Abstract
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.
Highlights
Acetogens thrive from the formation of acetate from hydrogen (H2) + carbon dioxide (CO2)
While homoacetate fermentation theoretically yields three molecules of acetate as sole product from C6 sugars, experimentally, acetate to C6 ratios of 2.6, 2.7, and 2.3–3 have been observed in growing cultures of the acetogens Moorella thermoacetica (Fontaine et al, 1942), Acetobacterium woodii (Heise et al, 1989) and T. kivui
To prove the involvement of CO2, concentrated cell suspensions of T. kivui were incubated at 65◦C with mannitol in the presence and in the absence of HCO3− in the medium
Summary
Acetogens thrive from the formation of acetate from hydrogen (H2) + carbon dioxide (CO2). In mannitol conversion by acetogens, electrons have to be deposited either internally on an intermediate of the sugar oxidation, yielding a more reduced product than acetate, or on an external electron acceptor. By a variety of physiological experiments with growing cells and cell suspension, we show unambiguously that T. kivui utilizes external CO2 as additional electron acceptor during growth on and conversion of mannitol; the biochemical and eco-physiological consequences are discussed. Non-growing cells of T. kivui in concentrated suspensions (which excludes that carbon and reducing equivalents were channeled into biomass), converted glucose to mainly acetate (supplementary Figure S1), with only minor amounts of H2 (0.2 mM; Figure 1C, for comparison calculated as if all H2 in was dissolved; n H2 in headspace/vol medium). Mannitol would be converted to acetate according to eq 5
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
