Certain species of dinoflagellates contain highly toxic substances which can cause the death of other marine creatures which eat them. Under certain conditions when an excess of nutrients is present and the sea is very calm, certain dinoflagellates multiply very rapidly and form a bloom. The water is often discoloured hence the name 'red tide'. Not all 'red tides' are red or toxic, but those that are occasionally cause the death of large numbers of fish. In recent years, many fish farms have suffered in this way. Examples of toxic dinoflagellates include Gymnodinium brevis, Goniaulax polyedra, and Exuviella baltica. In the Gulf of Mexico, tons of dead fish are periodically washed up along the Florida coastline. The fish are killed as they swim through blooms of the dinoflagellate Gymnodinium breve, rupturing the algae which release neurotoxins onto the gills of the fish (Anderson, 1994).
Sea birds, humans and marine mammals can also be poisoned indirectly when they eat filter-feeding shellfish that have accumulated the toxins. In the summer of 1968, cases of food poisoning in humans in the UK were traced to a dinoflagellate infection of mussels on the Northumberland coast, which also caused the death of seabirds, sand eels and flounders. In 1987, a large number of humpback whales died in Cape Cod Bay (USA). It was eventually shown that toxins from a dinoflagellate Alexandrium tamarense had killed the whales via their food web (Anderson, 1994). In the same month many fishermen, visitors and residents along the North Carolina coast, who had eaten local shellfish became ill with a variety of symptoms. In Canada, people who had eaten mussels from Prince Edward Island also became ill. This was traced to a toxin from a diatom, Pseudonitzchia pungens.
Many species of unicellular green algae are represented in the plankton. These are often motile or have a motile phase. Halosphaera (Figure 2.6) is one of the largest and belongs to the class Prasinophyceae. It consists of a single spherical cell with a tough elastic wall, and sometimes reaches nearly 1 mm in diameter. There is a large central vacuole in which a single nucleus is usually suspended, but fully grown cells may contain as many as eight nuclei. In the peripheral cytoplasm are numerous small, yellowish-green chromatophores giving the cell a vivid colour. Asexual reproduction involves a motile phase, repeated divisions of the protoplasm leading to the liberation of numerous four-flagellate spores. Sexual reproduction has not been recorded. Halosphaera is found throughout the North Atlantic but is commonest in tropical areas. It is sometimes carried into British waters in great numbers by the North Atlantic Drift and is often abundant in the northern North Sea in autumn.
Phaeocystis (Figure 2.7) belongs to a group of unicellular brown-coloured algae (class Haptophyceae). It is a minute biflagellate cell which develops into a colonial structure. The cells divide repeatedly, and extensive mucilaginous capsules form around them and bind them together in large gelatinous clumps up
Nucleus in central vacuole
Nucleus in central vacuole
Figure 2.6 Halosphaera.
to several centimetres in diameter. These are abundant in north temperate inshore waters in spring and summer. On occasion they are so numerous that they colour the water and give it a slightly slimy consistency. Fishermen call this 'weedy water' and it is apparently distasteful to some animals; for example, herring shoals seem to avoid this water and the catches are poor when Phaeocystis occurs in quantity on the fishing grounds. A Phaeocystis patch over 100 miles in extent was recorded from the North Sea in 1927.
Silico-flagellates are unicellular organisms belonging to the class Chryso-physeae which live predominantly in freshwater. The marine silico-flagellates are small, often about 100 ¡m in diameter, and have their protoplasm supported by an internal skeleton of interconnecting siliceous rods forming a capsular structure with outwardly radiating spines. There is usually a single flagellum. They are sometimes present in considerable numbers, mainly in the colder parts of the seas.
Blue-green algae (class Cyanophyceae) are also represented in the phyto-plankton. The commonest types form filaments of short chains of minute spherical
or oval cells with no definite chloroplasts, the pigments being diffused in the cytoplasm. In addition to chlorophyll, phycocyanin and phycoerythrin are present, giving the cells their bluish-green colour. Blue-green algae are different from all other classes of algae because, like bacteria, they have no organized nuclei. These plants have occasionally been found in large numbers at deep levels (see page 236) and may be able in some circumstances to feed saprophytically. Under certain conditions they form extensive blooms. With increasing eutrophi-cation of enclosed fresh and brackish water areas such as estuaries, and reservoirs, such blooms are now causing amenity problems. Toxins released by the algae can cause rashes and illness in bathers and other water users.
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