Depth variation of photosynthetic characteristics in unicellular algae

As we have noted earlier, depth adaptation of the photosynthetic system in phytoplankton is only to be expected in waters in which density stratification permits some of the cells to remain for long periods at depths where circulation is minimal but there is sufficient light for photosynthesis to take place. This situation exists throughout most of the ocean in the lower part of the euphotic zone. By what criteria can we recognize true depth adaptation of the phytoplankton photosynthetic system? The primary criterion is that cells from deep water should achieve a higher photosynthetic rate per unit cellular biomass than cells from the surface, in the light field prevailing in deep water. The deep-water cells would generally achieve this by investing an increased proportion of their cellular substance in light-harvesting pigments, possibly accompanied by lower respiration. A secondary criterion is that the deep-water cells should have a lower photosynthetic rate per unit cellular biomass than the surface cells at saturating light intensity: this would be a consequence of the deep-water cells, in the interests of biochemical economy, synthesizing only as much carboxylase as they can make use of at the low irradiance values existing at the greater depths.

On its own, a reduction in the light saturation parameter, Ek, is not an entirely satisfactory indication of adaptation to low irradiance.1486 Since Ek = P*m /a (see §10.2), any environmental factor that reduces the light-saturated photosynthetic rate per mg chlorophyll to a greater extent than it affects the slope of the initial part of the P (per mg chlorophyll) versus Ed curve, will reduce Ek. Yentsch and Lee (1966) showed, by analysing a large amount of published data for both natural and cultured phyto-plankton populations, that Ek tends to be linearly related to P*m and they emphasize the desirability of presenting Pm as well as Ek values in studies on light adaptation by phytoplankton. Nevertheless if, when measured under standardized conditions, one phytoplankton population is found to have a lower Ek value (or light saturation value) than another, then while we cannot on this ground alone conclude that its light-harvesting capability has increased, we may reasonably suppose that its ratio of light-harvesting capacity to carboxylation rate has increased, i.e. it has either increased its pigment content or divested itself of some of its surplus carboxylation capacity, or both, all of which can be regarded as forms of shade adaptation. It is generally found in the oceans that once thermal stratification has been established, phytoplankton isolated from the lower part of the euphotic zone have a lower Ek value than phytoplankton from near the surface.1297 Phytoplankton from the surface, 50 m and 100 m depth (100%, 10% and 1% of subsurface irradiance) in the Sargasso Sea in October, had Ek values of about 600, 300 and 60 mmol photons m-2 s-1 (Fig. 12.12a).1159 In the winter when the thermocline breaks down, the algae are circulated rapidly enough to prevent their becoming adapted to

Acquatic Ecosystem Images

Fig. 12.12 Depth adaptation by marine phytoplankton (after Ryther and Menzel, 1959). These P versus Ed curves for Sargasso Sea phytoplankton were determined by the 14C method at the temperature prevailing at the sea surface. Samples taken from 0 m (o), 50 m (©) and 100 m (•) depth. (a) October; water stratified with thermocline at 25 to 50 m. (b) November; water isothermal down to >150 m depth.

Fig. 12.12 Depth adaptation by marine phytoplankton (after Ryther and Menzel, 1959). These P versus Ed curves for Sargasso Sea phytoplankton were determined by the 14C method at the temperature prevailing at the sea surface. Samples taken from 0 m (o), 50 m (©) and 100 m (•) depth. (a) October; water stratified with thermocline at 25 to 50 m. (b) November; water isothermal down to >150 m depth.

the irradiance value at any depth, and it is found that the cells from all depths show the same degree of light adaptation and indeed are rather similar in their properties to summer surface phytoplankton (Fig. 12.12b).1159,1297 It can be seen from the curves in Fig. 12.12a that the phytoplankton from the lower part of the euphotic zone not only have a lower Ek, but are much more susceptible to photoinhibition by higher light intensity than are the surface phytoplankton.

Shimura and Ichimura (1973) found that phytoplankton from near the bottom of the euphotic zone in the ocean had a higher photosynthetic activity than surface phytoplankton when both were placed deep in the euphotic zone. In addition the phytoplankton from the deep layer showed a higher ratio of photosynthetic activity in green light to that in red light than surface layer phytoplankton. Thus the phytoplankton growing deep in the euphotic zone of the ocean appeared to be the better adapted to the dim, predominantly blue-green, light prevailing in that region. Similarly, Neori et al. (1984) found, for temperate and polar oceanic stations, that both absorption and chlorophyll a fluorescence excitation spectra of phytoplankton samples showed enhancement in the blue-to-green part of the spectrum (470-560 nm) relative to that at 440 nm, with increasing depth. They attributed this change to an increase in the concentration of photosynthetic accessory pigments, relative to chlorophyll a.

In nature in thermally stratified waters, the cells below the thermocline will be at a lower temperature than those near the surface and as we saw in an earlier section (§11.3), lowering the temperature automatically lowers Ek (by diminishing P*m without affecting a), quite apart from any alterations in the biochemical composition of the depth-adapted cells. That there are nevertheless inherent changes in the light relations of the cells is readily shown (as in the experiments in Fig. 12.12) by comparing the deep- and shallow-water phytoplankton at the same temperature. The in situ lowering of the Ek value of the deep phytoplankton due to lowered temperature is in fact of little ecological significance since the light intensity at that depth will be well below saturation in any case.

As we have noted earlier, the extent to which shade adaptation of the deep phytoplankton in the ocean represents true ontogenetic adaptation (i.e. physiological adaptation within species occurring throughout the illuminated water column) or is phylogenetic adaptation (the predominance in deeper water of particular species genetically adapted to low irradiance values) in most cases remains uncertain. However, Neori et al. (1984) believe that the changes in absorption and fluorescence properties with depth that they observed (see above) were predominantly due to shade adaptation within existing species: the depth distribution of species did not seem to account for the spectral changes, and furthermore, similar changes could be detected in phytoplankton samples within one day of transferring them to low light levels.

In the clear, colourless water of Lake Tahoe in which, as in the ocean, the illuminated region extends below the thermocline, Tilzer and Goldman (1978) found that the increase in chlorophyll content of the deep-water cells at the time of maximum thermal stratification (September) was accompanied by a diminution in the light saturation onset parameter: the Ek values were about 80, 50 and 15 W(PAR)m~2, for cells from 0, 50 and 105 m depth, respectively. The phytoplankton in this lake satisfied the primary criterion for true depth adaptation of the photosynthetic system. In the light field prevailing in deep water, the deep-water cells achieved a substantially higher photosynthetic rate per unit biomass than cells from the surface (Fig. 12.13). Their light-saturated rate of photosynthesis per unit biomass was not, however, significantly lower than that of the shallow-water cells, suggesting that the deep-water cells in this lake had not adopted the additional shade adaptation strategy of reducing their carboxylase content.

The less turbulent the upper mixed layer of a water body is, the more time the phytoplankton cells have to adapt as they undergo vertical

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