CAROTENOIDS 



two maxima at 445 my., and 485 mfx. ; further, the pigment extracted 

 from the coloured cells proved to be ^-carotene. 



Although the investigations just discussed are a good indication 

 that in fungi, as well as in higher plants, carotenoids may function by 

 mediating in photo-kinetic responses, the implications of Galston's work 

 must not be overlooked. It has already been stated that Galston • ». ' o 

 showed that action spectra measurements are not sufficiently precise 

 to distinguish between riboflavin and a carotenoid as the mediator in 

 the phototropic response of the higher plants (see p. 88); as 96 per cent, 

 of the auxin activity of Phycomyces is due to indole acetic acid, ' ^ it is 

 possible that riboflavin is the mediator in this fungus for it can sensitize 

 the photodestruction of indole acetic acid. ' ^ Further, Phycomyces 

 blakesleeanus grown in a medium containing diphenylamine and thus 

 containing only about 1-2 per cent, of its usual amount of carotene, is 

 strongly phototropic ; ' ^ this amount of carotene may, of course, be 

 sufficient for phototropic action. 



Against the suggestion that riboflavin is the mediator is the fact 

 that auxin (both indole acetic acid and auxin a) is not necessary for the 

 grovvth of Phycomyces. ' * and also no evidence has been yet presented 

 to indicate that riboflavin occurs in the photosensitive regions of the 

 fungus. 



(b) In reproduction. The work of Emerson and Fox, * ^ in which the 

 preferential accumulation of y-carotene in the male gametes of Allo- 

 myces was described, strongly suggests a function in the sexual processes. 

 Up to the present, however, no specific function in reproduction has 

 been demonstrated. 



BACTERIA* 



A large number of bacteria have been examined for carotenoids but 

 only a fraction of the pigments have been completely identified. How- 

 ever, the following general conclusions can reasonably be drawn : 



(i) As is the case of fungi, ^-carotene is by no means ubiquitous ; 



(ii) Lutein (xanthophyll) is conspicuous by its absence. It has been 

 reported only in Mycobacterium phlei ' ^ (this is disputed) ' * and 

 Micrococcus tetragenus. ' ' 



(iii) Many carotenogenic bacteria are characterized by specific 

 carotenoids, mostly xanthophylls. These xanthophylls some- 

 times occur in the form of methyl ethers {cf. torulene) ; 



* The nomenclature used in this section is that of Bergey — Manual of 

 Determinative Bacteriology, 6th Ed. (1948, London, Balli^re, Tindall, and Cox.) 



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