Dinoflagellata

The dinoflagellates (also known variously as Pyrrophyta, or 'fire algae') are chiefly marine planktonic types, comprising some 2000 species. This is another unicellular group, but one whose cells are often covered with armoured plates known as thecae (sing: theca). They are generally biflagellate, with the two dissimilar flagella lying in part within the longitudinal and lateral grooves that run around the cell (Figure 9.2). The beating of the flagella causes the cell to spin like a top as it moves through the water (the group takes its name from the Greek word 'to whirl'). Although many non-photosynthetic (chemoheterotrophic) types exist, most dinoflagellates are photosynthetic, containing

Figure 9.2 A dinoflagellate. Note the two flagella in perpendicular grooves. Each plays its part in the organism's locomotion

Transverse flagellum

Longitudinal flagellum

Theca

Box 9.1 Why would an alga want to glow in the dark?

The production of bioluminescence by several dinoflagellate species is thought to have a protective function. Such algae are the natural prey of copepods, tiny crustaceans found in astronomical numbers as part of the zooplankton. The bioluminescence could have an effect directly, by acting as a warning signal to the copepods, or indirectly, by making those crustaceans which had consumed glowing algae much more conspicuous to their own predators.

chlorophyll a and c plus certain carotenoids and xanthophylls, which give them a red/golden appearance. As a group, they are second only to the diatoms (see below) as the primary photosynthetic producers in the marine environment. Some dinoflag-ellates form endosymbiotic relationships with marine animals such as corals and sea anemones; these are termed zooxanthellae. An unusual feature of dinoflagellate ultrastructure is that the chromosomes contain little, if any, histone protein, and exist almost permanently in the condensed form.

Some tropical species of dinoflagellate emit light, the only algae to do so (see Box 9.1). Due to an enzyme-substrate (luciferin-luciferase) interaction, this can cause a spectacular glow in the water at night, especially when the water is disturbed, for example by a ship. Bioluminescence of this kind has proved to be a useful 'tagging' system for cells in biological research. Other marine dinoflagellates can produce metabolites that act as nerve toxins to higher animals. Shellfish such as mussels and oysters can concentrate these with no harm to themselves, but they can be fatal to humans who consume them. Sometimes, when conditions are highly favourable, an explosion of growth results in the development of huge 'red tides' of dinoflagellates in coastal waters. This produces a build-up of toxins, and may lead to the death of massive numbers of fish and other marine life. The greatly increased incidence of these blooms in recent decades is probably due to pollution by fertilisers containing nitrates and phosphates.

Reproduction by asexual means involves binary fission. In armoured forms, the theca may be shed before cell division, or split along suture lines; in either case, daughter cells must regenerate the missing sections. Sexual reproduction is known to occur in some dinoflagellates, and is probably more widespread. Gametes produced by mitosis fuse to produce a diploid zygote; this undergoes meiosis to reinstate the haploid condition in the offspring. In some species we see isogamy, the fusion of identical, motile gametes, while in others, anisogamy occurs, in which gametes of dissimilar size fuse. Fusion may occur between genetically identical gametes, or only when the gametes come from genetically distinct populations.

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