Effects of Light Intensity and Oxidized Nitrogen Sources on Hydrogen Production by Chlamydomonas reinhardii

Chlamydomonas reinhardii cells, after a period of dark anaerobic adaptation, evolve H(2) not only in the dark but also in the light. Our results show that high irradiances impair prolonged H(2) evolution, while under low irradiances or darkness H(2) evolution proceeds for more than 50 hours. NO(3) (-) and NO(2) (-) suppress H(2) evolution both in the dark or under low irradiance. Apparently the cells prefer these oxidized nitrogen sources to protons as electron acceptors, since both NO(3) (-) and NO(2) (-) become reduced to NH(4) (+), which is excreted to the culture medium in high amounts. H(2) evolution started once these oxidized anions were largely depleted from the medium. Moreover, H(2) evolution was consistently associated with NH(4) (+) excretion even if NH(4) (+) was already present in high amounts in the medium. This observation indicates that the cells utilize not only their carbohydrate but also their protein reserves as sources of reducing power for H(2) evolution. This conclusion was supported by the observation that when nitrogen-starved cells were made anaerobic in a nitrogen-free medium, they not only evolved H(2) at very high rates but excreted concomitantly NH(4) (+) up to concentrations in the millimolar range.