Chapter XVI 



PHYSIOLOGY, SYMBIOSIS 



Physiology 



It is impossible to separate entirely the physiology of the algae from 

 a discussion on the ecological factors controlling zonation. This is 

 particularly true of photosynthesis and respiration under different 

 environmental conditions, and reference should therefore be made 

 to the relevant pages (pp. 394? 408). Apart from the physiologico- 

 ecological studies of the dominant belt algae, our knowledge of algal 

 physiology is largely associated with the estabHshment around 1920 

 of bacterial free algal cultures. Such cultures had probably been ob- 

 tained earlier, but the intensive use of such cultures for physio- 

 logical work stems from about that time. These cultures have been 

 used for studies in the fundamental mechanisms of photosynthesis 

 and respiration, and also in growth studies (cf p. 43). The net 

 result has been that much of our knowledge at present is restricted 

 to unicellular algae, expecially members of the Chlorococcales, e.g. 

 Chlorella, Scenedesmus. Whilst one may expect that the larger algae 

 behave similarly, nevertheless there may be differences and an 

 extension of this work is highly desirable. 



Photosynthesis 



It has been shown that carotenoid pigments in Navicula, Chlor- 

 ella, Ulva and Monostroma absorb light that is wholly or largely 

 utilized in photosynthesis. It also appears that fucoxanthin in brown 

 algae absorbs hght that is used in photosynthesis and the phyco- 

 bilins in Rhodophyceae behave similarly. Despite this, however, it 

 seems that the Hght energy absorbed by these pigments is trans- 

 mitted to chlorophyll a, which must therefore be regarded as the 

 principal photosynthetic pigment bringing about the photolysis of 



water (see also p. 398). 



Our present knowledge of the mechanism of photosynthesis rests 

 very largely upon studies of algal cultures, because they form very 

 convenient material which responds rapidly to new techniques such 

 as the use of radio-isotopes. There is no reason to suppose that the 



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