In seawater medium, H2 evolution began after 0.5 h illumination and H2 evolution rate reached a peak value of 900 nl h−1 after ca. 2.5 h of illumination. Thereafter, H2 evolution rate exhibited a time-dependent decline and became undetectable after 5 h illumination. When sulphur was removed from the seawater medium, a remarkable increase in H2 evolution rate was observed, suggesting that sulphur plays a key role in modulating H2 production from P. subcordiformis. The maximum H2 evolution rate from S-deprived seawater medium was 11,720 nl h−1, a 13-fold increase when compared with that from the seawater medium. The accumulated H2 volume in seawater medium and S-deprived seawater was 3000 and 39,000 nl, respectively.