1228 THE TEMPERATURE FACTOR CHAP. 31 



temperatures without injury. Furthermore, unless the water in which 

 they Hve cools down perceptibly during certain periods of the day or sea- 

 son, these algae must be able to carry out photosynthesis at a tempera- 

 ture that would bring immediate and complete thermal inhibition — in fact, 

 thermal death — to most other plants. Photosynthesis of these "thermo- 

 philic algae" certainly is worth closer investigation, under natural as well as 

 under laboratory conditions. Inman (1940), who studied some algae 

 from Yellowstone Park geysers, was concerned primarily with the proof of 

 the spectroscopic identity of their chlorophyll with that of ordinary plants; 

 the only observation he made concerning their photosynthesis was that 

 they liberate oxygen when irradiated at room temperature. 



Desert plants, exposed to direct sunhght, sometimes are heated to 

 temperatures approaching those of hot springs. Their photosynthetic ap- 

 paratus, too, must remain uninjured by heat. MacDougal and Working 

 (1921) found that Opuntia actually continues to grow, and thus presumably 

 to carry out photosynthesis, at 58° C. ; growth is stopped, and shrinkage 

 ensues, when the temperature reaches 62°. Wood (1932) found that the 

 optimum of photosynthesis of some Australian desert plants lies between 

 40° and 50°, and that their net oxygen production does not become zero 

 until 55°. 



Extended heat resistance of thermophihc algae, bacteria and desert 

 succulents must be due to a different structure of the protoplasmic con- 

 stituents responsible for heat injury. Lipides might, perhaps, bear the 

 main responsibihty for this difference, because it is known that the ther- 

 mal stability of fats and lipides depends on the temperature at which they 

 have been formed in the organism. 



One chemical peculiarity of thermophilic algae was noted by Harvey 

 (1924): they contain no catalase, a unique occurrence in the whole plant 

 world. Harvey suggested that thermal algae have no need for catalase 

 because, at the high temperatures at which they live, the decomposition of 

 hydrogen peroxide proceeds rapidly enough by itself. This caused him to 

 speculate generally on the possibility of a primeval "life without enzymes" 

 in a medium hot enough for the organisms to dispense with catalysts. 

 However, thermophilic algae are more likely to represent adaptations of 

 normal species to high temperatures, than remnants of such prehistoric, 

 enzyme-free flora. (To prevent misunderstanding, it must be pointed out 

 that thermophihc algae contain many enzymes — e. g., oxidases — and are 

 only deficient in catalase.) 



A very interesting result was obtained by Sorokin and Myers (1953) in 

 the course of experiments on mass cultivation of Chlorella. By starting 

 with inocula from warm local surface water, and incubating the cultures 

 at 32° C, several strains were isolated which showed decided "thermo- 



