Reaction engineering analysis of the autotrophic energy metabolism of Clostridium aceticum

Abstract

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 60–90 mM Na⁺. The addition of the Na⁺-selective ionophore ETH2120 or the protonophore CCCP or the H⁺/cation-antiporter monensin revealed that a H⁺ gradient is used as primary energy conservation mechanism, which strengthens the exceptional position of C. aceticum as acetogenic bacterium showing a H⁺ dependent energy conservation mechanism as well as Na⁺ dependent growth.