Hydrogen production from sugars and complex biomass by Clostridium species, AK14, isolated from Icelandic hot spring

The hydrogen production ability of strain AK14 a moderate thermophilic bacterium was studied. According to 16S rRNA gene sequence analysis the strain belongs to genus Clostridium but is most closely related to Anaerobacter polyendosporus (95.1% similarity). Growth of strain AK₁₄ was observed at temperatures between 42 and 52°C with optimal growth at 50°C. Optimum pH for growth was at pH 6.5 but growth was observed from pH 4.5 to 7.5. Fermentation of glucose resulted in the production of acetate and butyrate (major) and ethanol (minor) as well as hydrogen and carbon dioxide. Effect of increased substrate (glucose) concentration was investigated and good correlation was observed between increased substrate loadings and end product formation up to 50 mM. At ≥ 50 mM glucose concentrations, end product formation levelled off and the substrate was not completely degraded. Also the pH at the end of fermentation dropped from 7.0 (control without glucose) to 5.3 (at ≥ 50 mM glucose). The ability to utilize various carbon substrates was tested, with positive growth observed on xylose, glucose, fructose, mannose, galactose, starch and xylan. The end products in all cases were the same as for glucose. By varying the liquid to gas phase during glucose fermentation under batch conditions a clear correlation was found between increased acetate, butyrate and hydrogen production at low gas-to-liquid conditions and less ethanol. Amount and identification of the end product formation from hydrolysates made of lignocellulosic (5.0 g L^-1 [dw]) biomass cellulose (from Whatman paper), newspaper, grass (Phleum pratense), barley straw (Hordeum vulgare), and hemp (stem and leaves of Cannabis sativa), was investigated. The biomass was pretreated with either a weak acid (HCl) or a weak base (NaOH) as well as enzymes (Celluclast® and Novozyme 188). The strain produced the most hydrogen (8.5 mol H₂ g^-1 VS) from cellulose hydrolysate but less from paper and lignocellulosic biomasses (between 0.26 to 3.60 mol H₂ g^-1 VS). The hydrogen production from lignocellulosic biomass was however enhanced significantly by acid and base pretreatment with the highest production from grass (6.23 mol H₂ g^-1 VS). Other end products were acetate, butyrate and ethanol.