Reaction engineering analysis of the autotrophic energy metabolism of Clostridium aceticum

Acetogenesis with CO₂:H₂ or CO via the reductive acetyl-CoA pathway does not provide any net ATP formation in homoacetogenic bacteria. Autotrophic energy conservation is coupled to the generation of chemiosmotic H⁺ or Na⁺ gradients across the cytoplasm membrane using either a ferredoxin:NAD⁺ oxidoreductase (Rnf), a ferredoxin:H⁺ oxidoreductase (Ech) or substrate-level phosphorylation via cytochromes. The first isolated acetogenic bacterium Clostridium aceticum shows both cytochromes and Rnf complex, putting it into an outstanding position. Autotrophic batch processes with continuous gas supply were performed in fully controlled stirred-tank bioreactors to elucidate energy metabolism of C. aceticum. Varying the initial Na⁺ concentration in the medium showed sodium-dependent growth of C. aceticum with a growth optimum between 60 and 90 mM Na⁺. The addition of the Na⁺-selective ionophore ETH2120 or the protonophore CCCP or the H⁺/cation-antiporter monensin revealed that an H⁺ gradient is used as primary energy conservation mechanism, which strengthens the exceptional position of C. aceticum as acetogenic bacterium showing an H⁺-dependent energy conservation mechanism as well as Na⁺-dependent growth.