THE ROLE OF CARBON-DIOXIDE IN LIGHT-ACTIVATED HYDROGEN-PRODUCTION BY CHLAMYDOMONAS-REINHARDTII

Light-activated hydrogen and oxygen evolution as a function of CO2 con centration in helium were measured for the unicellular green alga Chla mydomonas reinhardtii. The concentrations were 58, 30, 0.8 and 0 ppm C O2. The objective of these experiments was to study the differential a ffinity of CO2/HCO3- for their respective Photosystem II and Calvin cy cle binding sites vis-A-vis photoevolution of molecular oxygen and the competitive pathways of hydrogen photoevolution and CO2 photoassimila tion. The maximum rate of hydrogen evolution occurred at 0.8 PPM CO2, whereas the maximum rate of oxygen evolution occurred at 5 8 ppm CO2. The key result of this work is that the rate of photosynthetic hydroge n evolution can be increased by, at least partially, satisfying the Ph otosystem II CO2/HCO3- binding site requirement without fully activati ng the Calvin-Benson CO2 reduction pathway. Data are presented which p lot the rates of hydrogen and oxygen evolution as functions of atmosph eric CO2 concentration in helium and light intensity. The stoichiometr ic ratio of hydrogen to oxygen changed from 0.1 at 58 ppm to approxima tely 2.5 at 0.8 ppm. A discussion of partitioning of photosynthetic re ductant between the hydrogen/hydrogenase and Calvin-Benson cycle pathw ays is presented.