This study reports a factor 5.5 increase in hydrogen production by Anabaena Variabilis ATCC 29413 using Allen-Arnon medium compared with BG-11 and BG-11(0) media. The results were obtained with a flat panel photobioreactor made of acrylic and operated in two stages at 30 degrees C. Stage 1 aims at converting carbon dioxide into biomass by photosynthesis while Stage 2 aims at producing hydrogen. During Stage 1, the photobioreactor was irradiated with 65 mu mol/m(2)/s (14W/m(2)) of light and sparged with a mixture of air (95% by volume) and carbon dioxide (5% by volume). During Stage 2, irradiance was increased to 150 mu mol/m(2)/s (32W/m(2)) and the photobioreactor was sparged with pure argon. The parameters continuously monitored were (1) the cyanobacteria concentration, (2) the pH, (3) the dissolved oxygen concentration, (4) the nitrate and (5) the ammonia concentrations in the medium, and (6) the hydrogen concentration in the effluent gas. The three media BG-11, BG-110, and Allen-Arnon were tested under otherwise similar conditions. The maximum cyanobacteria concentrations during Stage 2 were 1.10 and 1.17 kg dry cell/m(3) with BG-11 and Allen-Arnon media, respectively, while it could not exceed 0.76 kg dry cell/m(3) with medium BG-110. Moreover, the heterocyst frequency was 5%, 4%, and 9% for A.variabilis grown in BG-11, BG-110, and Allen-Arnon media. The average specific hydrogen production rates were about 8.0 x 10(-5) and 7.2 x 10(-5) kg H-2/kg dry cell/h (1 and 0.9 LH2/kg dry cell/h at 1 atm and 30 degrees C) in media BG-11 and BG-110, respectively. In contrast, it was about 4.5 x 10(-4) kg H-2/kg dry cell/h (5.6 L H-2/kg dry cell/h at 1 atm and 30 degrees C) in Allen-Arnon medium. The maximum light to hydrogen energy conversion efficiencies achieved were 0.26%, 0.16%, and 1.32% for BG-11, BG-110, and Allen-Arnon media, respectively. The larger heterocyst frequency., specific hydrogen production rates, efficiencies, and cyanobacteria concentrations achieved using Allen-Arnon medium were attributed to higher concentrations of magnesium, calcium, sodium, and potassium in the medium. Finally, presence of vanadium in Allen-Arnon medium could have induced the transcription of vanadium based nitrogenase which is capable of evolving more hydrogen than molybdenum based one. (C) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.