Sunday, April 29, 2007

Metabolic shift analysis of the extreme thermophilic hydrogen producing Caldicellulosiruptor saccharolyticus

Karin Willquist1, Serve Kengen2, Fons Stams2, and Ed W. J Van Niel1. (1) Applied microbiology, Lunds University, Getingevagen 60, Lund, 222 41, Sweden, (2) Laboratory of Microbiology, Wageningen University, Hesselink van Suchtelenweg 4, Wageningen, 6703 CT, Netherlands

Caldicellulosiruptor saccharolyticus is an extreme thermophilic, gram-positive bacterium that is industrially interesting since it is able to produce nearly stoichiometric amounts of hydrogen, 90%, from a wide range of carbon sources. The organism also shows considerable adaptation abilities, which might be explained by the exceptional high number of transposases present in its genome. Data of sucrose fermentation suggest that at partial hydrogen pressures (pH2) above 10-20 kPa, a metabolic shift to lactate formation occurs, which is controlled by the redox-activation of lactate dehydrogenase. At pH2>50kPa the organism is able to fix carbon dioxide via a variation of the conventional Wood-Ljungdahl pathway: one carbon from pyruvate is combined with an external CO2 to form acetate. This assumption is supported by: (i) a decrease of the H2/acetate ratio from 1.85 to 0.5; (ii) NMR analysis showing that 13C-carbonate is exclusively accumulating in the carboxyl group of acetate; (iii) and the presence of genes coding for the Wood-Ljungdahl pathway in the genome. Control of the metabolic shift is required for a high-yield hydrogen production process. This research is a part of an integrated European project, HYVOLUTION, which is based on a bioprocess that employs thermophilic and phototrophic bacteria to provide high hydrogen production efficiency.

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