Le régime alimentaire des Thécamoebiens (Protista, Sarcodina)

Abstract

Tcslate amoeba are a heterogeneous and most likely polyphyletic group of free-living protists. Their common characteristic is the presence of a test. Testate amoebae have been observed in almost every aqualic habitat, mainly in the periphytic zone of fresh and salt water bodies, soils and mosses. These protozoa are of great interest in the study of ecosystems. Indeed, the presence of a rigid test makes their identification relatively easy and they are good biological indicators. Furthermore, these micro-organisms play a key role in the structure and the functioning of microbial communities at the surface of Sphagnum peatlands. Nevertheless, testate amoebae remain globally little studied. More precisely, only limited data exist on their feeding habits although this is a necessary preliminary key aspect to understand their role in microbiai trophic networks. The aim of this review therefore is to allow a synthesis of the observations at hand in the literature on the feeding behaviour of testate amoebae. An important part of this review is devoted to our own observations on the species living in Sphagnum. Although testate amoebae arc generally believed to be heterotrophic. several species arc mixotrophic. Strict heterotrophy however remains the dominant trophic mode of testate amoebae. In the literature, several authors consider testate amoebae as largely bacterivorous. It seems however that many species ingest other kinds of prey: paniculate organic matter, microalgae, plant cells, prolists, fungi and small metazoa. Furthermore, some species are polyphagous while others seem to have more specialised feeding habits. Little is known on prey identification by testate amoebae. In particular, it is difficult to tell if mechanisms of distant detection (e.g. chimiotactism) exist or if a physical contact is necessary. Our observations on living and on fixed individuals suggest that both mechanisms exist. Furthermore, it seems that the food selectivity by testate amoebae is influenced by the physiological state of prey organisms at the moment of their ingestion. As for naked amoebae, the ingestion of prey is done by invagination of the cytoplasmic membrane. However, the particularity of testate amoebae is that the ingestion and egestion lake place at the level of the pseudostome. but no clearly defined cytostome or cytoprocte exist. The ability of some testate amoebae species to ingest a wide range of prey sizes (0,2 to 1 000 μm) and of very different kinds likely represents an competitive advantage in some environments. In these conditions, it seems important to improve our knowledge on the biology and the ecology of these protozoa.