Agriculture Reference
In-Depth Information
(Darbyshire, 1976). Alabouvette et al. (1981) estimated that ca. -0.15 MPa (pF 3.2),
which corresponds to the filling of all pores of < 2 . is the theoretical lower limit for
water potential which allows protist activity. That is, small ciliates may swim freely in
the small micropores, and testate amoebae can immerse their pseudopods in the water
film covering solid particles.
Growth may be rapid, under favourable conditions generation time varies from two
to 48 hours for most species. The Testacea, however, have slower growth rates (of days
to weeks) and they always appear last in community successions after, respectively, flag-
ellates, naked amoebae and ciliates (Stout and Heal, 1967; Coûteaux and Ogden, 1988).
Feeding habits
Many flagellates are considered osmotrophic (Stout and Heal, 1967). However, most pro-
tists are mainly bacterial feeders and, of 258 species examined by Biczok (1965), 76 %
were at least partly bacteriophagous and 40 % could not use another food. Bacteriophages
are not generally considered to be specific in their diet (Coûteaux and Pussard, 1983).
They may however distinguish edible from inedible species, which are determined by the
presence of specific pigments (review in Stout and Heal, 1967). Under some circumstances,
the presence of edible bacteria has been reported to induce a rapid emergence from cysts
in certain protist populations (Kunicki-Goldfinger et al., 1967). Other food resources
may be used, either as an obligate food or as an alternative source of nutrition when
edible bacteria are exhausted. They include fungi, algae, yeasts, protists, small Metazoa
and also humic substances and cellulose (Lousier and Bamforth, 1990).
Water relations - Survival
Soil water potential is a major determinant of protist life. When the soil dries, the
osmotic potential in free water decreases and protists may cease activity if salinity
becomes too high. However, most tolerate salinities of up to 25-45 % o .
Protists are unable to move more than a few centimetres per day and may encyst when
conditions become unsuitable for their activity. True resistant cysts may withstand
extreme conditions of exposure to drought, temperature or acidity for many years.
Soil desiccation, increased salinity and the exhaustion of nutritive resources are
commonly considered as the main factors determining encystment. However, Coûteaux
et al. (1988), consider that cyst formation requires favourable nutritional conditions.
Following Singh (1941), Corliss and Esser (1974) and Coûteaux (1976) who found greater
numbers of cysts when feeding resources were not limited, they consider that encystment
starts as soon as cells have accumulated sufficient reserves. Conversely, where nutritional
resources are low, encystment is limited and many cells die if the soil dries out.
Community densities and biomasses
Most estimates of community density fall in the range 10 to 1000 individuals and
biomass between 50 and 3000 mg (Petersen and Luxton, 1982). Testate amoe-
bae are dominant in forest soils whereas naked amoebae replace them in cultivated soils.
Densities are highest in soils with elevated organic matter levels. Thus, amoebae may be
more numerous in woodlands with moder rather than mull-type humus (Volz, 1964).
This trend was confirmed by Schönborn (1982), although production was higher in
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