Phycology

These organisms are important members of the plankton in both fresh and marine waters, although a much greater variety of forms is found in marine members. Generally the Dinophyceae are less important in the colder polar waters than in warmer waters. The highly elaborate Dinophysales (Fig. 7.47(d), (e)) are essentially a tropical group.

A typical motile dinoflagellate (Figs. 7.1, 7.2) consists of an epicone and hypocone divided by the transverse girdle or cingulum. The epicone and hypocone are normally divided into a number of thecal plates, the exact number and arrangement of which are characteristic of the particular genus (Figs. 7.1, 7.3, 7.20(b), 7.24(b)). There is a longitudinal sulcus running perpendicular to the girdle. The longitudinal and transverse flagella emerge through the thecal plates in the area where the girdle and sulcus meet. The longitudinal flagellum projects out from the cell, whereas the transverse flagellum is wave-like and is closely appressed to the girdle. The cells can be photosynthetic or colorless and heterotrophic. Photosynthetic organisms have chloroplasts surrounded by one membrane of chloroplast E.R., which is not continuous with the outer membrane of the nuclear envelope. Chlorophylls a and c 2 are present in the chloroplasts, with peridinin and neoperidinin being the main carotenoids. About half the Dinophyceae that have been examined by electron microscopy have pyrenoids in the chloroplasts (Dodge and Crawford, 1970). The storage product is starch, similar to the starch of higher plants (Vogel and Meeuse, 1968), which is found in the cytoplasm. An eyespot may be present. The nucleus has permanently condensed chromosomes and is called a dinokaryotic or mesokaryotic nucleus.

Cell Structure

Theca

The thecal structure of motile Dinophyceae consists of an outer plasmalemma beneath which lies a single layer of flattened vesicles (Figs. 7.2, 7.3(c), 7.5) (Dodge and Crawford, 1970 ; Sekida et al., 2004). These vesicles, which normally contain cellulosic plates, give the theca its characteristic structure. The actual form and arrangement of the thecal plates varies from none in the phagotrophic Oxyrrhis marina, to very thick plates with flanges at the edges in Ceratium (Figs. 7.11, 7.48, 7.49) and Peridinium spp. (Figs. 7.2, 7.10).