Hydrogen production from switchgrass by thermophilic isolates with consolidated bioprocessing capabilities

Hydrogen (H₂) is considered the energy of the future and an ideal alternative fuel to the current energy scenario due to its high energy content (143 MJ/Kg and non-polluting nature.  Despite the many efforts to improve H₂ production via dark fermentation, challenges related to complex substrate utilization and low H₂ yields on glucose still preclude the scale-up of this process. In addition, glucose as a primary feedstock is not considered a cost-competitive as current market prices of glucose ($440-600/MT) are approximately 10-fold higher than the cost of lignocellulosics ($39-$60/MT). Hence, development of a cost-competitive large-scale H₂ production system requires the use of low-cost lignocellulosic biomass, such as switchgrass (SWG), utilizing robust and highly efficient microbes with consolidated bioprocessing (CBP) capabilities. In this work, the H₂ production capabilities of the extreme thermophile Caldicellulosiruptor saccharolyticus and other thermophilic isolates from Yellowstone National Park were examined on SWG using glucose and microcrystalline cellulose (MCC) as references. It was demonstrated that C. saccharolyticus can ferment untreated SWG in a single step to produce 11.2 mmol H₂/g SWG. In addition, bioH₂ production from glucose reached the theoretical maximum for dark fermentation of 4 mol H₂/mol glucose. The CBP approach presents opportunities for cost savings of capital and operational expenses in excess of 50%. Advantages of bioprocessing low-cost carbon feedstocks without any physicochemical or biological pretreatment by thermophilic microorganisms for bioH₂ production will be discussed.