Impact of different trace elements on metabolic routes during heterotrophic growth of C. ljungdahlii investigated through online measurement of the carbon dioxide transfer rate

Conversion of carbon dioxide in biogas into acetic acid by Clostridium thailandense immobilized on porous support materials

This study aimed to convert CO2 in biogas into acetic acid using immobilized Clostridium thailandense cells on various support materials, including activated carbon, expanded clay, and coir. Immobilized cells and free cells were evaluated for their CO2 conversion ability into acetic acid using H2 as an electron donor at an H2 to CO2 in biogas ratio of 2:1 (v/v), 30 °C, 150 rpm. Results showed that immobilized cells on activated carbon increased CH4 content to 96.9% (v/v), and acetic acid production to 15.65 mmol/L within 96 h. These values outperformed free cells. The activated carbon-immobilized cells could be reused two times without losing efficacy in the purification of biogas and acetic acid production. This work indicates that using the immobilized cells offers a sustainable approach to biogas upgrading, reducing the environmental footprint of biogas production by increasing its energy content and purity.

Deletion of genes linked to the C1-fixing gene cluster affects growth, by-products, and proteome of Clostridium autoethanogenum

Gas fermentation has emerged as a sustainable route to produce fuels and chemicals by recycling inexpensive one-carbon (C1) feedstocks from gaseous and solid waste using gas-fermenting microbes. Currently, acetogens that utilise the Wood-Ljungdahl pathway to convert carbon oxides (CO and CO2) into valuable products are the most advanced biocatalysts for gas fermentation. However, our understanding of the functionalities of the genes involved in the C1-fixing gene cluster and its closely-linked genes is incomplete. Here, we investigate the role of two genes with unclear functions-hypothetical protein (hp; LABRINI_07945) and CooT nickel binding protein (nbp; LABRINI_07950)-directly adjacent and expressed at similar levels to the C1-fixing gene cluster in the gas-fermenting model-acetogen Clostridium autoethanogenum. Targeted deletion of either the hp or nbp gene using CRISPR/nCas9, and phenotypic characterisation in heterotrophic and autotrophic batch and autotrophic bioreactor continuous cultures revealed significant growth defects and altered by-product profiles for both ∆hp and ∆nbp strains. Variable effects of gene deletion on autotrophic batch growth on rich or minimal media suggest that both genes affect the utilisation of complex nutrients. Autotrophic chemostat cultures showed lower acetate and ethanol production rates and higher carbon flux to CO2 and biomass for both deletion strains. Additionally, proteome analysis revealed that disruption of either gene affects the expression of proteins of the C1-fixing gene cluster and ethanol synthesis pathways. Our work contributes to a better understanding of genotype-phenotype relationships in acetogens and offers engineering targets to improve carbon fixation efficiency in gas fermentation.