Page 42 - Algae Anatomy, Biochemistry, and Biotechnology

General Overview 25

plates. In dinokont type dinoflagellates, these thecal plates generally form a bi-partite armor, con-
sisting of an upper, anterior half and a lower, posterior half, separated by a groove known as cin-
gulum where the transversal flagellum is located (Figure 1.38). A smaller groove, the sulcus,
extends posteriorly from the cingulum, and hosts the longitudinal flagellum. The two flagella
emerge from a pore located at the intersection of the two grooves. Very often they are important
components of the microplankton of freshwater and marine habitats. Though most are too large
(2–2000 mm) to be consumed by filter feeders, they are readily eaten by larger protozoa, rotifer,
and planktivorous fishes. Some Dinoflagellates are invertebrate parasites, others are endosymbionts
(zooxanthellae) of tropical corals. Dinoflagellates possess chlorophylls a, b, c 1 , and c 2 , fucoxanthin,
other carotenoids, and xanthophylls such as peridinin, gyroxanthin diester, dinoxanthin, diadinox-
antin, and fucoxanthin. The chloroplasts, if present, are surrounded by three membranes. Within the
chloroplasts the thylakoids are for the most part united in a stack of three. The chloroplast DNA is
localized in small nodules scattered in the whole chloroplast, with typical pyrenoids. A really
complex photoreceptive system is present in the dinophytes such as Warnowia polyphemus,
Warnowia pulchra,or Erythropsidinium agile consisting of a “compound eye” composed of a
lens and a retinoid. Most dinoflagellates are distinguished by a dinokaryon, a special eukaryotic
nucleus involving fibrillar chromosomes that remain condensed during the mitotic cycles. The prin-
cipal reserve polysaccharide is starch, located as grains in the cytoplasm, but oil droplets are present
in some genera. At the surface of the cell there are trichocysts which discharge explosively when
stimulated. Besides photoautotrophy, dinoflagellates exhibit an amazing diversity of nutritional
types because about half of the known species lack plastids and are therefore obligate heterotrophic.
Some are notorious for nuisance blooms and toxin production, and many exhibit bioluminescence.
Dinophyceae have generally a haplontic life history.

FIGURE 1.38 Dorsal view of Gonyaulax sp., a brackish water dinoflagellate.