Effects of cell density, carbon dioxide and molybdenum concentration on biohydrogen production by Anabaena variabilis ATCC 29413

• Low concentration of CO2 in headspace (4–8% vol/vol) enhanced hydrogen productivity.
• Glucose addition did not cause a high oxygen build-up in the headspace.
• Hydrogen productivity was highly optimized at elevated Mo6+ concentration of 1.6 mM.

This paper aims to determine the effects of cell density, carbon dioxide, and molybdenum concentration towards hydrogen production rate. Batch cultures of Anabaenavariabilis sp. were incubated in anaerobic environment under continuous indoor illumination of 70 μE m−2 s−1 at 35 °C. The optimal volumetric hydrogen production rate obtained was 44 μmol H2 mg chl a−1 h−1 occurred at cells density of 110 mg L−1, 5% carbon dioxide headspace concentration, and molybdenum concentration of 1.6 mM. The effect of organic carbon source (glucose) was also evaluated in the present study and it was found that the additional carbon produced the highest hydrogen production rate in all conditions. An increased concentration of molybdenum significantly enhanced the hydrogen productivity rate almost to that of glucose-supplemented culture at 49 μmol H2 mg chl a−1 h−1. However, further increase in molybdenum concentration beyond 1.6 mM showed no further improvement in the amount of hydrogen produced.