Increase in levels of photosynthetic spectral complexes by maintaining the plasmids harboring DNA encoding puhA, pufBA, or pucBAC in trans in Rhodobacter sphaeroides resulted in decrease of the photoheterotrophic production of H-2. However, removal of B875 or B800-850 light-harvesting (LH) complexes affected H-2 production differently. Lack of B875 complex following in-frame deletion of pufBA (mutant PUF1) not only slowed photoheterotrophic growth but also decreased H2 production, indicative of the essential requirement of the complex for LH process. However, the pucBA-deleted mutant, PUC1 lacking of B800-850 complex, increased H-2 production in comparison with its parental cell by approximately twofold, given irradiated with light (10W/m(2)) saturating the growth of wild type. The H-2 production of PUC1 did not increase in proportion to the light intensity, which is also observed with wild type. Thus, we suggest that light is not limited for the HI production of the cells under the experimental conditions employed in this work, but the cellular energy to be used for the formation of B800-850 complex may flow into the metabolism leading to the H-2 production in PUC1. (c) 2005 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.