Effect of medium composition on biohydrogen production by the extreme thermophilic bacterium Caldicellulosiruptor saccharolyticus

Up to now, the analysis of the effects of medium composition on biohydrogen production of Caldicellulosiruptor saccharolyticus was focused mainly on salt concentrations and complex compounds. Within this work we studied the effects of the presence of organic and/or inorganic nitrogen in the medium composition aiming to induce metabolic changes in C. saccharolyticus   to improve its hydrogen evolution rate (HER) and hydrogen specific productivity (qH₂)$\left(q_{{\text{H}}_2}\right)$. Biohydrogen productivities and hydrogen to substrate yield (Y₍H_2/s₎)$\left(Y_{\left({\text{H}}_2/\text{s}\right)}\right)$ of C. saccharolyticus   on xylose in batch mode were higher working in a complex medium than in a defined one; but no significant difference could be settled according to hydrogen to carbon dioxide yields (Y₍H_2/CO₂₎)$\left(Y_{\left({\text{H}}_2/{\text{CO}}_2\right)}\right)$. The specific growth rate of C. saccharolyticus on complex medium was settled at 0.1 h⁻¹ operating in chemostat mode to achieve the highest H₂-productivities under stable conditions. In chemostat mode on xylose, a reduction of the ammonium feed concentration in a defined medium until N-limiting conditions involved higher qH₂$q_{{\text{H}}_2}$ comparing with a straight C-limiting growth.