The Coccolithophoridae (Figure 2.9) are one of the more important groups of nanoplankton. They are minute, unicellular organisms, mostly some 5-20 [m in diameter and containing a few brown chromatophores (Thomas, 1993; Winter and Siesser, 1994). They are characterized by tiny calcareous plates which cover their outer surface. The plates are usually extremely finely and elaborately sculptured. At their simplest, they are oval discs, but in some species, the plates form long projections from the surface of the cell, often of bizarre design. They have complex life histories usually involving several morphologically different phases. Some have both a motile uni- or bi-flagellate phase and a non-motile
Figure 2.9 A coccolithophore.
pelagic phase, and some have a benthic filamentous phase. Coccoliths are widely distributed and are sometimes so numerous near the surface that they impart a slight colouration to the water. The condition which herring fishermen call 'white water', and which is regarded as indicating good fishing, is sometimes due to swarms of coccoliths. The calcareous plates of disintegrated coccoliths are a conspicuous component of the deep-sea sediment in some areas.
Coccoliths have occasionally been found in surprisingly large quantities far below the photosynthetic zone, sometimes very numerous between 200 and 400 m, and even in considerable abundance at depths of 1000-4000 m. This deep-water distribution suggests that some coccoliths can feed to some extent by methods other than photosynthesis, perhaps by absorption of organic solutes or even by ingestion of organic particles. In some areas they may be an important source of food for some of the animals at deep levels (see page 236).
The seas contain many minute unicellular organisms which swim by means of one or more flagella, and are loosely termed microflagellates (Thomas, 1993). The majority are within the size range 1-20 ¡m in diameter, but there are a few larger species up to about 100 ¡m. Most of them contain chlorophyll, and there is no doubt that they are important as primary food producers. There are also many colourless saprophytic forms. The life histories are not well known. Some have been observed to reproduce by fission, while others are spores of larger algae. Non-motile cells, similar in general appearance to microflagellates but lacking flagella, are also known; and in some cases the life history includes both motile and non-motile pelagic stages.
Although the number of bacteria in seawater varies greatly with time and place, in coastal waters they sometimes contribute a significant part to the total biomass of the plankton. Bacterioplankton are often associated with floating organic debris known as 'marine snow' (see page 233). Bacteria and other microorganisms are a constituent of much importance in marine ecosystems, functioning in many roles as decomposers, saprophytes and pathogens; regenerating nutrients, producing dissolved gases and ectocrine compounds, some of which may be essential for the normal growth of other organisms. They are a major source of food for protozoa and filter-feeding animals.
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