Using the technique of in situ photoprecipitation, a comparative study of the structurally homologous ionic species hexachloroplatinate ([PtC&12-), hexachloroosmiate ( [OsC&12-), and hexachloroiridates was conducted for photoconversion to metallocatalysts for photosynthetic hydrogen evolution. As with earlier [PtC1612-studies, [osc16l2-can be photoconverted to a metallocatalyst at photosystem I (PSI), although at a rate about 50% slower than that of [PtC&]2-. However, once photoprecipitated, its catalytic action for H2 production was 3 times as high as that of metallic platinum. Simultaneous photoevolution of 02 and H2 was observed in [osc16l2--photoprecipitated thylakoids. Maximum hydrogen evolution rate was 113 nmol-h-'.mg chl-'. Surprisingly, neither [IrC&12--nor [IrC&13-treated thylakoids were able to produce H2. Analysis indicated that [IrC1612-was able to accept only one electron by transformation to [IrC&]3-which was completely unable to acquire subsequent electrons from PSI. The inability of [IrC&l" to be reduced to metallic iridium is presumably due to a high energy level barrier of [IrC&]3-reduction.
Contrary to the prediction of the Z-scheme model of photosynthesis, experiments demonstrated that mutants of Chlamydomonas containing photosystem II (PSII) but lacking photosystem I (PSI) can grow photoautotrophically with O2 evolution, using atmospheric CO2 as the sole carbon source. Autotrophic photosynthesis by PSI-deficient mutants was stable both under anaerobic conditions and in air (21 percent O2) at an actinic intensity of 200 microeinsteins per square meter per second. This PSII photosynthesis, which was sufficient to support cell development and mobility, may also occur in wild-type green algae and higher plants. The mutants can survive under 2000 microeinsteins per square meter per second with air, although they have less resistance to photoinhibition.
–Sustained hydrogen photoevolution from Chlamy domonas reinhardtii and C. Moewusii was 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 H2 photoevolution. 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 H2 occurring at light on. This burst of H2 corresponded 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.
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