Chromatic adaptation in eukaryotic algae

Red algae do show changes in their pigment composition in response to changes in the spectral quality of the light field in which they are grown. The direction of the pigment change, however, quite apart from its quantitative extent, seems to depend on the intensity of the light. Brody and Emerson (1959) determined the ratio of phycoerythrin to chlorophyll in the unicellular red alga Porphyridium cruentum grown in green light (546 nm - absorbed mainly by phycoerythrin) or blue light (436 nm -absorbed mainly by chlorophyll), at low (^0.1 Wm~2) or high (25-62 W m~2) intensity. At low irradiance the cells grown in green light had a phycoerythrin/chlorophyll ratio more than twice that of the cells grown in blue light. In three species of red macroalgae - Corallina elongata, Plocamium cartilagineum and Porphyra umbilicalis - collected from a rocky shore on the coast of Malaga (Spain), and incubated in sea water for six hours under different light qualities, Lopez-Figueroa and Niell (1990) found that the synthesis of phycoerythrin was preferentially stimulated by green light, and synthesis of phycocyanin was preferentially promoted by red light. In a Cryptomonas species isolated from the deep chlorophyll layer in the western Pacific Ocean, Kamiya and Miyachi

Chromatic Adaptation

Fig. 12.8 Effect of spectral quality of light field during growth on the photosynthetic pigment composition of the unicellular red alga Porphyridium cruentum (by permission, from Ley and Butler (1980), Plant Physiology, 65, 714-22). The spectra are normalized to the same absorbance value at 676 nm. The letter labelling each spectrum indicates the light field in which the cells were grown: R = red, B = blue, L = low-intensity white, H = high-intensity white, G = green.

Fig. 12.8 Effect of spectral quality of light field during growth on the photosynthetic pigment composition of the unicellular red alga Porphyridium cruentum (by permission, from Ley and Butler (1980), Plant Physiology, 65, 714-22). The spectra are normalized to the same absorbance value at 676 nm. The letter labelling each spectrum indicates the light field in which the cells were grown: R = red, B = blue, L = low-intensity white, H = high-intensity white, G = green.

(1984) found the phycoerythrin/chlorophyll ratio to be much higher in cells grown in green light than in cells grown in blue or red light, the same low irradiance (0.8 Wm~2) being used in each case. We may regard these pigment changes as complementary chromatic adaptation: the cells increased the proportion of that pigment which best absorbed the light to which they were exposed.

In the Porphyridium cells grown at high monochromatic irradiance, however, the position was reversed: the phycoerythrin/chlorophyll ratio of cells grown in green light was less than 50% of that in cells grown in blue light. Similarly, Ley and Butler (1980) found that cells of P. cruentum grown in high-intensity red (112 mmol photons m~2s_1) or blue (50 mmol photons m~2s_1) light have about twice the phycoerythrin/ chlorophyll ratio of cells grown in high-intensity green (99 mmol photons m~2s_1) light (Fig. 12.8). In the multicellular red alga Porphyra, Yocum and Blinks (1958) found that plants exposed to high-intensity blue light (436 nm, 24 Wm~2) for ten days contained more phycoerythrin and less chlorophyll than plants exposed for the same time to high-intensity green light (546nm, 17.5Wm~2).

These changes in pigment composition induced by high-intensity monochromatic illumination are accompanied by changes in the photo-synthetic characteristics of the plants. Yocum and Blinks (1958) found that marine red algae that had just been collected or had been kept under green light for ten days showed low photosynthetic efficiency in the region of the chlorophyll red absorption band, the action spectrum falling well below the absorption spectrum between 650 and 700 nm. Plants which had been kept for ten days under blue light, however, were highly efficient in the red region (as well as showing some increase in the blue region), the action and absorption spectra now approximately coinciding between 650 and 700 nm. Clearly, what had resulted from the exposure to blue light (itself absorbed by chlorophyll) was an increase in the efficiency of utilization of light absorbed by chlorophyll.

