Biohydrogen Production by the Thermophilic Bacterium Caldicellulosiruptor saccharolyticus: Current Status and Perspectives

Abstract

Caldicellulosiruptor saccharolyticus is one of the most thermophilic cellulolytic organisms known to date. This Gram-positive anaerobic bacterium ferments a broad spectrum of mono-, di- and polysaccharides to mainly acetate, CO2 and hydrogen. With hydrogen yields approaching the theoretical limit for dark fermentation of 4 mol hydrogen per mol hexose, this organism has proven itself to be an excellent candidate for biological hydrogen production. This review provides an overview of the research on C. saccharolyticus with respect to the hydrolytic capability, sugar metabolism, hydrogen formation, mechanisms involved in hydrogen inhibition, and the regulation of the redox and carbon metabolism. Analysis of currently available fermentation data reveal decreased hydrogen yields under non-ideal cultivation conditions, which are mainly associated with the accumulation of hydrogen in the liquid phase. Thermodynamic considerations concerning the reactions involved in hydrogen formation are discussed with respect to the dissolved hydrogen concentration. Novel cultivation data demonstrate the sensitivity of C. saccharolyticus to increased hydrogen levels regarding substrate load and nitrogen limitation. In addition, special attention is given to the rhamnose metabolism, which represents an unusual type of redox balancing. Finally, several approaches are suggested to improve biohydrogen production by C. saccharolyticus.

Thermococcus kodakarensis : the key to affordable biohydrogen production

“…However, H 2 formation from reduced pyridine nucleotides (NAD(P)H) is unfavorable as the Gibbs energy under standard conditions is positive. Only at relatively high temperatures and at low H 2 partial pressures, the reaction becomes exergonic [48]. Nevertheless, H 2 formation from NAD(P)H is possible, which can also be concluded from the H 2 /hexose ratios found for certain (hyper)thermophiles.…”

“…Microbial dark fermentation Current sugar-to-H 2 yields by microbial dark fermentation vary from less than 1 to almost 4 mole of H 2 /mole of hexose consumed ( Table 1.1) [13,14,43,48,49]. The differences in yield are mainly due to the types and relative proportions of the products generated by fermentation.…”

“…1.5). However, complete anaerobic oxidation of glucose is thermodynamically difficult, which is indicated by the positive value for the Gibbs energy (∆ r G°) associated with this reaction, and is only possible at very low partial H 2 pressures [48]. Moreover, since dark fermentation is performed by microorganisms, fermentations have been optimized by evolution to produce biomass (ATP) and not H 2 .…”

“…To enhance the economy of the microbial dark fermentation, it is necessary to improve the sugar-to-H 2 yield. Current sugar-to-H 2 yields by microbial dark fermentation vary from less than 1 to almost 4 moles of H 2 /mole of hexose consumed [13,14,43,48,49], the latter corresponding to about a third of the theoretical yield of 12 moles. The production of 4 H 2 /hexose is regarded as the upper limit ("Thauer limit", [51]), of standard metabolisms, and increased yields require further oxidation of substrates to CO 2 .…”

“…The production of 4 H 2 /hexose is regarded as the upper limit ("Thauer limit", [51]), of standard metabolisms, and increased yields require further oxidation of substrates to CO 2 . Complete anaerobic oxidation of glucose, however, is thermodynamically difficult and is only possible at very low partial H 2 pressures [48].…”