TEMPERATURE COEFFICIENT AND HEAT OF ACTIVATION 1231 



photosynthesis for a while, but resume it afterward, also can be inter- 

 preted as an example of individual thermal adaptation. (This phenomenon 

 contrasts with the gradual suspension of photosynthesis in chilled plants, 

 observed by Ewart, and mentioned on page 1219.) 



Similar questions have been much discussed in connection with the 

 increase in the abundance of algae observed as one proceeds northward in 

 European waters. Kniep (1914) and Harder (1915) suggested that this 

 is due to the greater excess of photosynthesis over respiration at the lower 

 temperatures. This theory received support from experiments by Ehrke 

 (1931) ; but Lampe (1935), who conducted experiments on numerous green, 

 red and brown algae, under laboratory conditions and in natural habitats, 

 concluded that the average light intensity under natural conditions often 

 is not low enough to permit application of the Kniep-Harder theory. 

 Furthermore, he observed that some algae can adjust themselves wdthin a 

 few days to higher or lower temperatures; the position of the optimum of 

 net organic synthesis is shifted, by this adaptation, nearer to the tempera- 

 ture of the medium. These algae can act as cryophiles in winter and as 

 thermophiles in summer. Only some deep water red algae, and the umbro- 

 philic green algae, have rigid cryophilic characteristics, and actually in- 

 crease in weight more rapidly in cold water (and dim hght). 



C. Temperature Coefficient and Heat of Activation 



OF Photosynthesis* 



As stated before, if we leave aside the two extreme ends of the "bio- 

 kinetic range" of photosynthesis, and restrict ourselves to its middle part, 

 where no time-dependent inhibition effects occur, we can obtain a short 

 segment of a temperature curve that apparently reflects the direct effect of 

 temperature on the kinetic mechanism of photosynthesis. The shape of 

 this temperature curve depends on several parameters, such as light in- 

 tensity and carbon dioxide supply. This was not sufficiently realized by 

 earlier plant physiologists, who gave figures for the "temperature coef- 

 ficient" of photosynthesis without identifying the specific conditions 

 under which this coefficient was determined. In weak light and in the 

 presence of an adequate supply of carbon dioxide, the rate of the over-all 

 process of photosynthesis is practically equal to that of the primary photo- 

 chemical reaction, and the temperature curve reflects the influence (or, 

 more likely, lack of influence) of temperature on this primary reaction. 

 When the supply of carbon dioxide is low, the temperature coefficient may 

 be essentially that of the supply process, e. g., of the diffusion of carbon 

 diox-ide through an aqueous phase. In this case, Qio (the proportional 

 increase in rate caused by an increase in temperature by 10° C.) will be of 



* Bibliogi-aphy, page 1255. 



