THERMAL ADAPTATION 



1227 



photosynthesis is independent of temperature, the net oxygen evolution of 

 all plants — cryophilic as well as thermophilic — must decrease with tem- 

 perature (because of accelerated respiration). In cryophilic plants, this 

 behavior is accentuated by the early "thermal saturation" of true photo- 

 synthesis (which is not paralleled by an equally early saturation of respira- 

 tion). Furthermore, these plants live in regions where the average inten- 

 sity of sunlight is low, and this shifts their maximum efficiency under 

 natural conditions toward lower temperatures. In July, the average 

 illumination at Disco is only 450 lux, a light intensity at which the opti- 

 mum of net oxygen production by Salix lies close to 0° C. 



ro . 



E 

 E 



to 

 to 



LlI 



X 



>- 



o 



h- 

 o 



X 

 Q- 



10 15 20 25 30 35 40 

 TEMPERATURE, "C. 



Fig. 31.8. Variation in rate of photosynthesis with tem- 

 perature at high light intensities for Nitzschia closterium 

 (O) and N. palea (A) (after Barker 1935). 



The temperature optima of true photosynthesis probably never lie as 

 low as one may think from the consideration of the net gas exchange of 

 cryophihc plants. Barker (1935) (c/. fig. 31.8) found that the optimum of 

 true photosynthesis of the marine diatom Nitzschia closterium lies at 27° C, 

 although this organism usually lives at 8-12°. The fresh water diatom 

 Nitzschia palea had an optimum at 33°. (This high optimum temperature 

 probably enables the species to withstand the high temperatures that shal- 

 low waters may reach on warm summer days.) 



At the opposite extreme from the arctic plants, adapted, as far as their 

 net organic synthesis is concerned, to temperatures near the freezing point, 

 we find some algae (e. g., Phormidium) as well as certain sulfur bacteria 

 thriving in hot springs, with temperatures up to 80 or 90° C. (c/., for ex- 

 ample, Harvey 1924). Their cells must be capable of sustaining such high 



