FORM AND FUNCTION IN MARINE PHYTOPLANKTON
Authors: SOURNIA ALAIN 
Published: January 2008

SummaryWhat is presented here is a tentative synthesis of morphological, cytological, physiological and ecological data on planktonic algae, which I hope will help in the understanding of mutual relationships. Emphasis is put on the marine phytoplankton although effort has been made to include the more significant limnological information.(1) All the algal classes, but two, are present in the marine plankton – which makes 13 classes. Many or most of them possess one or several features that are commonly viewed as animal characters, and so Bacillariophyceae (diatoms) are the more typical ‘algae’ in marine plankton. Coincidently or not, they have received much more attention than any other class.(2) Both structurally and morphodynamically, colonies of cells often appear as something else or something more than sums of cells.(3) There is a profuse variety of flagellar types and flagellar appendages, whose functional significance is open to investigation. In general, swimming velocity (ca. 1 m h‐l) exceeds sedimentation rate (ca. 0·7 m day‐1) by one order of magnitude or more.(4) Very few phytoplankters can truly be described as “naked”. Cell coverings fall into eight major categories which differ by chemical composition, structure and ontogeny. An additional, external organic coating may be widespread. Mucus and microfibrils may also be more common than previously thought.(5) The variability of chloroplast morphology and ultrastructure has not been explored for functional relationships.(6) Evidence for the presence of intracellular bacteria and viruses is rapidly increasing.(7) The suspension of algal cells in the medium depends on a number of morphological factors whose effects are often opposite. The sinking rate increases with increasing cell size and is maximum for spheroid (not spherical) shapes; colonies sink faster than the individual cells. The incidence of various shapes, appendages, mucilage and cell orientation is, essentially, intricate and/or insufficiently known. Lipidic inclusions are no longer viewed as floating devices but the ancient theory of ionic exchange has been revived. As now understood, the suspension of phytoplankton is no more a matter of floating, but rather exploring different layers and being tossed around by physical entrainment. However, questions remain the same: how do the “morphological adaptations” contribute to this, and how do different forms compare to each other?(8) As far as the absorption of nutrients is concerned, there is an advantage for phytoplankters to be small and either motile or rapidly sinking. The permeability of the various cell coverings has been ignored. The advantage of being small is confirmed by the consideration that growth rates and all the metabolic rates decrease with increasing size. However, the balance of photosynthesis against respiration for varying sizes is a complex problem.(9) After an extensive review of the literature, the existence of several “shade species” is confirmed (without ensuring that light is the responsible factor). These taxa exhibit the full range of shapes, sizes, structures and behaviour, so that the relevant morphological adaptations, if any, are at least polymorphic.(10) Although grazing certainly moulds the size spectrum and the algological spectrum of phytoplankton to a large extent, the effect of a given morphological feature can hardly be generalized, except that long appendages, mucilages, and probably colonies discourage grazers. The role of bioluminescence and trichocyst expulsion may also be considered.(11) The hope of correlating cell size with a single factor of the environment, whether temperature or something else, should be abandoned since many more factors are involved. On the other hand, multi‐parametric models can justify or predict which cell size predominates under a given set of conditions (note the works of H. J. Semina and co‐workers, T. R. Parsons and M. Takahashi, and E. A. Laws).(12) The ratio of surface area to volume of the cell is a meaningful physiological index. Its relative conservation among the vagaries of sizes and shapes, and its ecological regulation need further investigation.(13) Considering the profusion of data that has accumulated on the structure and functioning of planktonic algae, and realizing that sophisticated techiques are available for both kinds of studies, now is the time, it seems, for fruitful research into the relationships between form and function. Such research will certainly increase our understanding of specific variability, adaptation and diversity.