producers in biocathodes. We have studied the biological H2 production in biocathodes operated at -1.0 V vs. Ag/AgCl, using a highly comparable technology and using CO2 as the sole carbon feedstock. Ten different bacterial strains were chosen from genera Rhodobacter, Rhodopseudomonas, Rhodocyclus, Desulfovibrio, and Sporomusa, all described as hydrogen-producing candidates. Monospecific biofilms were formed on carbon cloth cathodes and hydrogen evolution was monitored over time using a microsensor. Eight over ten bacterial strains showed electroactivity and H2 production rates increased significantly (2 to 8-fold) compared to abiotic conditions for two of them (Desulfovibrio paquesii DSM 16681 and Desulfovibrio desulfuricans DSM 642). D. paquesii DSM 16681 exhibited the highest production rate (45.6 ± 18.8 µM·min-1) compared to abiotic conditions (5.5 ± 0.6 µM·min-1), although specific production rates (per unit biomass) were similar to those observed for other strains. Our results demonstrated many microorganisms are suspected to participate in hydrogen production, but inherent differences among strains did occur. These differences have to be considered for future developments of resilient biofilm coated cathodes as a stable hydrogen production platform in microbial electrosynthesis.
The application of bacteria-coated cathodes for sustainable H2 production may not be efficient enough to maintain H2 biosynthetic requirements for highly efficient producing strains. Here, we applied genetic engineering tools intending to further increase the H2 production ability of D. paquesii. [Fe]-only hydrogenase and tetraheme cytochrome c3 were selected as genes of interest to be overexpressed in D. vulgaris DSM 644 and D. paquesii DSM 16681. Different conditions and described protocols were tested towards implementing the proper mechanisms to ensure overexpression of the selected genes, but no successful results were obtained by the time of completion of this Ph.D. Thesis. Even though, an extensive and critical revision of the key factors to be controlled was done to address the existing limitations of the process.
Following an architecture analogy, studying H2 production in BES has been the keystone of this Thesis, and model processes (such as methanogenesis, bioelectro-H2 production, and homoacetogenesis) the needed rib-vaults to complete the desired structure. Different methods have been used throughout. In the process of completion of the dome, the presented approaches might have contributed to a better understanding of the key role of H2 during microbial electrosynthesis and derive some conclusions. First, enhancing the current knowledge of extracellular electron transfer may lead to better control of reductive BES. Second, the required H2 supply for sustainable electrochemical bioprocesses may be provided in a more efficient way using bio-H2 evolving microorganisms. Finally, the application of synthetic biology and defined consortia should be considered for new and promising contributions in the METs field.
