Search

NMR analysis of (13)C-labelling patterns showed that the Embden-Meyerhof (EM) pathway is the main route for glycolysis in the extreme thermophile Caldicellulosiruptor saccharolyticus. Glucose fermentation via the EM pathway to acetate results in a theoretical yield of 4 mol of hydrogen and 2 mol of acetate per mole of glucose. Previously, approximately 70% of the theoretical maximum hydrogen yield has been reached in batch fermentations. In this study, hydrogen and acetate yields have been determined at different dilution rates during continuous cultivation. The yields were dependent on the growth rate. The highest hydrogen yields of 82 to 90% of theoretical maximum (3.3 to 3.6 mol H(2) per mol glucose) were obtained at low growth rates when a relatively larger part of the consumed glucose is used for maintenance. The hydrogen productivity showed the opposite effect. Both the specific and the volumetric hydrogen production rates were highest at the higher growth rates, reaching values of respectively 30 mmol g(-1) h(-1) and 20 mmol l(-1) h(-1). An industrial process for biohydrogen production will require a bioreactor design, which enables an optimal mix of high productivity and high yield.

In recent years a number of hyperthermophiles with the ability to utilize sugars as source for carbon and energy have been isolated. Analysis of their central metabolism may reveal adaptations to the extreme environment, or give information about the evolution of the primary pathways involved. The best studied representative is Pyrococcus furiosus , which has become the model organism of the heterotrophic hyperthermophiles. This deeply branched archaeon utilizes a modified Embden-Meyerhof Pathway, which involves a set of unprecedented ADP-dependent kinases, and a unique glyceraldehyde-3-phosphate: ferredoxin oxidoreductase. Moreover, pyruvate is converted via acetyl-CoA to acetate, involving an ADP-forming acetyl-CoA synthetase, which is not encountered in Bacteria. Reductant generated by ferrodoxin-linked enzymes is released either by S 0 -reduction to H 2 S, by proton reduction to H 2 or by the formation of alanine. Yield studies suggest that in addition to ATP synthesis by substrate level phosphorylation in the ultimate acetate-forming step, there are alternative energy conserving systems. The ADP-dependent Embden-Meyerhof pathway is probably shared by other members of the Thermococcales . In contrast, an ATP-dependent Embden-Meyerhof pathway is operating in the S 0 -respiring archaeon Thermoproteus tenax , although it involves a PP i -dependent phosphofructokinase. Finally, hyperthermophilic bacteria such as Thermotoga maritima utilize a classical Embden-Meyerhof pathway. Thus, the presence of the different versions of the Embden-Meyerhof pathway in these deeply rooted microbes indicates that the hypothesis that the Entner-Doudoroff pathway is more primitive is not correct.