International Journal of Hydrogen Energy 31 (2006) 1496 – 1503 www.elsevier.com/locate/ijhydene Hydrogen production by Clostridium thermocellum 27405 from cellulosic biomass substrates David B. Levin a, b , Rumana Islam c , Nazim Cicek c , Richard Sparling d , ∗ a Department of Biology, University of Victoria, Victoria, BC, Canada V8W 3P6 b Institute for Integrated Energy Systems, University of Victoria, Victoria, BC, Canada V8W 3P6 c Department of Biosystems Engineering, University of Manitoba, Winnipeg, MB, Canada R3T 5V6 d Department of Microbiology, University of Manitoba, Winnipeg, MB, Canada R3T 2N2 Available online 24 July 2006 Abstract Hydrogen (H 2 ) and end-product synthesis by Clostridium thermocellum were investigated in batch cultures using cellulosic sources (-cellulose, shredded filter paper, and delignified wood fibers (DLWs)) and cellobiose at low (0.1gl -1 ), medium (1.1gl -1 ), and high (4.5gl -1 ) added substrate concentrations. Cellulosic substrates produced higher total amounts of H 2 in high substrate concentration cultures, but better H 2 yields at both low and medium substrate concentrations. DLW was shown to be an effective substrate providing an average yield of 1.6mol H 2 mol -1 glucose. Acetate, ethanol, lactate, and formate were the primary fermentation end products. Acetate yields per mole hexose were highest in low substrate concentration cultures and yields shifted toward increased lactate at high substrate concentrations. On average, the ratio of acetate to ethanol (4:3) stayed roughly constant under all growth conditions tested, while lactate, which was a minor product at the end of fermentation under low and medium sugar concentrations, represented > 30% of the organic end products at the end of the fermentation in the presence of high levels of substrate. Since these were unstirred cultures, development of H 2 supersaturation may help explain this shift. At low and medium substrate concentrations, H 2 production and yields were similar or greater in cultures containing cellulosic substrates compared with cellobiose. Overall, delignified wood was found to be the best candidate for H 2 production.  2006 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. Keywords: Clostridium thermocellum; Biohydrogen; Dark fermentation; Cellulose; Cellobiose 1. Introduction Environmental concerns about reducing greenhouse gases have stimulated the search for clean energy sources. Biologically derived organic materials and residues currently constitute a large source of waste ∗ Corresponding author. Tel.: +1 204 474 8320; fax: +1 204 474 7603. E-mail addresses: dlevin@uvic.ca (D.B. Levin), rislam1975@yahoo.com (R. Islam), Nazim_Cicek@umanitoba.ca (N. Cicek), Richard_Sparling@umanitoba.ca (R. Sparling). 0360-3199/$30.00  2006 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2006.06.015 biomass [1]. Use of these biomass-rich resources for bioenergy and related bioproducts could contribute to the displacement of fossil fuels as our primary energy source and could reduce GHG emissions. Hydrogen (H 2 ) is a clean bioenergy substitute for fossil fuels, pro- ducing water as its only by-product when it burns. The higher heating value of hydrogen is 3042 cals m -3 , the highest of all known fuels [2,3]. Currently 90% of H 2 is being produced from methane reformation or electrol- ysis of water and contributes to only 3% total energy consumption. These processes are both energy intensive and not environment-friendly [2].