161 



R. P. Levine 



Wild type cells fix carbon dioxide by photoreduction after being adapted 

 to hydrogen in the dark for a period of 15 minutes. The maximuna rate of 

 photoreduction was obtained at a light intensity of about 1500 lux. At intensities 

 greater than 2000 lux the cells reverted to photosynthesis. However, photo- 

 reduction was obtained at a light intensity of 10, 000 lux in the presence of 1 x 

 10"^M DCMU. At this concentration DCMU causes a 99 per cent inhibition of 

 photosynthetic oxygen evolution and carbon dioxide fixation. 



At 1500 lux the mutant strains ac-115 and ac-141 gave rates of carbon 

 dioxide fixation and hydrogen consumption comparable to the wild type rate, 

 whereas the rates of ac-21 and ac- 208 were negligible. The addition of DCMU 

 had little effect upon photoreduction by ac-115 and ac-141, and photoreduction 

 was obtained at 10, 000 lux in the absence of DCMU. 



Both ac- 1 15 and ac- 141 resemble a mutant strain of Scenedesmus de- 

 scribed by Bishop^^"^) that can carry out photoreduction but does not evolve 

 oxygen by photosynthesis and has no Hill reaction with p-benzoquinone. 



The Hill reaction 



Further differences between the mutant strains were revealed in a study 

 of the Hill reaction both by whole cells and chloroplast fragments^ \ As 



seen in Table I, ac-115 and ac-141 have no Hill reaction with a variety of Hill 

 oxidants; ac-21 and ac-208 show Hill reaction activity, except that the latter 

 does not have activity when the oxidant is potassium ferricyanide. 



Table I 



Hill reaction, TPN photoreduction, and photophosphorylation by 

 wild type and mutant strains of C. reinhardi 



Hill reaction* TPN reduction* * Photophosphorylation***" 



Vitamin DPIP & 

 Strain DPIP Cyt c FeCy a+ b PMS K3&FMN Ascorbate 



wild type 55.2 28.8 324 35.4 7.8 124 50 8 



* fimoles Hill oxidant reduced/hr/mg chlorophyll 



** ^imoles TPNH/hr/mg chlorophyll 

 *** /Ltmoles Pi esterified/hr/mg chlorophyll 



+ a signifies TPN reduction when the source of electrons is from water and 



b signified TPN reduction when the source of electrons is from DPIP and 

 ascorbate 



