The evidence taken together seems to me to lead to the conclusion that chromatic adaptation is a major factor influencing the depth distribution of the three types of benthic marine algae, but that it is not the only one, and in some instances other factors prevail. The fact that all algae that grow at depths where the light is predominantly green or blue-green have pigments, whether specialized carotenoids such as fucoxanthin or siphonaxanthin, or biliproteins such as R-phycoerythrin, which enhance their absorption in that spectral region, cannot in my view be plausibly regarded merely as coincidence. The algae evolved in the sea; this means that they evolved in an environment throughout most of the illuminated part of which, the light field had a predominantly blue-green character. Given the enormous importance of absorption spectra in determining the efficiency of collection of light energy, it is hard to believe that the types of pigment system that evolved would not be influenced by, and thus correspond to, the spectral character of the prevailing light. The argument that the specialized algal pigment systems are merely adaptations to low irradiance I do not find acceptable. That they are adaptations to limiting light levels seems unquestionable, but if the low-intensity light field has a markedly non-uniform distribution across the spectrum, then for efficient utilization of cellular resources it is better to make pigments whose absorption bands are well placed to harvest the dim light. It is in principle possible to achieve any required level of light collection at any depth with, say, just the green pigment mixture found in surface-dwelling Chlorophyta provided the pigment concentration per unit area is made high enough: the fact that the algae have in the main not chosen that, biochemically expensive, form of adaptation to low irradiance, but have evolved pigments absorbing in the spectral regions where the light actually occurs, indicates that they are adapted not just to low light, but to low light of a particular spectral character.
Thus, the predominance of red algae in the deepest part of the euphotic zone can be plausibly attributed to their possession of phycoerythrin - the most efficient way (in terms of quanta collected per unit of protein invested) of harvesting the dim blue-green light prevailing at those depths. The failure of green algal species lacking siphonaxanthin to penetrate to great depths we may consider to be a consequence of their comparatively poor absorption capacity for the prevailing underwater light field. The greater depth distribution of the siphonaxanthin-containing green species, and the brown algae, we may reasonably regard as being due to the enhanced capacity for absorption in the plentiful 500 to 550 nm waveband, conferred by their specialized carotenoids.
Chromatic adaptation is not, of course, the whole story. We noted early on that there is no reason why it should have anything to do with the relative success of the different algal groups near the surface. The presence of non-siphonaxanthin green species near the surface is not an example of chromatic adaptation, but their failure to penetrate the depths is such an example. The predominance of coralline reds near the surface in rough-water locations in South Australia has everything to do with their resistance to wave movement and nothing to do with their pigment composition. The presence of significant levels of green algae in the 20 to 35 m zone in the clear water of Pearson Island, South Australia (Fig. 12.5), we may regard as an example of chromatic adaptation, but their much poorer performance overall in South Australian than in the optically similar Mediterranean or central Pacific waters indicates that other factors such as, perhaps, temperature can prevail over chromatic adaptation in determining algal depth distribution. Within the deep-water red-alga zone, the transition at the lowest depths from fleshy macroscopic red algae to encrusting coralline red algae1202 has no obvious explanation in terms of pigment differences. In the very dim light available at those depths, production rates can only be very low, in which case the rates of loss by grazing and respiration become very important. It may be that the encrusting coralline reds are less susceptible to grazing than the fleshy species and/or have a lower respiration rate. The predominance of seagrasses on sandy bottoms down to great depths in clear colourless water, e.g. to 35 m in the St Vincent Gulf, South Australia1206 or 50 m in the Red Sea,814 is not due to their being chromatically better adapted than algae (although they should be reasonably well adapted to the bluish light prevailing in such habitats) but to their ability, by means of their roots and rhizomes, to colonize these unstable substrates.
In short, phylogenetic chromatic adaptation is by no means the only factor responsible for the depth distribution of the different plant groups under water: we should, however, continue to regard it as one important factor in that set of interacting factors that finally determines that distribution.
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