ULTRAVIOLET LIGHT 



345 



There is no doubt that in the spectral region used by Richter 

 (<300m/i) any but a very short exposure is fatal to the organism in 

 general and to the photosynthetic apparatus in particular. 



Meier (1936) has studied the action of ultraviolet rays of different 

 wave length and intensity on Chlorella, and constructed spectral toxicity 

 curves, in which the duration of exposure required for the lethal effect 

 (at an intensity of 1000 erg/cm.Vsec.) was plotted against wave length. 

 At 302 mn, this duration was of the order of 10^ sec. ; it decreased very 

 rapidly below 300 ran, and reached a minimum (110 sec.) at 260 m^u. At 

 still shorter waves, the effect became somewhat weaker, with a secondary 

 peak around 240 mn. 



Killing the cells by ultraviolet light of course means complete cessa- 

 tion of photosynthesis, even if the location of the primary attack is not 

 in the photosynthetic apparatus 

 proper. An indication that ultra- 

 violet light (X = 236 mn) does attack 

 a colorless component of this appa- 

 ratus (either directly, or through 

 chlorophyll as sensitizer) can be seen 

 in the experiments of Arnold (1933). 

 He irradiated Chlorella pyrenoidosa 

 suspensions with the light from a low- 

 voltage mercury arc consisting mainly 

 of the resonance line 253.6 m^. Fig- 

 ure 36 shows the decrease in the rate 

 of photosynthesis of irradiated cells 

 with time, and also the comparative 

 indifference of the respiratory system 

 to this irradiation. The abscissae 

 are logarithms of the maximum rates 

 of photosynthesis after exposure to 



ultraviolet light relative to the rate prior to inhibition. Arnold inter- 

 preted this ratio as the proportion of "reduction centers" (enzyme 

 molecules?) which have survived irradiation. 



The linear decrease in log (N/No) with time shows that the rate of 

 deactivation is proportional to the number of surviving "centers" (as in 

 a radioactive decay process) ; this indicates that deactivation is achieved 

 by a single absorption act, and does not require a cumulative effect of 

 several quanta. Arnold determined the absorption of ultraviolet light 

 by the irradiated suspension, and calculated that complete deactivation 

 requires the absorption of about six quanta for each chlorophyll molecule 

 in the suspension. This figure — which does not claim any precision 

 beyond that of the order of magnitude — indicates that the number of 



5 10 15 ^^ 



Tim«, minutei 



Fig. 36.— Effect of ultraviolet 

 light on Chlorella (after Arnold 1934). 

 R, respiration (no effect); P, photo- 

 synthesis (exponential decline of rate 

 with time). 



