Depth variation of pigment composition in macrophytes

Ramus et al. (1976) suspended samples of two green (Ulva lactuca and Codium fragile) and two red algal species (Chondrus crispus and Porphyra umbilicalis) for seven-day periods at depths of 1 and 10 m in harbour water at Woods Hole, Massachusetts, USA. Pigment compositions were determined. The samples were then reversed in position - the 1 m samples being lowered to 10 m, and vice versa - and after a further seven-day period, further pigment analyses were carried out. In all four species the level of photosynthetic pigments increased in the plants held at 10 m, relative to those held at 1 m, the increase for chlorophyll a being about l.4-fold in the red algae and 2.3- to 3.4-fold in the green. In U. lactuca, chlorophyll b increased about five-fold, so that the b/a ratio rose by 50%, but in C. fragile the b/a ratio remained the same. In the red algal species the increase in phycoerythrin in the 10 m relative to the 1 m samples was greater than the increase in chlorophyll a, so that the phycoerythrin/ chlorophyll ratio rose by 50 to 60% with increased depth.

The light field at 10 m depth at this site, being predominantly yellow-green, differed greatly from that at 1 m depth in spectral composition as well as intensity. In an attempt to determine the effects of diminished intensity alone, Ramus et al. analysed the pigments from algae taken from sunny and shaded sites in the intertidal region.1102 Shade increased pigment levels to about the same extent as submersion to 10 m depth. In U. lactuca and P. umbilicalis, the chlorophyll b/a and the phycoerythrin/ chlorophyll ratios, respectively, did increase in the shade but to a lesser extent than that induced by increased depth. Ramus et al. concluded that the greater shifts in pigment ratio associated with increased depth were indeed a response to the changed spectral composition of the light, not just to the lowered intensity.

Wheeler (l980a) carried out transplantation experiments with the sporophytes of the giant kelp Macrocystis pyrifera (Phaeophyta). When plants growing at 12 m depth were moved up to 1 m depth, then over a l0-day period the pigment content per unit area of thallus diminished markedly, the decreases being 52% for chlorophyll a, 35% for

Caption for Fig. 12.10 (cont.)

Jeffrey and Vesk (1977), Journal of Phycology, 13, 27l-9). Upper: electron micrograph of chloroplast in cell grown in white (4W m~2) light. Inset is light micrograph of whole cells. Lower: electron micrograph of chloroplasts in cell grown in blue-green (4W m~2) light showing increased number of three-thylakoid compound lamellae. Inset is light micrograph of whole cells showing increased number of chloroplasts.

chlorophyll c and 68% for fucoxanthin. When the plants were replaced at 12 m depth pigment levels increased again to their original values in 18 days. Readaptation to increased depth was accompanied by an increase in the molar ratio of fucoxanthin to chlorophyll a from 0.50 to 0.77 but by a decrease in the chlorophyll c/a ratio from 0.50 to 0.31. When samples of the intertidal brown algae Ascophyllum nodosum and Fucus vesiculosus were suspended at 4m depth for seven days, concentrations of fucoxanthin and chorophylls a and c per g fresh mass rose 1.5- to three-fold.1103 However, in these species fucoxanthin increased somewhat less than chlorophyll a so that the fucoxanthin/chlorophyll a ratio diminished by about 20%: the chlorophyll c/a ratio remained the same during depth adaptation.

Interesting though transplantation experiments are, the alternative approach of studying plants of a given species growing naturally at different depths is of greater ecological relevance. Plants of the siphonous green alga Halimeda tuna growing in the Adriatic Sea at 5 m had a total chlorophyll content per g dry mass, which was 50% greater than that in plants growing at 2 m depth:1263 the chlorophyll a/b ratio fell only slightly with increased depth, from 2.03 to 1.86. The chlorophyll content of the green benthic alga Udotea petiolata growing in Spanish coastal water increased with depth by 50% over the range 5 to 20m, most of the increase taking place between 5 and 10 m:1045 the a/b ratio did not change with depth. In a high-arctic fjord (Young Sound, NE Greenland), the chlorophyll a content of leaf blades of the brown macroalga, Laminaria saccharina in August (ice cover absent), increased with depth, being 0.68, 1.13, 1.25 and 2.87 mgmgdry wt-1, at 2.5, 5, 10 and 15m, respectively.138 In the case of the giant kelp, Macrocystis pyrifera, a single plant can extend through 30 m or more of depth, so that different parts of the same plant are exposed to very different light climates. In plants ~18 m long, growing in a kelp forest in Baja California, Mexico, Colombo-Pallotta et al. (2006) found that in tissue sampled from the same plant at seven different depths over the range 0 to 18 m, the concentration (per unit area of blade) of chl a increased by 42%, chl c by 67%, but fucoxanthin by 110%, so that the fucoxanthin/chl a ratio increased by nearly half. Tissue near the surface was well supplied with the photoprotective xanthophyll cycle carotenoids, zeaxanthin and antheraxanthin, but these diminished with depth and were present at negligible levels in tissue collected below 9 m. In addition, the absorption of the tissue in the UV (310 nm), relative to the chlorophyll a 675 nm peak, diminished exponentially, by about 50% with depth over the range 0 to 10 m, which we may take as indicative of decreased synthesis of UV-protective pigments.

Wiginton and McMillan (1979) studied a number of seagrass species growing at various depths off the Virgin Islands and the coast of Texas, USA. Three of the species showed no significant increase in chlorophyll content with increasing depth down to their maximum depths of 12 to 18 m. In a fourth species, Halophila decipiens, however, which penetrated to 42 m, the chlorophyll content changed only slightly from 7 to 18 m, but approximately doubled between 18 and 42 m. The chlorophyll a/b ratio fell from 1.72 at 18 m to 1.49 at 42 m. Plants of this species from a shallow but shaded site had the same chlorophyll content and a/b ratio as plants from 42 m depth: this was taken to indicate that the pigment changes associated with increased depth were due to the decrease in total intensity rather than to the change in spectral quality. Plants of the isoetid species Littorella uniflora growing at 2.3 m depth (20% of subsurface irradiance) in a Danish lake contained about 65% more chlorophyll (dry weight basis), and had a lower chlorophyll a/b ratio (2.6 compared to 3.2) in their leaves than plants growing at 0.2m depth (70% of subsurface irradiance).1265

In the brown alga, Dictyota dichotoma, growing in Spanish coastal water, Perez-Bermudez et al. (1981) found that the chlorophyll a and c contents (on a mass basis) increased by 17 and 53%, respectively, between 0 and 20 m depth, the a/c ratio decreasing from 2.35 to 1.80: at 10 m depth the values were intermediate. The fucoxanthin content, on the other hand, decreased by 42% between 0 and 10 m depth, and then increased again to its original value between 10 and 20 m depth: the possible functional significance of this is discussed in a later section (ยง12.5). Plants growing in a shaded site at the surface contained 35% more chlorophyll a, 63% more chlorophyll c and 20% more fucoxanthin than plants growing in a sunny site. In a study on the red alga Chondrus crispus growing from 0 to 20 m depth off Massachusetts, Rhee and Briggs (1977) found that the phycoerythrin/chlorophyll ratio in midsummer remained approximately constant down to about 10 m depth, but approximately doubled between 10 and 13 m, remaining at this value at greater depth. On the basis of the studies that have so far been carried out, it appears that many benthic aquatic species can increase the content and/or alter the proportions of their photosynthetic pigments with increasing depth.

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