A marine green alga, Platymonas subcordiformis, was demonstrated to photobiologically evolve hydrogen (H2) after the first stage of photosynthesis, when subjected to a two-phase incubation protocol in a second stage of H2 production: anaerobic incubation in the dark followed by the exposure to light illumination. The anaerobic incubation induced hydrogenase activity to catalyse H2 evolution in the following phase of light illumination. H2 evolution strongly depended upon the duration of anaerobic incubation, deprivation of sulphur (S) from the medium and the medium pH. An optimal anaerobic incubation period of 32 h gave the maximum H2 evolution in the second phase in the absence of sulphur. Evolution of H2 was greatly enhanced by 13 times when S was deprived from the medium. This result suggests that S plays a critical role in the mediation of H2 evolution from P. subcordiformis. A 14-fold increase in H2 production was obtained when the medium pH increased from 5 to 8; with a sharp decline at pH above eight. H2 evolution was enhanced by 3050% when supplementing the optimal concentrations of 25mM acetate and 37.5mM glucose. 便攜式車載氣相色譜儀