Plants of the subtidal red alga Ptilota serrata collected from 24 m depth off northeastern America showed light saturation of photosynthesis at about 116mmolphotonsm~2s_1, whereas plants from 6m saturated at about 182 mmolphotonsm^s-1.877 At very low light intensities (714 mmol photons m~2s_1) both kinds of plant achieved the same photo-synthetic rate per g dry mass but at higher intensity the deep-water plants levelled off first and achieved a maximum photosynthetic rate only half that of the shallow-water plants (Fig. 12.11). It seems likely that the shallow-water plants contained a more active carboxylation system.
A similar adaptive response has been observed by Glenn and Doty (1981) in the tropical red alga Eucheuma striatum. They attached thalli of this alga (grown in shallow water) at a series of points to a line extending from 1 to 9.5 m depth on a Hawaiian reef: irradiance diminished by about a factor of ten over this range. After a month the thalli were recovered for photosynthetic measurements. A progressive decrease in photosynthetic capacity (light-saturated photosynthetic rate per g dry mass) with depth
was observed, the value in thalli from 9.5 m being about half that in material from 1 m. Thus, Eucheuma seems to divest itself of surplus carboxylating capacity as depth increases: a response that should be of benefit to the alga.
Fairhead and Cheshire (2004) studied the seasonal and depth-related variation in the photosynthetic characteristics of the brown macroalga, Ecklonia radiata, growing subtidally on a rocky substrate at West Island, South Australia. The light intensity required for saturation decreased with depth. For example, in spring (September) Emax values were 469, 383, 116 and 77mmolphotonsm~2s_1, respectively, for algae collected from 3, 5, 10 and 12m depth. In winter, when average irradiance is lower, the photosynthetic efficiency (expressed in terms of a) at sub-saturating light increased, and the irradiance required for saturation of photosynthesis, and for light compensation of respiration, both decreased. In a high-arctic fjord (Young Sound, NE Greenland), Borum et al. (2002) found that the light saturation onset parameter and the light compensation point of the brown macroalga, Laminaria saccharina, in (ice-free) August both decreased with depth: Ek and Ec were 170 and 22 mmol photons m~2s_1, respectively, at 2.5 m depth, and 17 and 4.2 mmol photons m~2s_1 at 10 m depth.
In the case of the giant kelp Macrocystis pyrifera, as we noted earlier, a single frond can extend from the holdfast on the sea floor up through as many as 30 m of water column and thus through a very considerable light gradient. Wheeler (1980a) found that in immature fronds, 12 to 14 m long, growing in 12 m of water in coastal water off Santa Barbara (California), the light-saturated photosynthetic rate per unit area rose progressively with distance from the holdfast, 1.5- to three-fold, reaching a maximum at about 10 to 12 m. Thus, over this part of the frond the photosynthetic capacity per unit area rose with diminishing depth, which we may regard as an adaptive response since it is only at the lesser depths that there is enough light to support a high photosynthetic rate per unit area. In single plants of M. pyrifera in Baja California, extending from the holdfast at 18 m depth to the surface, Colombo-Pallotta et al. (2006) found the light-saturated photosynthetic rate per unit area of blade to increase progressively with diminishing depth, rising by ^60% between 18 and 0 m. Consistent with the depth variation in the content of xantho-phyll cycle photoprotective carotenoids, noted earlier, non-photochemical quenching of chl a fluorescence was very marked in tissue from the surface, and still evident in tissue from 3 and 6 m, but virtually absent in tissue from below 9 m depth, confirming that this alga invests in photoprotection of its photosynthetic system only where its fronds encounter it near the surface, and not at greater depth.
In coastal waters of southeastern Florida, two Halophila seagrass species, H. johnsonii and H. decipiens, occur together subtidally, but only the former species is also found in the shallower intertidal waters. Durako et al. (2003) found that the onset of photoinhibition occurred at lower irradiances for H. decipiens (537-820 mmol photons m~2s_1) than for H. johnsonii (11412670 mmol photons m~2s_1), and that furthermore, leaves of H. johnsonii contained a UV-absorbing (peak, 345 nm), and therefore possibly photo-protective, pigment absent from H. decipiens. Halophila johnsonii therefore appears to be better adapted to the light stresses of shallow waters than H. decipiens. However, since these species, although closely related, are not identical, this should properly be regarded as an example of phylogenetic, rather than ontogenetic, adaptation.
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