The principle of morphological conservation of a favourable surface-to-volume ratio, which holds equally for the phytoplankton of marine and inland waters, might be more strongly compelling were it not for the fact that another common evolutionary trend - that of embedding vegetative cells in swathes of mucilage - represents a total antithesis. The formation of globular colonies is prevalent among the freshwater Cyanobacteria, Chlorophyta and the Chryso-phyta. It is also observed in the vegetative life-history stages of the haptophyte, Phaeocys-tis, though, generally, the trait is not common among the marine phytoplankton. In many instances, the secondary structures are predominantly mucilaginous and the live cells may occupy as little as 2% of the total unit volume in Coenochloris and Uroglena and scarcely exceeds 20% in Microcystis or Eudorina. It was originally supposed to provide a low-density buoyancy aid but it has since been shown that any advantage is quickly lost to increased size (see Chapter 2). In some instances, the individual cells are flagellate (as in Uroglena and Eudorina) and the flagella pass through the mucilage to the exterior, where their coordinated beating propels the whole colony through the medium. Because the surface offers little friction, the mucilage is said to be helpful in assisting rapid passage and migration through weakly turbulent water. Certainly, in the case of the colonial gas-vacuolate Cyanobacteria that are able to regulate their buoyancy (e.g. Micro-cystis, Snowella, Woronichinia), larger colonies float more rapidly than smaller ones of the same density (Reynolds, 1987a). Merely adjusting buoyancy then becomes a potentialy effective means of recovering or controlling vertical position in the water (Ganf, 1974a). It is interesting that, in the two buoyancy-regulating filamentous species of
Cyanobacteria included in Table 1.2 (Anabaena circinalis, Aphanizomenon f los-aquae), the supposed advantage of the filamentous habit is sacrificed through a combination of aggregation, coiling and mucilage production to the attainment of rapid rates of migration (Booker and Walsby, 1979).
Provided colonies have a simultaneous capacity for controlled motility, there are good tele-ological grounds for deducing circumstances when massive provision of mucilage represents a discrete and alternative adaptation to a plank-tic existence. However, the idea that streamlining is more than a fortuitous benefit is challenged by the many non-motile species that exist as mucilaginous colonies. There are other demonstrable benefits from a mucilaginous exterior, including defence against fungal attack, grazers, digestion or metal toxicity, and there are circumstances in which it might assist in the sequestration or storage of nutrients or in protecting cells from an excessively oxidative environment (see Box 6.1, p. 271). Even mucilage itself, essentially a matrix of hygroscopic carbohydrate polymers immobilising relatively large amounts of water, is highly variable in its consistency, intraspecifi-cally as well as interspecifically.
Thus, doubts persist about the true function of mucilage investment. However, a consistent geometric consequence of mucilage investment is that the planktic unit is left with an exceptionally low surface-to-volume ratio (i.e. area II, towards the left in Fig. 1.7).
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