A novel unicellular hydrogen-producing green alga was isolated from fresh water pond in Pathumthani province, Thailand. Under light microscope, this alga was identified as belonging to the genus Tetraspora. Phylogenetic analysis of 18S rRNA sequence revealed that the green alga, identified as Tetraspora sp. CU2551, is closely related to other unicellular green algal species. Tetraspora sp. CU2551 had the shortest doubling time when grown in Tris-acetate-phosphate (TAP) medium under a light intensity of 48-92 mu E/m(2)/s and a temperature of 36 C. Hydrogen production increased with increasing pH from 5.75 to 9.30; however, almost no production was observed at a pH of 5.25. Addition of 0.5 mM P-mercaptoethanol to the TAP medium stimulated hydrogen production about two-fold. During the hydrogen production phase, the use of TAP medium lacking both nitrogen and sulfur resulted in about 50% increase in the hydrogen production. This was in contrast to only a small increase in the production when either nitrogen or sulfur was omitted in TAP medium. The stimulation of hydrogen production by 0.5 mM beta-mercaptoethanol under nitrogen- and sulfur-deprived conditions occurred only when the cells were grown at a light intensity lower than 5 mu E/m(2)/s with no effects at higher intensities. Maximal calculated hydrogen production, 17.3-61.7 mu mol/mg Chl a/h, is a very high production rate compared to other green algae and makes Tetraspora sp. CU2551 an interesting model strain for photobiological hydrogen production. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.