Chlorophylls and other pigments

It is generally found (with a few exceptions) among the algae, both unicellular and multicellular, that as the light intensity during growth decreases (even without alteration of spectral composition), the content of their photosynthetic pigments increases: two- to five-fold increases are commonly observed. Many studies of this have now been carried out, in a wide taxonomic range of algal species: reviews may be found in Richardson, Beardall and Raven (1983), and Falkowski and LaRoche (1991). A convenient parameter in terms of which to express this phenomenon is the carbon:chlorophyll a ratio of the cells (C:chl a). On the basis of an analysis of literature data for eight diatoms, two green algae, one euglenid and two cyanobacteria, Geider (1987) concluded that C:chl a increases linearly (i.e. decreasing pigment content) with increased light level at constant temperature, but decreased exponentially (i.e. increasing pigment) with increased temperature at constant light level.

Accessory photosynthetic pigments also increase as the growth light intensity is decreased, and indeed generally do so to a greater extent than chlorophyll a.326 In the higher plants and green algae, the ratio of chlorophyll b to a increases with diminishing light intensity. For example in the green flagellate Dunaliella tertiolecta, simultaneously with a 2.6-fold rise in chlorophyll a content, the a/b ratio fell from 5.6 to 2.3 when the growth irradiance was reduced from 400 to 20 mmol photons m~2s_1.382 In the marine dinoflagellate Glenodinium, as the light intensity during growth was lowered over the range 30 to 2.5 Wm~2, the chlorophyll a content per cell rose progressively, by about 80%, but the cellular concentration of the light-harvesting peridinin/chl a protein rose seven-fold.1080 It has been shown with cultures of the cryptomonads Chroomonas387 and Cryptomonas,1351 the cyanobacteria Anacystis,1413 Oscillatoria409 and Synechococcus,674 the unicellular rhodophyte Porphyridium169,313,799 and the red macro alga Griffithsia1425 that the ratio of biliprotein pigment to chlorophyll a increases with diminishing light intensity: the increases can be several-fold.

Corresponding changes have been observed in the field. In plants of Chondrus crispus growing in sunlit sites at 3 to 4 m depth, the ratio of phycoerythrin to chlorophyll remained high during the winter but underwent a 60% fall in late spring/early summer: in plants in shaded sites at the same depth the ratio remained high during the summer.1122 When the sunlit plants became shaded due to a dense growth of an epiphytic diatom in August they regained much of their phycoerythrin. In the sublittoral red alga Gracilaria compressa, in the Adriatic Sea, the distal portions of the fronds, which received direct illumination, were yellow-green in colour and contained 0.065% (dry mass) chlorophyll and only traces of phycoerythrin. The proximal portions, which were shaded, were purplish-red, and contained 0.085% chlorophyll and 0.82% phycoerythin.195 In littoral algal turf communities, there are marked changes in average ambient light exposure within short distances along the thalli, and these are accompanied by marked differences in photosynthetic pigment composition. In the case of two such turf species in the Hawaiian islands, Ahnfeltiopsis concinna and Laurencia mcdermidiae, Beach and Smith (1996) observed striking pigment alterations within the <10 cm lengths of individual thalli: tissue in the understory region of the turf was red to purple-black in both species, whereas the canopy (exposed) tissue was yellow-orange in A. concinna and green in L. mcdermidiae. The canopy tissue had lower levels of biliprotein, but increased concentrations of carotenoid and UV-absorbing compounds.

In a brackish eutrophic lake, Veerse Meer, in the Netherlands, a thick mat, consisting of five to seven layers of the green macroalga, Ulva spp., forms each year. With increasing depth (diminishing light) within the mat, the layers were found to have increasing absorbance at all wavelengths over the 400 to 700 nm range, attributable to increases in the concentrations of chlorophylls a and b, and lutein.860 In dense stands of the tropical seagrass, Thalassia testudinum, in the Mexican Caribbean, Enriquez et al. (2002) found that photosynthetic pigment content and leaf absorbance decreased from the basal (more shaded) to the apical (more exposed) regions of the leaves.

An apparent exception to the rule that accessory pigments increase more than chlorophyll a as growth light levels decrease is fucoxanthin. In the brown algae Sphacelaria and Laminaria, and in the diatoms Nitschia and Phaeodactylum, fucoxanthin was observed to increase somewhat less than chlorophyll a as light intensity diminished.174,325,1214

The radiant intensity in full sunlight is so high that, quite apart from causing photoinhibition, it can be quite lethal, in part because of the inability of some plants safely to handle the very high rates of energy absorption by chlorophyll and other pigments, resulting in photo-oxidation of cell material. Carotenoids, however, can as discussed earlier (§10.1) in various ways exert a protective effect against such photo-oxidation,240 and another adaptive response of algae to high light intensity is, as well as reducing the levels of photosynthetic light-harvesting pigments in the manner we have discussed, to increase the cellular concentration of photoprotective carotenoids. The halophilic unicellular chlor-ophyte Dunaliella salina, which occurs in salt ponds, when grown in full sunlight makes so much photoprotective b-carotene that the cells turn red. This extra b-carotene is not coupled in to the photosynthetic system, and in fact simply acts as a colour filter, with the consequence that the cells show greatly diminished photosynthetic activity in the blue spectral region.823 In Dunaliella bardawil, which also accumulates b-carotene in high light, the pigment is concentrated in oily globules in the interthyla-koid spaces of the chloroplast: the b-carotene protects the alga against the photoinhibitory effects of blue light (which it absorbs), but not against the effects of intense red light.96 Some unicellular algal species, such as those that can give a red tinge to the surface of snow in high mountains, accumulate photoprotective carotenoids outside the chloroplast: these are referred to as secondary carotenoids. Haematococcus lacustris, an alga of shallow freshwater bodies, accumulates globules of secondary carote-noids, mainly astaxanthin, around the periphery of the cell, when exposed to high light intensities.518

Paerl, Tucker and Bland (1983) found that the ratio of carotenoid to chlorophyll in surface blooms of the blue-green alga Microcystis aeruginosa rose progressively during the summer, to a high value, and attributed this to the carotenoids having a protective role. The carotenoid that reached the highest concentration was zeaxanthin, a xanthophyll, which is in fact now thought to exert an important photoprotective function in plants generally, but by mechanisms other than simple interception of the light.230 In the marine cyanobacterium Synechococcus, Kana et al. (1988) found that as the light intensity during growth varied from 30 to 2000mmolphotonsm~2s-1, so the cellular content of b-carotene and chlorophyll a diminished several-fold in parallel, but the zeaxanthin concentration remained the same. These authors suggested that in this case b-carotene is entirely part of the photosynthetic system, whereas zeaxanthin by contrast has a wholly photoprotective function.

We have discussed earlier (§10.2) the role of the mycosporine-like amino acids in protecting some algae against the damaging effects of the UV component of solar radiation. In an Arctic fjord (Spitsbergen, Norway) Aguilera et al. (2002) found that coinciding with the increase in underwater radiation during the sea-ice break-up in June, there was, in two red algal species - Palmaria palmata and Devaleraea ramentacea - an increase in the concentration of MAAs, in addition to a decrease in the photosynthetic accessory pigments, phycocyanin and phycoerthyrin.

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