Physiological Ecology of Free-Living Protozoa

Abstract

There are about 20,000 species of the single-celled animal-like organisms we call protozoa, and they are the most abundant phagotrophs in the biosphere. One milliliter of sea water contains about 1000 heterotrophic flagellates (Sherr and Sherr, 1984; Fenchel, 1988), freshwater sediments contain about 10,000 ciliates (Finlay, 1980, 1982), and organically rich habitats such as activated sludge plants support at least 105 ciliates and flagellates per milliliter (Curds, 1973). The calcareous and siliceous oozes that cover most of the marine benthos are largely composed of the sedimented shells and skeletons of planktonic foraminifera and radiolaria, and at least one wonder of the ancient world, the great pyramid of Cheops at Gizeh, consists almost entirely of the compacted shells of the fossil foraminiferan Nummulites gizehensis (see Haynes, 1981).