FLASH YIELD OF BACTERL\ AND HYDROGEX-ADAPTED ALGAE 1481 



reason for the effect of long dark intervals on the flash yield in some or- 

 ganisms (or under certain conditions, such as cyanide poisoning). One 

 could extend the Weller-Franck hypothesis to the Hill reaction by assum- 

 ing that the supply of the oxidant (ferricyanide, or an intermediate in its 

 reduction) can become rate-limiting under certain conditions. Gilmour 

 et al. offered a different concept. They suggested that the occurrence, 

 after long and intense flashes of one (or two) zero-order hmiting reactions, 

 together with a (temperature independent) first-order reaction different 

 from that which determines the yield of short flashes, can be interpreted 

 by assuming a "reservoir" in which a part of the energy-rich products, 

 formed in the flash, can be reversibly stored, escaping the back reaction and 

 permitting the "Emerson-Arnold enzyme" to operate more than once in 

 the wake of a single flash. Gilmour et al. elaborated a chemical mechanism 

 for the filling and emptying of the "reservoir" which could justify the 

 postulate that the reservoir has no effect on the yield of brief flashes (or 

 on the rate in continuous light) but increases the yield of "long" flashes 

 of the same total energy. In essence, this mechanism amounts to a com- 

 bination of the Franck-Emerson-Arnold reaction system with a side reac- 

 tion feeding into a reservoir, from which the Emerson-Arnold system can 

 draw after a flash. If the reaction feeding into the reservoir is enzymatic, 

 the amount of photoproducts drained into the reservoir during the flash 

 will depend not only on the integrated intensity of the flash, but also on its 

 duration (as in Tamiya's model). The addition to the flash yield, provided 

 by the stored photoproducts, ^vill then increase with the duration of the 

 flash, and can be expected to depend also on temperature. The Emerson- 

 Arnold reaction will nevertheless remain the bottleneck through which all 

 photoproducts must pass, and which determines both the yield in steady 

 light and the maximum yield of instantaneous flashes. 



Further details of the reservoir filling and emptying mechanism (includ- 

 ing two catalysts, one of which is photochemically activated) were sug- 

 gested by Gilmour et al. to account for the above-mentioned complex fea- 

 tures of the flash yield-dark time curves ; experiments were also made (and 

 interpreted by the same model) on the flash yield in chloroplast preparations 

 partially inactivated by heat, cold, or ultraviolet light. 



8. Flashing-Light Experiments with Bacteria and Hydrogen-Adapted 



Algae 



The only result available on the photosynthesis of 'purple bacteria in 

 flashing light is the observation of Arnold, quoted by van Niel (1941) and 

 recorded in Table 32.1, that the ratio P"'^''7BChlo is of the order of 1/400. 

 This result can be interoreted as evidence that the limiting catalyst, E^. 



