The hydrogen production by Chlorella vulgaris exposed to four different light patterns was explored. Cultures of immobilized, sulfur-deprived cells were purged with N(2) and either kept in the dark for 72 h, kept in the dark for 24 h before being exposed to light (intensity of 120 mu mole/m(2)/s) for 72 h, exposed to light for 72 h, or kept in the light for 24 h before being subjected to darkness for 48 h. The latter condition provided for the highest total hydrogen production of 530 +/- 5 ml/l of medium and a maximum hydrogen release rate of 34.8 ml/h/l. Growth for 72 h under partial light conditions was essential for continuous and enhanced hydrogen production. The addition of glucose to the sulfur-deficient medium increased hydrogen production by 18 times under partial light condition. To increase the hydrogen productivity, a carbon source may be added to enhance hydrogen production. (C) 2010 Elsevier Ltd. All rights reserved.