General features of phytoplankton

Despite being drawn from a diverse range of what appear to be distantly related phyloge-netic groups (Table 1.1), there are features that phytoplankton share in common. In an earlier book (Reynolds, 1984a), I suggested that these features reflected powerful convergent forces in evolution, implying that the adaptive requirements for a planktic existence had risen independently within each of the major phyla represented. This may have been a correct deduction, although there is no compelling evidence that it is so. On the other hand, for small, unicellular microorganisms to live freely in suspension in water is an ancient trait, while the transition to a full planktic existence is seen to be a relatively short step. It remains an open question whether the supposed endosymbiotic recombinations could have occurred in the plankton, or whether they occurred among other precursors that subsequently established new lines of plank-tic invaders.

It is not a problem that can yet be answered satisfactorily. However, it does not detract from the fact that to function and survive in the plankton does require some specialised adaptations. It is worth emphasising again that just as phytoplankton comprises organisms other than algae, so not all algae (or even very many of them) are necessarily planktic. Moreover, neither the shortness of the supposed step to a plank-tic existence nor the generally low level of structural complexity of planktic unicells and coeno-bia should deceive us that they are necessarily simple organisms. Indeed, much of this book deals with the problems of life conducted in a fluid environment, often in complete isolation from solid boundaries, and the often sophisticated means by which planktic organisms overcome them. Thus, in spite of the diversity of phy-logeny (Table 1.1), even a cursory consideration of the range of planktic algae (see Figs. 1.1-1.5)

Figure 1.3

Coenobial phytoplankters. Colonies of the diatoms (a) Asterionella formosa, (b) Fragilaria crotonensis and (d) Tabellaria flocculosa var. asterionelloides. The fenestrated colony of the chlorophyte Pediastrum duplex is shown in (c). Scale bar, 10 |im. Original photomicrographs by Dr H. M. Canter-Lund, reproduced from Reynolds (1984a).

reveals a commensurate diversity of form, function and adaptive strategies.

What features, then, are characteristic and common to phytoplankton, and how have they been selected? The overriding requirements of any organism are to increase and multiply its kind and for a sufficient number of the progeny to survive for long enough to be able to invest in the next generation. For the photoautotroph, this translates to being able to fix sufficient carbon and build sufficient biomass to form the next generation, before it is lost to consumers or to any of the several other potential fates that await it. For the photoautotroph living in water, the important advantages of archimedean support and the temperature buffering afforded by the high specific heat of water (for more, see Chapter 2) must be balanced against the diffi-cuties of absorbing sufficient nutriment from often very dilute solution (the subject of Chapter 4) and of intercepting sufficient light energy to sustain photosynthetic carbon fixation in excess of immediate respiratory needs (Chapter 3). However, radiant energy of suitable wavelengths (photosynthetically active radiation, or PAR) is neither universally or uniformly available in water but is sharply and hyperbolically attenuated with depth, through its absorption by the water and scattering by particulate matter (to be discussed in Chapter 3). The consequence is that for a given phytoplankter at anything more than a few meters in depth, there is likely to be a critical depth (the compensation point) below which net photosynthetic accumulation is impossible. It follows that the survival of the phytoplankter depends upon its ability to enter or remain in the upper, insolated part of the water mass for at least part of its life.

This much is well understood and the point has been emphasised in many other texts. These have also proffered the view that the essential characteristic of a planktic photoautotroph is to minimise its rate of sinking. This might be literally true if the water was static (in which case,

Figure 1.4

Filamentous phytoplankters. Filamentous coenobia of the diatom Aulacoseira subarctica (a, b; b also shows a spherical auxospore) and of the Cyanobacteria (c) Gloeotrichia echinulata, (d) Planktothrix mougeotii, (e) Limnothrix redekei (note polar gas vacuoles), (f) Aphanizomenon flos-aquae (with one akinete formed and another differentiating) and Anabaena flos-aquae (g) in India ink, to show the extent of mucilage, and (h) enlarged, to show two heterocysts and one akinete. Scale bar, 10 |im. Original photomicrographs by Dr H. M. Canter-Lund, reproduced from Reynolds (1984a).

neutral buoyancy would provide the only ideal adaptation). However, natural water bodies are almost never still. Movement is generated as a consequence of the water being warmed or cooling, causing convection with vertical and horizontal displacements. It is enhanced or modified by gravitation, by wind stress on the water surface and by the inertia due to the Earth's rotation (Coriolis' force). Major flows are compensated by return currents at depth and by a wide spectrum of intermediate eddies of diminishing size and of progressively smaller scales of turbulent diffu-sivity, culminating in molecular viscosity (these motions are characterised in Chapter 2).

To a greater or lesser degree, these movements of the medium overwhelm the sinking trajectories of phytoplankton. The traditional view of planktic adaptations as mechanisms to slow sinking rate needs to be adjusted. The essential requirement of phytoplankton is to maximise the opportunities for suspension in the various parts of the eddy spectrum. In many instances, the adaptations manifestly enhance the entrainabil-ity of planktic organisms by turbulent eddies. These include small size and low excess density (i.e. organismic density is close to that of water, ~1000 kg m-3), which features do contribute to a slow rate of sinking. They also include

Figure 1.5

Colonial phytoplankters. Motile colonies of (a) Volvox aureus, with (b) detail of cells, (c) Eudorina elegans, (d) Uroglena sp. and (e) Dinobryon divergens; and non-motile colonies, all mounted in India ink to show the extent of mucilage, of (f) Microcystis aeruginosa, (g) Pseudosphaerocystis lacustris and (h) Dictyosphaerium pulchellum. Scale bar, 10 |im. Original photomicrographs by Dr H. M. Canter-Lund, reproduced from Reynolds (1984a).

mechanisms for increasing frictional resistance with the water, independently of size and density. At the same time, other phytoplankters show adaptations that favour disentrainment, at least from weak turbulence, coupled with relatively large size (often achieved by colony formation), streamlining and an ability to propel themselves rapidly through water. Such organisms exploit a different part of the eddy spectrum from the first group. The principle extends to the larger organ isms of the nekton, - cephalopods, fish, reptiles and mammals - which are able to direct their own movements to overcome a still broader range of the pelagic eddy spectrum.

All these aspects of turbulent entrainment and disentrainment are explored more deeply and more empirically in Chapter 2. For the moment, it is important to understand how they impinge upon phytoplankton morphology in a general sense.

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