Thermophilic consolidated bioprocessing for sustainable hydrogen production

Hydrogen (H₂) is considered the “energy of the future” due to its non-polluting nature and high energy content (143 MJ/Kg) which is 3.3-fold higher than that of gasoline. An economically-viable, large-scale production of bio-H₂ would require development of advanced H₂-producing technologies like Consolidated Bioprocessing (CBP). CBP has been proposed as the most cost-efficient and ultimate industrial configuration for low-cost hydrolysis and fermentation of cellulosic biomass with average cost saving of 50%. CBP of lignocellulosic feedstock for H₂ production using thermophiles offers several techno-economic advantages: increased conversion rates due to improved mass transfer rates, substrate accessibility and solubility at elevated temperatures; favorable thermodynamics of stochiometric H₂ yields at higher temperatures; diminished possibility of contamination by unwanted microorganisms; reduced formation of by-products. The CBP H₂ production capabilities of the extreme thermophile Caldicellulosiruptor saccharolyticus DSM 8903 were examined on switchgrass (SWG) and municipal solid waste (MSW), with glucose and microcrystalline cellulose (MCC) serving as references. H₂ production from glucose reached the theoretical maximum for dark fermentation of 4 mol H₂/mol glucose. The H₂ yield on MCC, SWG and MSW peaked at 9.4, 11.2, and 3.3 mmol H₂/g substrate, respectively. C. saccharolyticus was able to produce H₂ from mechanically-comminuted SWG directly without any physicochemical or biological pretreatment. As pretreatment is the single most cost-intensive processing step in biomass bioconversion (25% of total costs on average), combining four processing steps (pretreatment, enzyme production, saccharification, fermentation) into a single biorefinery operation makes C. saccharolyticus a promising CBP candidate for cost-efficient, environmentally-friendly and sustainable production of H₂.