The nature of adaptive changes that take place within the photosyn-thetic system under these various moderate-to-high intensity, spectrally selective light regimes has been clarified by the detailed study of Ley and Butler (1980) on Porphyridium cruentum. By careful analysis of the absorption spectra and the fluorescence behaviour they have been able to arrive at conclusions concerning the absorption characteristics of, and energy transfer between, the two photosystems in cells grown in light of various spectral compositions. In red algae, chlorophyll a is the main light-harvesting pigment for photosystem I and phycoerythrin is the main pigment in photosystem II. Cells growing in red or blue light will be receiving much more excitation energy in chlorophyll than in phycoery-thrin, which, given a photosynthetic system initially adapted to a broad spread of wavelengths, will lead to a much greater input of energy into photosystem I than photosystem II. To ensure a balanced operation of the photosynthetic system, the cells must increase the absorption cross-section of photosystem II relative to that of photosystem I. This they achieve partly by increasing the ratio of phycoerythrin to chlorophyll, but also by putting a much larger proportion of their chlorophyll into photosystem II. In addition, Ley and Butler found that these red- or blue-light-grown cells have a diminished probability of energy transfer from photosystem II to photosystem I, which helps to keep excitation energy in photosystem II. The increased effectiveness of red light in Porphyra, observed by Yocum and Blinks after prolonged exposure of the alga to blue light, we may now plausibly explain in terms of an increased proportion of chlorophyll in photosystem II, and therefore a more balanced functioning of the photosystems in red light. Cells growing in green light will, if they are initially adapted to a mixture of wavelengths, receive far more excitation energy in photosystem II than I. Their adaptive response is to lower the absorption cross-section of photosystem II relative to that of photosystem I by reducing the phycoerythrin/chlorophyll ratio, and including nearly all their chlorophyll in photosystem I: PS I chlorophyll/ PS II chlorophyll is ~20 in green-light-grown, compared to ~1.5 in redor blue-light-grown cells.806 In addition, the green-light-grown cells show a greater probability of energy transfer from photosystem II to I than cells grown in red or blue light.

The red- or blue-light-grown cells have a higher phycoerythrin content than the green-light-grown cells on a per cell basis, as well as in proportion to chlorophyll. It might be argued that although it makes sense to put more chlorophyll into photosystem II, to make more phycoerythrin in red light is not a useful adaptive response since this biliprotein has little absorption above 600 nm and so will not collect the red photons. However, it may be that what the cell 'detects' is not specifically the spectral nature of the incident light, but simply the fact that photosystem I is receiving more energy than photosystem II, and so it responds with a general increase in the pigment complement of photosystem II. Red-dominated light fields do not occur naturally in the marine environment and so we should not expect red algae to show specific adaptation to them.

From the work of Ley and Butler it seems that the general principle on which chromatic adaptation to relatively high intensity, spectrally confined, light fields takes place is adjustment of the composition and properties of the photosystems in such a way as to lead to their being excited at about the same rate, and thus to efficient photosynthesis. The complementary chromatic adaptation observed by Brody and Emerson (1959) in P. cruentum exposed to low-intensity blue and green light, since it operates in a contrary fashion so far as pigment changes are concerned, must have a different basis. At low light intensity the total rate of supply of excitation energy rather than imbalance between the photosystems becomes the major constraint on photosynthesis. It may be that the best strategy for the algae growing in dim green or blue-green light is to make whatever pigments will best capture the available light, to incorporate these predominantly into photosystem II (biliproteins are in this photosystem, anyway) and transfer a proportion of the absorbed energy to photosystem I.

The fact that the chromatic adaptation in cyanobacteria, involving changes in the ratio of phycoerythrin to phycocyanin, is entirely of the complementary type is not surprising. Both the pigments involved are biliproteins and both feed their excitation energy to photosystem II, so the problem of imbalance between the photosystems does not arise. Growth of the blue-green alga Anacystis nidulans in red (1 > 650 nm) light does lead to a 75% fall in chlorophyll content with little change in phycocyanin content.655 We may now interpret this as an attempt by the cells to reduce excitation of photosystem I to the point at which it is in balance with photosystem II. In the cyanobacterium Synechococcus 6301, the phycocyanin/chlorophyll ratio increases in red light (absorbed by chlorophyll a, mainly associated with photosystem I) and decreases in yellow light (absorbed by phycocyanin, associated exclusively with photosystem II). Manodori and Melis (1986) interpreted these changes as representing the achievement of a balanced excitation of the two photosystems by adjustment of photosystem stoichiometry: the actual antenna size of the individual photosystems apparently did not alter.

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Responses

  • gerard
    What is chromatic adaptation of algae?
    3 years ago
  • Grace
    What is chromatic adaption in plants?
    2 years ago
  • Mezan Tesmi
    Which alga chromatic adaptation is seen?
    1 year ago
  • Lexi
    What is chromatic adaption / algae?
    1 year ago

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