Hydrogen is a clean energy alternative to fossil fuels. Photosynthetic bacteria produce hydrogen from organic compounds under anaerobic, nitrogen-limiting conditions through a light-dependent electron transfer process. In this study, the hydrogen production efficiency of phototrophic bacteria, Rhodobacter capsulatus and its Hup mutant strain (an uptake hydrogenase deleted strain) were tested on different initial acetate concentrations at fluctuating temperatures with indoor and outdoor photobioreactors. Acetate was effectively metabolized and H(2) was produced at a high rate. Increasing the initial acetate concentration resulted in a shift in the utilization kinetics of acetate from first order to second order. The effects of fluctuating temperature and day/night cycles on hydrogen production were also studied in indoor and outdoor conditions using acetate as the carbon source. Temperature fluctuations and day/night cycles significantly decreased hydrogen production. It was found that the Hup mutant strain of R. capsulatus has better hydrogen productivity than the wild type parent in outdoor conditions. (C) 2009 Elsevier B.V. All rights reserved.