Tetraselmis subcordiformis (formerly called Platymonas subcordiformis), a marine green alga, was previously demonstrated to photo-biologically produce hydrogen when treated with CCCP (Carbonyl Cyanide m-Chlorophenylhydrazone). The current results of our studies showed that there was a peak yield of hydrogen by T. subcordiformis at the stationary stage of its algal growth curve, suggesting that starvation of an essential element induced biochemical changes that enhanced hydrogen production by T. subcordiformis. Further investigation indicated that among nitrogen, sulphur and phosphorus (which are major components in the cultivation medium), nitrogen deprivation in the medium reduced the protein content inside the cells of T. subcordiformis, but increased the carbohydrate content over 4 times, resulting in 5.5 times increase in the hydrogen yield. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.