CAROTENOIDS IN PLANTS 



torqiuitum. ^** This pigment, which has the same absorption spectrum 

 as neurosporene (see p. 107), is much less strongly adsorbed on alumina 

 than the latter. It is possible that if neurosporene is accepted as being 

 tetrahydrolycopene, e-carotene is tetrahydrocarotene. 



The position of the xanthophylls in diatoms is by no means clear. 

 Seybold and Egle^=^^ and Heilbron^^s j^^y^. repeatedly established 

 the presence of lutein (xanthophyll) undfucoxanthin in diatoms. Pace ^ * ' 

 agrees with the presence of lutein but considers the other xantho- 

 phylls which are present to be xanthophylls characteristic of the 

 higher plants. He claims to have detected isoltitein, cryptoxanthin 

 and, possibly, zeaxanthin and violaxaiithin " b ". A similar claim has 

 been made by Handke. ^ ^ ^ Strain ^ * ^ considers this false and he and 

 his colleagues claim the existence of diatoxanthin and diadinoxanthiriy 

 two specihc xanthophylls very similar to, but apparently quite distinct 

 from zeaxanthin and lutein respectively. ^ * ^ These pigments have not 

 yet been isolated. The possibility does exist that they are m-isomers 

 of lutein and zeaxanthin. Strain et al. also consider that in diatoms 

 there are several normally-occurring fucoxanthins, (e.g. fucoxanthins a, 

 b, and c, ^** the latter two have recently been renamed w^ofucoxan- 

 thins A and B ^ * ^) and deny that they are either artefacts or the results 

 of post-mortem changes (see p. 133). All the pigmtii-s observed by 

 Strain et al. occurred in all diatoms examined : viz. Navicula torquatum, 

 Isthmia nervosa, Nitzschia closterium, N. palea, Steplianopyxis tiirris and 

 Thalassiosira gravida. W^assink and Kersten ^ ^ ' have found carotene, 

 fucoxanthin, and possibly diatoxanthin in Nitzchia dissipata ; fucoxan- 

 thin exists as a protein complex with an absorption maximum at 

 500 mfx. The change from brown to green which this alga undergoes 

 on boiling is explained by denaturation of the protein complex. The 

 absorption spectra of boiled and unboiled diatoms are, however, only 

 slightly different. ^ ^ ' 



Chrysophyceae 



The only work recorded is that by Heilbron ^ ^ ^ who examined the 

 carotenoids of a mixture of thre^ algae, Apistonema cartcri, Thallo- 

 chrysis litoralis and Gloeschysis maritima. ^-carotene, lutein (xantho- 

 phyll), and fucoxanthin were isolated. These pigments were also isolated 

 by Heilbron's group from Nitzschia cLosteriutn (above) and this similar- 

 ity is added proof for Pascher's ^ ^ " thesis that the Chrysophyceae 

 and Dacillariophyceae are closely related and probably derived from a 

 common ancestry. Pascher also considers that the Xanthophyceae are 

 closely related to these two classes but it will be remembered that the 

 carotenoids of a typical member of Xanthophyceae are different. 



131 



