 Water splitting , in which water is decomposed into its component protons , electrons , and oxygen , occurs in the light reactions in all photosynthetic organisms . Some such organisms , including the alga <unk> <unk> and cyanobacteria , have evolved a second step in the dark reactions in which protons and electrons are reduced to form H2 gas by specialized hydrogenases in the chloroplast . Efforts have been undertaken to genetically modify cyanobacterial hydrogenases to efficiently synthesize H2 gas even in the presence of oxygen . Efforts have also been undertaken with genetically modified alga in a bioreactor . 
