The majority of phytoplankton cells are subjected to continually varying irradiance as they circulate through the mixed layer and do not have the time to change their pigment composition in accordance with the prevailing light field. When, as in some of the clearer, less coloured waters, light penetrates so well that the euphotic zone is deeper than the mixed layer, however, then in the stable water below the thermocline the phytoplankton can remain at approximately the same depth for long periods and there is evidence that they do indeed adapt to the light conditions prevailing at such depths. We noted earlier (§11.1) that phyto-plankton isolated from the deep chlorophyll maximum in the oceans have about twice as much chlorophyll per unit biomass as phytoplankton from the surface layer.687 This is not necessarily adaptation within a species: whether members of the same species are found both in a highly pig-mented form in the oceanic deep chlorophyll maximum and in a less-pigmented form near the surface, or whether the taxonomic composition of the phytoplankton population is entirely different at the two depths, is in most cases not known. However, in the cyanophyte, Prochlorococcus, which can occur throughout the illuminated water column, there is evidence for a much greater chlorophyll content in the cells in the DCM than in the surface layer.1289,1407
In inland waters with their usually more rapid attenuation of light with depth, it is typically the case that the depth of the mixed layer is greater than or equal to that of the euphotic zone, so there is little opportunity for depth adaptation by the phytoplankton. There are, however, some lakes such as Crater Lake, Oregon, USA, and Lake Tahoe, California-Nevada, USA, with water so clear and colourless that the euphotic zone is deeper than the mixed layer and a deep chlorophyll maximum develops at 75 to 100 m depth in the growing season.769,1364 Even in lakes in which light does not penetrate deeply, if there is for some reason particularly stable thermal stratification so that circulation is restricted to a very shallow layer, then the euphotic depth can exceed the mixed depth. This appears to be the case in Lake Lovojarvi in Finland: in the summer this was found to have an epilimnion about 2.5 m deep but with photosynthesis extending down to about 3.5m.659 A chlorophyll maximum, consisting largely of cells of the blue-green alga Lyngbya limnetica with a particularly high chlorophyll content, developed at about 3.5 m.
In Lake Tahoe, Tilzer and Goldman (1978) showed that in June and September (particularly the latter) the chlorophyll content of the phyto-plankton (mainly diatoms) per unit biomass increased with depth. In September, for example, the chlorophyll a content (mg per mg wet mass) was 0.61, 1.8 and 3.06 at 0, 50 and 105m depth, respectively. Since one diatom species, Fragilaria vaucheriae, constituted about 58% of the total biomass at all three depths, it seems likely that the increase in chlorophyll content of the total biomass represented true adaptation within this, and perhaps other, species. In June also, when three other species constituted most of the biomass, a doubling of chlorophyll a content between 20 and 50 m was accompanied only by comparatively small changes in the population composition, suggesting that chlorophyll content had increased within species. In Lake Lovojarvi, in contrast, there was a marked change in population composition between the mixed layer and the chlorophyll maximum in the hypolimnion, with Lyngbya being unimportant in the former and dominant in the latter: the chlorophyll maximum in this lake we may regard as being due to phylogenetic rather than ontogenetic adaptation. Similarly, in clear-water lakes in northwestern Ontario, Canada,389 the deep chlorophyll maximum (4-10 m depth) was in each case dominated by a single large, colonial chrysophycean flagellate species (the particular species varying from one lake to another), presumably adapted to this particular niche.
Given the demonstrated in vitro pigment adaptation of marine phyto-plankton species to blue light, and taking into account also the data for Lake Tahoe (optically similar to ocean water), we may reasonably suspect that the deep chlorophyll maximum in oceanic waters is caused at least in part by increased pigment synthesis promoted by the low intensity and/or the predominantly blue character of the prevailing light, within species that occur over a range of depths. It seems likely that there are present, in addition, specialized, highly pigmented species for which the bottom of the euphotic zone is the preferred niche. Studies of the taxonomic composition of the phytoplankton population as a function of depth in stratified oceanic waters could provide information on the relative importance of phylogenetic and ontogenetic adaptation in the establishment of the deep chlorophyll maximum.
Not all unicellular algae are planktonic. Any species that normally occur attached to, or within, fixed structures, such as the surface of sediments or rocks, or epiphytically upon larger algae, are thereby freed from the continually varying character of the light field experienced by the plankton and so have the opportunity to adapt to the light field prevailing at the depths where they are located. In McMurdo Sound, Antarctica, in the austral spring Robinson et al. (1995a) found the molar fucoxanthin/chl a ratio of the diatom-dominated algal community in the platelet layer immediately underneath the 2.2 m thick ice layer, and at the benthic surface (26m depth), to be 1.22 and 1.61, respectively, which may be compared with 0.74 to 0.92 for surface-ice algae, and a literature value of 0.61 to 0.83 from a survey of 51 diatom species.1290 Robinson et al.
interpreted the high fucoxanthin/chl a ratios as an adaptive complementary pigmentation response, resulting in enhanced absorption of the predominantly green light. It may be noted that since the surface ice, platelet layer and benthic diatom populations have different taxonomic compositions, these pigment changes should be regarded as essentially phylogenetic, rather than ontogenetic, in nature.
Studies have also been carried out on the symbiotic dinoflagellates (zooxanthellae) that occur within corals. There is a general tendency, but to varying extents in different coral species, for the pigment content of the zooxanthellae to increase with depth. Leletkin et al. (1981) compared the zooxanthellae of the coral Pocillopora verrucosa growing at 20 and 45 m depth in the Timor Sea. Those from coral at 45 m contained about 1.5 times as much chlorophylls a and c, b-carotene and diadinox-anthin, and 2.4 times as much peridinin per cell as zooxanthellae from coral at 20 m. The number of chlorophyll molecules per photosynthetic unit was 42% higher in the cells from 45 m than in those from 20 m, but the number of photosynthetic units per cell was about the same. Thus the pigment adaptation to increased depth appears largely to consist of an increase in the pigment complement per photosynthetic unit.380
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