Rhodobacter sphaeroides O.U.001 is a photosynthetic non-sulfur bacterium which produces hydrogen from organic compounds under anaerobic conditions. Halobacterium salinarum is an archaeon and lives under extremely halophilic conditions (4 M NaCl). H. salinarum contains a retinal protein bacteriorhodopsin in its purple membrane which acts as a light-driven proton pump. In this study the Rhodobacter sphaeroides O.U.001 culture was combined with different amounts of packed cells of H. salinarum S9 or isolated purple membrane fragments in order to increase the photofermentative hydrogen gas production. The packed cells of H. salinarum have the ability to pump protons upon illumination due to the presence of bacteriorhodopsin. The proton gradient produced may be used for the formation of ATP or protons may be used for H-2 production by R. sphaeroides. Similar to intact cells purple membrane fragments may also form vesicles around certain ions and may act like closed systems. The hydrogen production experiments were carried out using 400 ml water-jacketed-glass column stirred photobioreactors. In combined systems 10-200 nmol of bacteriorhodopsin was used. Hydrogen gas production was enhanced by four- to sixfold in combined systems of H. salinarum packed cells with R. sphaeroides O.U.001 cell. Stirring both increased the total gas produced and enhanced the rate of hydrogen production. The light energy conversion efficiency was increased from 0.6% to 2.25% in combined systems. (c) 2006 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.