Homoacetogenic Conversion of Mannitol by the Thermophilic Acetogenic Bacterium Thermoanaerobacter kivui Requires...

Acetogenic microorganisms utilize organic substrates such as sugars in addition to hydrogen (H₂) + carbon dioxide (CO₂). Recently, we reported that the thermophilic acetogenic microorganism Thermoanaerobacter kivui is among the few acetogens that utilize the sugar alcohol mannitol, dependent on a gene cluster encoding mannitol uptake, phosphorylation and oxidation of mannitol-1-phosphate to fructose-6-phosphate. Here, we studied mannitol metabolism with resting cells of T. kivui; and found that mannitol was "fermented" in a homoacetogenic manner, i.e., acetate was the sole product if HCO₃ ^- was present. We found an acetate:mannitol ratio higher than 3, indicating the requirement of external CO₂, and the involvement of the WLP as terminal electron accepting pathway. In the absence of CO₂ (or bicarbonate, HCO₃ ^-), however, the cells still converted mannitol to acetate, but slowly and with stoichiometric amounts of H₂ formed in addition, resulting in a "mixed" fermentation. This showed that-in addition to the WLP-the cells used an additional electron sink-protons, making up for the "missing" CO₂ as electron sink. Growth was 2.5-fold slower in the absence of external CO₂, while the addition of formate completely restored the growth rate. A model for mannitol metabolism is presented, involving the major three hydrogenases, to explain how [H] make their way from glycolysis into the products acetate or acetate + H₂.