Evidence against chromatic adaptation

On the basis of the relation between water optical type and algal distribution outlined above, we would predict that in any clear, colourless coastal water, with maximum penetration in the blue waveband, green algae should be certainly a major, and probably a dominant, component of the algal biomass throughout much of the middle and lower sublittoral. The water bathing the coast of South Australia, in the region of the Great Australian Bight and the Gulf of St Vincent, is of a clear, colourless oceanic type: there is little river run-off in this dry region. We might thus expect the depth distribution of benthic algae to be similar to that described above for Caribbean, central Pacific or Mediterranean locations. In fact, a series of thorough studies by Shepherd and Womers-ley (1970, 1971, 1976), and Shepherd and Sprigg (1976) has shown this not to be so: the distribution is in reality much more similar to that found on northern European coasts, with the mid-sublittoral zone dominated by large brown algal species, giving way in the lower sublittoral to a dense cover of red algae. In calmer waters the brown algae dominate the upper sublittoral too, whereas on rough-water coasts they are replaced in this zone by a short turf of the (presumably) surge-resistant coralline red alga Corallina.1477 In terms of biomass, green algae are generally a minor component of the algal community at all depths. At one site (Pearson l Great Australian Unfit

Fig. 12.5 Variation of biomass per unit area of rock face of green, brown and red algae with depth in a clear, colourless southern hemisphere temperate water (Pearson Island, Great Australian Bight, 34° S). Plotted from data of Shepherd and Womersley (1971).

Island, Great Australian Bight), however, where the water was particularly clear and colourless (oceanic water type IA), the Chlorophyta (mainly Caulerpa sp.) constituted a significant fraction of the community from 20 to 35 m (Fig. 12.5) but even here never attained the dominant position that green algae occupy in the underwater flora of Malta or Eniwetok atoll. The poor showing of the green algae in these waters cannot be explained on optical grounds.

An alternative possibility is temperature, an environmental parameter that is known to be a major determinant of seaweed geographical distribution. According to Liining (1990), in his book on Seaweeds: Their Environment, Biogeography and Ecophysiology:

the worldwide distribution patterns of seaweeds are mainly determined by global temperature gradients. Deeply imprinted temperature demands, evolved to mirror the geological cycle of cooling and heating of the earth at higher latitudes, keep species of algae apart.

The South Australian waters are comparatively cool, attaining temperatures of 18 to 20°C at the surface in summer. The temperature of the surface water off Malta at the time of the study by Larkum et al. was about 27°C.267 It may be that the siphonaxanthin-containing green algae, being perhaps mainly of tropical origin, require, as a group, higher temperatures for growth than the brown and red algae.

Our consideration of chromatic adaptation has so far been couched in qualitative terms: in terms, for example, of whether a particular pigment absorption band is located within or outside the predominant waveband of light penetrating to a given depth. Dring (1981) has attempted to carry out a quantitative test of the theory by calculating how the amount of photosynthesis per unit irradiance might be expected to change with depth in different kinds of algae in waters of various optical types. The calculations for each alga were carried out using a measured photosyn-thetic action spectrum for the alga and a series of spectral distributions of irradiance at increasing depth computed for a given water type using the spectral transmission data of Jerlov (1976). In effect, what is being calculated is the extent to which the matching of the active pigment composition to the spectral distribution of available light, i.e. chromatic adaptation, should result in increased or diminished efficiency of utilization of available light with increasing depth, and therefore increased or diminished competitive ability of one algal type compared to another. The results of such calculations can then be compared with observed algal depth distribution data to see if the predictions of the theory correspond to observation.

The findings are somewhat mixed. Some of the predictions accord reasonably well with observation. For example, for coastal waters types 3 and 9 (both of which would have sufficient yellow colour to cause rapid attenuation of the blue waveband) calculations predict that with increasing depth there should be a decrease in photosynthetic effectiveness of green algae such as Ulva sp., little change for kelp (brown algae) such as Laminaria saccharina, and an increase for red algal species having phy-coerythrin as the main biliprotein. This fits in quite well with the algal distributions in such waters on northern European coasts where the maximum depths to which the algae penetrate are generally found to be in the order red > brown > green.

On the other hand, these calculations and earlier ones by Larkum et al. (1967) indicate that in colourless oceanic waters, at the deepest limits of algal growth, green algae and thin-bladed brown algae should perform as well as or better than the phycoerythrin-containing red algae. We have in fact already noted that at least in warm oceanic waters green algae do indeed penetrate deeply and are sometimes dominant throughout most of the sublittoral zone. Nevertheless, at extreme depth in such waters they are eventually replaced by red algae. A problem with these calculations is that the validity of the results is highly dependent on the accuracy of the action spectrum attributed to the alga. Dring used action spectra data measured with just one wavelength at a time. As we saw in a previous chapter (§10.3), such action spectra can be misleading because some wavelengths of light when presented singly are incapable of exciting both photosystems to the same extent. The errors arising in this way are not great in the case of the green and brown algae but can be substantial in the case of the red algae in which photosystems I and II have rather different action spectra. If red algal action spectra are measured with supplementary green light (to ensure that photosystem II is always functioning) then substantially higher relative activity in the blue region is obtained.405 It may be that if the calculations were carried out again with a corrected red algal action spectrum, the increased activity in the blue might be sufficient to give the red algae a significant advantage over the green algae in the blue-green light field at the bottom of the euphotic zone in oceanic water.

Another objection to the chromatic adaptation theory is based on the fact that if any particular pigment mixture, whether it be that of the green, the brown or the red algae, is raised to a high enough concentration per unit area of thallus, then there is eventually almost complete light absorption at all wavelengths and the algae become virtually black. Absorption by chlorophyll, for example, in the green waveband, although certainly low, is not infinitesimal and at high chlorophyll concentrations becomes substantial. A mature ivy leaf containing about 60 mgcm-2 chlorophyll absorbs not only, as we might expect, ~100% of the red light incident upon it, but also ^70% of the green (550 nm) light it receives.722 Ramus et al. (1976) argue that if a seaweed is optically thick as are, for example, Codium fragile (green) and Chondrus crispus (red), then it does not matter what colour it is, and they go on to conclude that the red algae are phylogenetically no better adapted to utilize the ambient light at great depth than are their green counterparts. That optically thick algae have roughly the same light-harvesting capacity (approaching 100% at all wavelengths) whatever the nature of their pigments is certainly true. What must be borne in mind, however, is that if an alga is faced with the problem of absorbing light from an ambient field that is rich in the green waveband, then it is much more efficient in terms of the biochemical economy of the cell to achieve this with a pigment such as phycoerythrin, which has its absorption peaks in that waveband, than to do it with chlorophyll, which absorbs only weakly. The protein cost (remembering that chlorophyll has to be part of a pigment-protein) of achieving high absorption in the green with chlorophyll will be much greater than the protein cost of achieving it with phycoerythrin. It is true that within each of the three algal groups there are some species that have adopted the strategy of having a thick, deeply coloured, thallus and thus absorbing most of the incident light at all wavelengths, and it would be reasonable to say that algae in this category do not make use of chromatic adaptation: as Ramus et al. point out, it does not matter which set of pigments they contain. In most marine algal species, however, absorption is far from complete in parts of, or throughout, the spectrum, and so the degree to which their spectral absorption matches the spectral quality of the field is of great significance for the efficient utilization of the incident radiation.

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