A long food chain

Even if protista are rather small in size (usually between few microns to a few millimetres), they are very important for natural organisation, given the important role that photosynthetic protozoa play in the food chain of the aquatic organisms.
They mainly feed on bacteria and thus play an essential role in the food chain, since they are the main producers on which all other organisms depend. The number of these organisms belonging to marine plankton can be huge; they have been found even as at 1,000 and 5,000 metres deep. This group includes, for instance, diatoms, which, as we said before, are equipped with a siliceous shell.
After the organism dies, it can fall and settle on the seabed, thus originating, especially in the coldest seas, some siliceous deposits that are called “infusorial earth”, used for instance to polish silver and to make toothpaste.
Diatoms are food for many forms of life, such as small crustaceans, called Copepoda, which in turn are food for the shoals of herrings.
Plankton is also composed of other protozoa, such as Radiolaria, large amounts of which lived in the past geological ages. After death, their shells settle on the seabed, building up a thick, hard rock, known as radiolarite, which, along with siliceous deposits from diatoms, compose the “infusorial earth”.

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