738 



John D. Spikes and Dennis C. Hall 



Chlorella during gamma irradiation, with Hill reaction activity 

 being protected to a slightly greater extent than photosynthetic 

 activity. 



Chlorophyll destruction in gamma- irradiated algae was rela- 

 tively very much less than the inactivation of Hill reaction and 

 photosynthetic activities. Therefore decrease in activity covild 

 not be correlated in any simple way -with the loss of bulk 

 chlorophyll. The extractability of chlorophyll from the algae 

 into 95^ methanol was increased by gamma irradiation. Chloro- 

 phyll in solution in 95^ methanol was much more sensitive to 

 destruction by gamma radiation than chlorophyll in situ in the 

 algal cell. 



The Hill reaction and photosynthetic activities of both 

 i^Ltraviolet and gamma- irradiated algae were retained during 

 post- irradiation storage in much the same way as in unirradiated 

 algae. Activities were lost rapidly during storage in the dark 

 at higher temperatures, and slowly in the light at low tempera- 

 tures. There was no evidence of photoreactivation in gamma- 

 irradiated algae. 



DISCUSSION 



The present work demonstrates the photodynamic inactivation of 

 Chlorella photosynthesis and Hill reaction in the presence of 

 certain dyes. These dyes presumably penetrate through the cell 

 membrane and into the chloroplast in order to act as sensitizers. 

 The apparent lack of activity of dues such as Rhodamine B, which 

 have been shown to photosensitize other systems, may thus result 

 from slow penetration into Chlorella cells rather than from lack 

 of activity. Some dyes inhibited photosynthetic activity by 

 dark reactions; Hill reaction activity was much less sensitive. 

 Presumably this inhibition results from the ability of these 

 dyes to associate or react with celliilar constituents involved 

 in carbon dioxide- fixation pathways. Such dyes might be useful 

 specific inhibitors of particular steps in these pathways. 



The study of photodynamic processes may be justified in the 

 context of photosynthetic mechanism from several points of view. 

 First, a number of workers have demonstrated that chlorophyll- 

 sensitized endogenous photodynamic processes occur in photo- 

 synthetic plants under certain conditions. Plants therefore 

 survive exposure to light only by possessing devices to minimize 

 photodynamic action. Secondly, there may be useful analogies 

 betvAeen simple photodynamic systems and photosynthesis which 



