CONTROL OF PHOTOSYNTHETIC REDUCTANT: THE ROLE OF LIGHT AND TEMPERATURE ON SUSTAINED HYDROGEN PHOTOEVOLUTION BY Chlamydomonas sp. IN AN ANOXIC, CARBON DIOXIDE‐CONTAINING ATMOSPHERE*

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Abstract
–Sustained hydrogen photoevolution fromChlamy domonas reinhardtiiandC. Moewusiiwas measured under an anoxic, CO2‐containing atmosphere. It has been discovered that light intensity and temperature influence the partitioning of reductant between the hydrogen photoevolution pathway and the Calvin cycle. Under low incident light intensity (1‐3 W m‐2) or low temperature (approx. 0°C), the flow of photosynthetic reductant to the Calvin cycle was reduced, and reductant was partitioned to the hydrogen pathway as evidenced by sustained H2photoevolution. Under saturating light (25 W m‐2) and moderate temperature (20±5°C), the Calvin cycle became the absolute sink for reductant with the exception of a burst of H2occurring at light on. This burst of H2corresponded to the expression of about 450 electrons for each photosynthetic electron transport chain. These results suggest that the hydrogen pathway and the Calvin cycle compete for reductant under anoxic conditions and that partioning between the two pathways can, to a certain extent, be controlled by the appropriate choice of experimental conditions.