Industrial production of phototrophic microorganisms is often hindered by low productivity due to limited light availability and therefore requires large land areas. This letter demonstrates that supply of hydrogen gas (H-2) increases in phototrophic biomass productivity compared to a culture growing on light only. Experiments were performed growing Synechocystis sp. in batch bottles, with and without H-2 in the headspace, which were exposed to light intensities of 70 and 100 mu mol/m(2)/s. At 70 mu mol/m(2)/s with H-2, the average increase in biomass was 96 mg DW/L/d, whereas at 100 mu mol/m(2)/s without H-2, the average increase in biomass was 27 mg DW/L/d. Even at lower light intensity, the addition of H-2 tripled the biomass yield compared to growth under light only. Photoreduction and photosynthesis occurred simultaneously, as both H-2 consumption and O-2 production were measured during biomass growth. Photoreduction used 1.85 mmol of H-2 to produce 1.0 mmol of biomass, while photosynthesis produced 1.95 mmol of biomass. After transferring the culture to the dark, growth ceased, also in the presence of H-2, showing that both light and H-2 were needed for growth. A renewable H-2 supply for higher biomass productivity is attractive since the combined efficiency of photovoltaics and electrolysis exceeds the photosynthetic efficiency.