ALGAL PIGMENTS AND THEIR SIGNIFICANCE

Summary The last 15 years have seen notable advances in knowledge of the distribution of chlorophylls, carotenoids and other pigments in the algae, in the structures and function of individual pigments, and in their correlation with morphological characteristics. The algal classes are often associated with specific colouring matters or fairly well‐defined mixtures thereof, so that detailed knowledge of the pigmentation is not without phylogenetic value. The Chlorophyceae contain chlorophyll a and b , zeaxanthin and lutein, together with a minor proportion of less typical carotenoids. In general the quantities and proportions do not diverge greatly from those found in higher plants; green algae cultured under abnormal or artificial conditions often develop unusual pigmentation though experience is here rather limited. In view of what has become known of the role played by carotenoids in the sexual reproduction of Chlamydomonas eugametos , particular interest is attached to the differential pigmentation of gametes, antheridia and oogonia of many of the Chlorophyceae. Quantitative and qualitative examination of Xanthophyceae, Chrysophyceae, Dinophyceae, Cryptophyceae, Chloromonadineae and Euglenineae is less complete, but it is certain that chlorophyll a , carotenes, and xanthophylls are still the main pigments though other carotenoids which have hitherto appeared to be characteristic of other algal classes (e.g. fucoxanthin) or higher plants (e.g. flavoxanthin and taraxanthin) are occasionally encountered. Some of the algae of these classes contain characteristic carotenoids (diato‐ and diadinoxanthin in diatoms, peridinin and dinoxanthin in dinoflagellates) though their general distribution has yet to be examined; the detailed chemical nature of these materials remains obscure, and its elucidation is complicated by the pigments assuming labile forms which may be present in the algae or formed by chemical treatment. The diatoms are distinguished also by chlorophyll c which has been found as well in some of the Phaeophyceae. The brown algae have long been thought to contain no more than traces of chlorophyll b , a view substantiated by modem analyses, so that the new chlorophyll may replace chlorophyll b of other algae and higher plants, and is perhaps to be correlated with photosynthetic activity as indicated by typical reserve materials. The Phaeophyceae, in addition to carotenoids common to most algae, are characterized by fucoxanthin and nearly related pigments which assume a degree of photo‐synthetic activity: quantitatively, however, the brown algae are not richer in carotenoids when compared with other algae, for an excess of fucoxanthin is balanced by a deficiency in other polyene pigments. The Rhodophyceae contain no chlorophyll b , but here the deficiency is to some extent made good by a recently discovered pigment, chlorophyll d. The total chlorophyll content is nevertheless low compared with that of the green algae, part of the photosynthetic activity being due to phycobilins which occur also in the Cyanophyceae. The blue‐green algae are in turn outstanding in exhibiting a series of new carotenoids, perhaps of photosynthetic value; of these myxoxanthin and myxoxanthophyll seem to be as characteristic of the Cyanophyceae as, for example, fucoxanthin is of the Phaeophyceae. The chemistry of the algal pigments which are common in higher plants has been adequately reviewed elsewhere, but a section of the present account is devoted to the salient features of purely algal colouring matters, especially chlorophyll c and d , the phycobilins, and characteristic algal carotenoids mentioned above. An outstanding result of algal pigment research has been the demonstration that here phycobilins and carotenoids are not, as has been assumed of similar pigments of higher plants, merely precursors or degradation products of chlorophyll, but possess a degree of photosynthetic activity enabling the organism to take advantage of conditions of habitat where chlorophyll is less efficient. Much more remarkable, however, is the demonstration of the effect of certain carotenoids on the reproductive processes in Chlamydomonas , a brief review showing that such diverse phenomena as formation of flagella in gametes, motility, fertilization and relative sexuality are brought about by a group of pigments of which our knowledge is, in view of their complex nature, unusually complete. My thanks are due to Prof. I. M. Heilbron, D.S.O., F.R.S., for his interest and encouragement during the preparation of this review.