A comparative study of the cell surface and the mechanism of macromolecule internalization in three phytoflagellates

A cytocheraical investigation on the cell surface of three phytoflagellates, one chlorophyte, Dunaliella tertiolecta and two dinoflagellates, Amphidinium carterae and Prorocentrum roicans showed that they all possess an anionic and heterogenous oligosaccharide coat on the external side of the plasma membrane. Studies with lectins gave clues to the nature of the sugar residues of their glycoconjugates. In particular, they may contain N-acetylglucosamine and/ or sialic acid, N-acetylgalactosamine and D-mannose or glucose. Among other properties, the carbohydrate moieties of the cell surface components were found to play an important role in cell recognition, cell to cell adhesion and in endocytosis. All three phytoflagellates were able to internalize macromolecules such as horseradish peroxidase, cationized ferritin, lectins and lectin-colloidal gold conjugates. In Dunaliella pinocytosis was observed for the first time, and was found to occur in the vicinity of the flagella, away from the cup-shaped chloroplast. During fluid-phase and adsorptive endocytosis, macromolecules were trapped within uncoated or coated pits and then delivered, via endosomes, to the lysosomal system. The reverse process of metabolic secretion as part of membrane recycling is also described. In Prorocentrumf exogenous macromolecules were internalized via surface pores, while in Amphidinium the ligands were taken in by plasma membrane-derived vesicles with subsequent incorporation into the peripheral cytoplasmic vacuole. In both dinoflagellates, the major site of macromolecule internalization was the flagellar canal-pusule system. Morphological and morphometric studies on Prorocentrum showed pusule size changes in response to different levels of salinity and temperature. A three-dimensional model of Amphidinium pusule is proposed based on sequential sections. It is suggested that a fibrillar collar system in conjunction with the flagellar-beating plays an important role in regulating the flow of materials in and out of the pusule. It appears that the pusule is a complex structure involved in osmoregulation, cellular uptake and possibly secretion. With regard to the relationship between the ultrastructure of the cell covering and the mechanism of endocytosis, two hypothetical evolutionary lines are suggested, one includes those organisms such as Dunaliella, amoeba and some other protozoa that lack a rigid cell wall and a specialized compartment for macromolecule internalization; the other comprises those forms such as dinoflagellates, cryptomonads, euglenoids and some kinetidal protozoa that possess a complex cell covering and a specific compartment for macromolecule uptake.Science, Faculty ofZoology, Department ofGraduat