The role of light intensity on biohydrogen production from glucose by Clostridium beijerinckii, Clostridium acetobutylicum, and Rhodobacter sphaeroides was studied to evaluate the performance and possible application in co-culture fermentation system. The applied source of light had spectrum similar to the solar radiation. The influence of light intensity on hydrogen production in dark process by C. acetobutylicum was negligible. In contrast, dark fermentation by C. beijerinckii bacteria showed a significant decrease (83%) in produced hydrogen at light intensity of 540 W/m(2). Here, the redirection of metabolism from acetic and butyric acid formation towards lactic acid was observed. This not yet reported effect was probably caused by irradiation of these bacteria by light within UVA range, which is an important component of the solar radiation. The excessive illumination with light of intensity higher than 200 W/m(2) resulted in decrease in hydrogen production with photofermentative bacteria as well. (C) 2015 Elsevier Ltd. All rights reserved.