Enhanced dark fermentative H2 production by agar-immobilized cyanobacterium Aphanothece halophytica

Abstract

Cell immobilization is one of the techniques used to improve H2 productivity in cyanobacteria. In this study, H2 production by immobilized cells of unicellular halotolerant cyanobacterium Aphanothece halophytica was investigated and optimized. The results showed that immobilized cells of A. halophytica had higher H2 production than free cells under nitrogen-deprived condition. Among various support material types used, agar-immobilized cells showed the highest H2 production rate. Under nitrogen deprivation, the optimal conditions of cell immobilization for H2 production were 3% (w/v) agar concentration, 0.2 mg dry cell weight per mL of gel solution, and 0.125 cm3 of agar cube. The optimum pH of medium and incubation temperature for H2 production by agar-immobilized cells were pH 7.4 and 40 °C, respectively. Using a large glass vial and headspace volume resulted in enhancement of H2 production by agar-immobilized cells. Finally, H2 production by agar-immobilized cells was analyzed for three consecutive cycles. H2 production could be maintained at the highest level after two cycles when half of immobilized cells were replaced with fresh immobilized cells. These findings indicate that the enhanced H2 production of the unicellular halotolerant cyanobacterium A. halophytica can be achieved by immobilization method, thus providing the possibility to improve H2 production by cyanobacteria in the future.