A marine green microalga, Platymonas subcordiformis, photo-synthetically generates H-2 but only transiently at a negligible yield when exposed to light after a period of dark anaerobic incubation. A protonophore uncoupler, carbonyl cyanide m-chlorophenylhrazone ( CCCP) significantly increased the yield of H2 photo-production. CCCP optimally at 15 muM gave 4.9 ml H-2 after 8 h light irradiation in 1 l algal cell culture at 1.8 x 10(6) cells ml(-1). The H-2 yield at 15 muM CCCP was increased by 240-fold when compared to the control. This improvement may be by CCCP disrupting the proton motive force thus facilitating proton transfer across the thylakoidal membrane.