Enhanced hydrogen production by Nostoc sp. CU2561 immobilized in a novel agar bead

Abstract

A large number of microalgae isolated from Thailand were screened for their hydrogen production capacity. The selected highly efficient microalga, identified as Nostoc sp. CU2561, was investigated for the conditions under which the cells had maximal hydrogen production rate. Nostoc sp. CU2561 showed highest hydrogen production rate when grown in BG11 medium deprived of nitrogen and sulfur (BG11-N-S). To further improve hydrogen production, newly invented agar beads were used as matrix for cell immobilization. The highest hydrogen production rate by 1.5% (w/v) agar bead immobilized cells was obtained using cell concentration of 0.125 mg dry wt mL−1. Agar bead–immobilized cells showed superior hydrogen production rate, 1.5-fold higher when compared with agar cube–immobilized cells, and 5- and 10-fold higher when compared with those in alginate bead–immobilized and alginate bead–suspended cells, respectively. Supplementation of 0.5% (w/v) fructose increased hydrogen production rate of agar bead–immobilized cells approximately 1.7-fold, whereas the reducing agent β-mercaptoethanol increased hydrogen production rate by about 8.2-fold. Overall, Nostoc sp. CU2561 immobilized in agar bead showed the highest hydrogen production rate when incubated in BG11-N-S containing 5 mM β-mercaptoethanol with the highest hydrogen production rate of 18.78 ± 1.44 μmol H2 mg−1 chl a h−1. In addition, agar bead–immobilized cells could continuously produce hydrogen for 3 cycles. The hydrogen production by immobilized cells could be prolonged up to 120 h during the first cycle. This study provides a potential of new immobilization strategy using cyanobacteria immobilized in agar bead for hydrogen production which can be applied to scale up at industrial level in the future.