Abstract
Nitrogenase produces hydrogen as an inevitable byproduct of the reaction. In cynobacterial nitrogenase-based photobiological hydrogen production, the presence of hydrogenase seems to decrease the productivity by reabsorbing the hydrogen. Many cyanobacteria have both uptake (Hup) and bidirectional hydrogenase (Hox). Nostoc sp. PCC 7422 has both hup and hox genes, but its Hox activity is extremely low. We have constructed ΔhupL mutant and found that the hydrogen production (120 μmoles H2/mg Chl/hr) of the mutant was 3 times that of the wild-type. Because no significant change was observed in nitrogenase between these two, the above difference is attributed to inactivation of reabsorption in the mutant. The mutant cells accumulated hydrogen to about 30% (v/v). The maximum level was affected by gas phase, cell density, cell culture etc. After hydrogen accumulation, the culture started to produce hydrogen although at a lower rate when the gas phase was renewed to Ar.
