To identify optimal hydrogen production conditions using growing cultures of Rhodobacter sphaeroides DSM 158 the effects of varying the reactor's volumetric power input (0.01-1.4 kW m(-3)) and irradiation intensity (5-2500 Wm(-2)) were investigated in batch and continuous production modes. Irradiation intensity had a greater effect on hydrogen production than volumetric power input. Hydrogen production and photofermentative biomass formation were maximized by irradiation at 2250 W m(-2) with a volumetric power input of 0.55 kW m(-3). The bacterial dry weight (2.64 g L-1) and rate of hydrogen production (195 mL L-1 h(-1)) achieved under these conditions were greater than any that have previously been reported for batch-mode hydrogen production by R. sphaeroides. Continuous mode experiments (D = 0.1 h(-1)) yielded a bacterial dry weight, hydrogen production rate, productivity and hydrogen yield of 2.35 +/- 0.18 g L-1, 165 +/- 6.2 mL L-1 h(-1), 3.96 L L-1 d(-1) and 36.6%, respectively. (C) 2014 Elsevier Ltd. All rights reserved.