As revealed in numerous studies, protozoa are highly resistant to many external factors, including low temperatures (Sukhanova 1968; Lozina-Lozinsky 1972). Under natural conditions, low temperatures have a considerable influence on the character of metabolism and the related morpho-functional processes in the protozoa cells. Affected by near-zero temperatures and the concomitant dehydration and altered chemism of the environment, protists use different survival strategies (Bradbury 1987; Gutierrez et al. 2001):
(i) In one strategy, organisms do not undergo cell differentiation, keep the general morphology of vegetative stage unchanged, and at the same time maintain metabolism at a sufficient level, until the action of the adverse factor ends. Lowering temperature below the optimum triggers protective mechanisms inside the cell, such as the increase in the content of trehalose, unsaturated fatty acids and polyols, and the synthesis of cold-resistant enzymes (Poljansky 1963; Lozina-Lozinsky 1972; Mazur 1984; Robinson 2001; Clegg 2001; Podlipaeva et al. 2006).
(ii) Alternatively, protozoa turn to the mechanisms based on cell differentiation, and pass into a more stable state, which essentially differs from the vegetative state. Accordingly, survival will be achieved by almost complete suspension of metabolic activity; that is why the strategy of this second type is often called crypto-biosis (from Greek "hidden life", according to the term given by Keilin in 1959). In many organisms, transition to the state of physiological resting is accompanied by the formation of specific morphological structures (Goldovskij 1986; Ushatinskaja 1990); in protozoa, these are resting cysts (Gutierrez et al. 1990; Hausmann et al. 2003).
Encystation of protozoan cells is accompanied by the processes of differentiation, which are characterized by alterations such as considerable dehydration of the cytoplasm, autophagic activity, deposition of storage substances, formation of a protective envelope, and changes in the organization of the nuclear apparatus (Lozina-Lozinsky 1972; Corliss and Esser 1974; Walker et al. 1980; Ushatinskaya 1990; Gutierrez et al. 1990; Guppy and Withers 1999). The resting cysts of cili-ates (e.g., Colpodidae) accumulate a large amount of disaccharides, trehalose and/or sucrose (Potts 1994). In the process of dehydration, it is suggested that these polyhydroxylic compounds substitute for the aqueous hydration envelope around macromolecules and intracellular organelles, thus protecting them from damage (Clegg 1986).
The resting cysts of protozoa are protected from adverse environmental effects by a multilayer water- and gas-tight envelope (Ushatinskaya 1990; Gutierrez et al. 2001). Encysted acanthamoebas, for example, are resistant to biocides, chlorination and antibiotics (De Jonckheere and Van de Voorde 1976; Khunkitti et al. 1998; Turner et al. 2000; Lloyd et al. 2001). Little is known about the macromolecular composition of different envelope layers; their major components are proteins, glycoproteins and carbohydrates (Tomlinson and Jones 1962; Neff and Neff 1969; Gutierrez et al. 2003; Matsusaka and Hongo 1984; Benitez et al. 1991; Izquierdo et al. 1999).
The cultivation of protozoa isolated from permafrost showed that all ancient amoe-bas, ciliates and a part of heterotrophic flagellates formed resting cysts — as, according to the literature data, do their modern counterparts of analogous species and genera. However, there are some species in the fauna of ancient heterotrophic flagellates
(Goniomonas truncata, Spumella elongata, Colponema edaficum, Bodo curviflis, B. designis, B. repens, B. minimus, Phalansterium solitarium, Spongomonas uvella, Salpingoeca globulosa, Cercomonas angustus, Heteromita minima, Protaspis simplex, Apusomonas proboscidea), which have never been reported to have resting cysts in their life cycle (Zhukov 1993; Mylnikov, personal communication).
Electron microscopy study of ancient ciliates (Colpodea) and heterolobose amoebas (Acanthamoeba) has revealed that, in different species, the number of structurally distinct layers in the envelope of their cysts varies from two to four, which was also observed in the envelope of modern specimens of those species and genera (Frenkel 1987; Díaz et al. 2000; Gutierrez et al. 2003; Chavez-Munguia et al. 2005). Acanthamoebas form a two-layer envelope, which consists of the outer ectocyst and the inner endocyst (Fig. 8.5). There may be pores on the cyst surface, so-called ostioles. The pores are covered with a protective cap, operculum, which is made of the same material. The cyst envelope of the ciliate Colpoda inflata (Colpodea) consists of an ectocyst, mesocyst (intermediate layer between ecto- and endocyst), endocyst and granular layer or metacyst (Fig. 8.5).
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