Titolo della tesi: Bioelectrochemical conversion of CO₂ to acetate: innovative strategies for controlling methanogenesis in mixed and pure cultures
Anthropogenic activity has led to an increase in CO2 concentrations, significantly contributing to the intensification of the greenhouse effect. To address this issue, carbon capture and storage (CCS) technologies have been developed to separate and retain CO2 from industrial gases. However, relying solely on geological and oceanic storage is insufficient due to the limited number of demonstration plants and the potential environmental risks. A promising alternative is carbon capture and utilization (CCU) technologies, which enable the transformation of CO2 into high-value-added products. In this context, the use of microorganisms as biocatalysts is an attractive option due to their low operational costs and reduced environmental impact. In particular, autotrophic non-photosynthetic organisms, such as acetogens, can convert CO2 into acetate using hydrogen as an electron donor. This process can also generate longer-chain fatty acids, such as butyrate, valerate, and caproate, which have applications in various industrial sectors.
At the core of this biochemical reaction, bioelectrochemical systems (BES) are applied to overcome the limitations of low mass transfer of H2 in the liquid phase and the chemical inertia of CO2 in gas fermentation systems. This technology, known as microbial electrosynthesis (MES), represents an innovative solution to improve the efficiency of CO2 utilization processes. However, acetogens compete directly with methanogens, microorganisms that are thermodynamically more efficient in utilizing the same substrates (H2 and CO2) and are also capable of metabolizing acetate to produce methane. This competition reduces the effectiveness of CO2 conversion processes. Currently, long-term inhibition of methanogenesis is primarily achieved through the use of 2-bromoethanesulfonate (BESA), which limits the large-scale development of this technology due to its high costs.
In this thesis, alternative strategies to BESA usage were explored, including the selection of an acetogenic microbial consortium derived from waste sludge through the development of a selection and enrichment protocol using bioaugmentation with Acetobacterium woodii, demonstrating 40-day methanogen inhibition. Additionally, a model was developed to understand the competition between acetogens and methanogens, proposing an approach to separately describe acetogenic and methanogenic biomass. Kinetic constants were also extrapolated to evaluate the ability of acetogens to maintain a competitive advantage over time in both hydrogenophilic and bioelectrochemical conditions in semi-continuous reactors. It was shown that a dilution factor (D) corresponding to half the maximum specific growth rate (µmax) of acetogens maintained acetogenesis efficiency at 40% and methanogenesis at 10% under hydrogenophilic conditions, and at 64.2% and 2.3%, respectively, in bioelectrochemical conditions at -0.7 V vs SHE. Subsequently, it was hypothesized that applying the selection protocol could be combined with the metabolic versatility of acetogens in bioelectrochemical conditions to promote the formation of an acetogenic biofilm and facilitate the washout of methanogens. This hypothesis was validated during a research period in the Department of Biotechnology at TU Delft, in the Netherlands, where microbial community characterization of the biofilm showed that acetogens were dominant over methanogens, with acetate production efficiency approximately 20 times higher than methane production. Lastly, pioneering studies were conducted on the application of pure extremophilic cultures in bioelectrochemical conditions, focusing on the halophile Acetohalobium arabaticum and on the alkolophile Thindallia Arobium, whose growth conditions (i.e. high salinity and pH 10) are incompatible with the development of methanogens, thus eliminating the need for sterilization in an industrial scaling-up perspective. The study conducted showed that Acetohalobium arabiticum has a higher acetate production rate under bioelectrochemical conditions than hydrogenophilic conditions.
This research proposes innovative solutions for CO2 valorization and the optimization of acetogen application in bioelectrochemical processes.