The effects of external factors on both H(2) production and bidirectional Hox-hydrogenase activity were examined in the non-N(2)-fixing cyanobacterium Synechocystis PCC 6803. Exogenous glucose and increased osmolality both enhanced H(2) production with optimal production observed at 0.4% and 20 mosmol kg(-1), respectively. Anaerobic condition for 24 h induced significant higher H(2)ase activity with cells in BC11(0) showing highest activities. Increasing the pH resulted in an increased Hox-hydrogenase activity with an optimum at pH 7.5. The Hox-hydrogenase activity gradually increased with increasing temperature from 30 degrees C to 60 degrees C with the highest activity observed at 70 degrees C. A low concentration at 100 mu M of either DTT or beta-mercaptoethanol resulted in a minor stimulation of H(2) production. beta-Mercaptoethanol added to nitrogen- and sulfur-deprived cells stimulated H(2) production significantly. The highest Hox-hydrogenase activity was observed in cells in BG11(0)-S-deprived condition and 750 mu M beta-mercaptoethanol measured at a temperature of 70 degrees C; 14.32 mu mol H(2) mg chl alpha(-1) min(-1). (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.