Form and Function in Marine Phytoplankton

Abstract

Summary What 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...

A biodiversity‐inspired approach to aquatic ecosystem modeling

“…The initial high-turbulence, high-nutrient environment is replaced by a stratified depleted environment as the year progresses, with distinct niches for different groups of species (respectively: diatoms, coccolithophorids, dinoflagellates) along this trajectory. Margalef explained this succession in terms of differential species-specific affinities for limiting nutrients, which in turn have been linked to morphology and cell size (Sournia 1982a;Aksnes and Egge 1991;Chisholm 1992). The concept of affinity-driven succession applies seamlessly to (near-surface) seasonal succession as observable in our results: As the bloom persists, colonizing species with low nutrient affinities are replaced by species with higher nutrient affinity.…”

“…Nutrient-harvesting biomass includes compounds directly affecting nutrient consumption (e.g., membrane-bound transporters), as well as any co-occurring machinery. If one accepts that the capacity for nutrient uptake is determined by the surface area of the cell (Munk and Riley 1952;Sournia 1982a), nutrient-harvesting biomass must comprise the ''shell'' of the cell, i.e., the cell wall and membrane as well as transporters. Then the ratio of nutrient-harvesting biomass to structural biomass can serve as proxy for the surface-to-volume ratio, and for isomorphic species its reciprocal can be a proxy for cell size (Kooijman 2000).…”

Benthic-pelagic exchange of microalgae at a tidal flat. 2. Taxonomic analysis

“…Evidence that sinking occurs is well known, although much of the evidence comes from stratified, open-water environments (Kiørboe 1993) where turbulent currents are weak and the risk of sinking out of the euphotic zone is high. , Smayda (1970), andSournia (1982) have summarised the mechanisms aiding suspension/flotation of phytoplankton in the sea as morphological (cell shape, size, colony formation, protuberances), physiological (lipid reserves, ionic regulation) and physical. For non-motile phytoplankton such as diatoms, the distribution of cells is closely coupled with physical mixing processes.…”

Some size relationships in phytoflagellate motility

“…lies on two data sets: Throndsen (1973) and Table 2 in Sournia (1982) which is a compilation of data from several authors. Both sources comprise only data for marine species.…”

“…Logically, Table 2. Swimming velocity (tm -s -') and size (diameter of spherical equivalent, jim) of marine flagellates (data from Throndsen, 1973 andSournia, 1982).…”