The carotenoids

The carotenoids are another class of photosynthetic pigment, which extend absorption still farther into the 'window', at the short-wavelength end. Chemically they are quite distinct from the chlorophylls, being C40 isoprenoid compounds. There are far more known carotenoids than there are chlorophylls. The distribution of the various chloroplast carotenoids among different plant groups is shown in Table 8.2. b-carotene is present in all except the Cryptophyta. In higher plants and green algae, which rely mainly on chlorophylls for light harvesting, the molar ratio of carotenoid to chlorophyll (a + b) is about 1:3. Other classes of algae, apart from those that contain biliproteins, depend to a greater extent on their carotenoids to capture light, and this is evident in the pigment composition. The molar ratio of carotenoid to chlorophyll (a + c) is about 1:0.5 in the diatom Phaeodactylum,514 1:1.4 in the dinoflagellate Gyrodinium627 and among the brown algae is about 1:2 in Hormosira698 and 1:0.5 in Laminaria.16 Even among the red algae, which have biliproteins, the caro-tenoid:chlorophyll molar ratio varies from 1:2.6 to 1:1.121

The structures of some of the most important carotenoids in higher plants and algae are shown in Fig. 8.12. Their ability to absorb visible

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Fig. 8.11 Absorption spectra of chlorophyll c1 and c2 in acetone containing

2% pyridine with a pathlength of 1cm (Jeffrey, S.W., unpublished data).

(Chlorophyll c1 (2.68 mgmF1) ; chlorophyll c2 (2.74 mgmF1)--.)

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Fig. 8.11 Absorption spectra of chlorophyll c1 and c2 in acetone containing

2% pyridine with a pathlength of 1cm (Jeffrey, S.W., unpublished data).

(Chlorophyll c1 (2.68 mgmF1) ; chlorophyll c2 (2.74 mgmF1)--.)

quanta is due to their possessing systems of up to 11 conjugated double bonds. They absorb at the short-wavelength end of the visible range, which is why their characteristic colours are yellow, orange or red. The absorption spectrum of b-carotene is shown in Fig. 8.13.

Of particular importance in aquatic ecosystems is fucoxanthin, which, together with its various derivatives, is the principal carotenoid in the brown algae (Phaeophyta), diatoms (Bacillariophyceae), Haptophyta and Chrysophyceae, and in some dinoflagellate species. While it commonly occurs only in its unmodified form it is frequently accompanied by derivatives with hexanoyl or butanoyl residues esterified to a hydroxyl group on carbon atom 19' and/or with a keto group at position 4 of the left-hand ring.349

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