Cell-wall morphology correlated with vertical migration in the non-motile marine dinoflagellate Pyrocystis noctiluca

Abstract

We report an ultrastructural study of the morphological changes in cells of the marine dinoflagellate Pyrocystis noctiluca Murray, which correlate with its vertical migration pattern. Cells alternate between a large, highly vacuolated, positively buoyant, vegetative cyst surrounded by a dinosporin-containing wall and a smaller, more compact, negatively buoyant, cellulose-bounded cell. The cyst wall is composed of two layers: a thin smooth outer layer, thought to be composed of dinosporin, and a thick inner layer that likely to be cellulosic. One or two thecate cells are formed from within the cysts. Thecate cells are smaller, more compact and contain many small translucent bodies. They are surrounded by a typical dinoflagellate amphiesmal layer composed of membranes and cellulose plates. The amphiesmal layer appears only in recently divided cells and exists for only one night. By the following day, the cellulose wall has been replaced by a new dinosporin wall synthesized from beneath the cellulose thecal layer. The cyst stage is suggested as being optimized for photosynthesis, whereas the compact, negatively buoyant, thecate form is thought to allow nutrient uptake in deeper waters. Vertical migration in this species is thus correlated with the presence of dinosporin wall during most of its stay in the upper waters, alternating with a brief thecate wall in deeper nutrient-rich waters. This is the first report correlating dinoflagellate vertical migration with changes in cell-wall composition.