Studies on the Biology of Soil Ciliates
The soil ciliate fauna consists of at least three major groups: (a) Species found wholly or chiefly in granular soils and litters and which may also be found in moss or sphagnum. These species are often larger than other soil species and structurally more specialized. (b) Species also commonly fond in sewage, faeces or polluted water. These are chiefly small holotrichous ciliates which feed on bacteria. They tolerate a wide range of pH, salinity, temperature, oxygen and carbon dioxide tensions. They generally have a well developed cyst physiology. (c) Species of common fresh water genera which differ from the fresh water species chiefly in size. They are generally small holotrichous ciliates fairly widespread in soil although they have not yet been recorded from other habitats. Parasitic species such as Balantidium and anaerobic species such as Trimyema occur very rarely. Suctorians occur occasionally. In the laboratory most species grow best in a medium nearest their natural environment, viz. soil or peat extract. This is particularly true of the first and third groups. The second group grows well in enriched media which are not tolerated by the other species. In a laboratory culture most species are found associated with the bottom detritus but one species, Blepharisma steini, common in litters and occasionally pigmented, avoids the bottom of the culture and is found only at the surface. Soil ciliates are not affected by pH changes over a wide range. In a soil laboratory culture there is less variation than in ordinary laboratory infusions, pH and Eh are fairly stable, and after the first week the bacterial flora consists principally of Pseudomonas. The ciliate fauna is present for about six weeks, the first species appearing within a few days, the majority being present after the first week, and finally the carnivorous species Dileptus alone being present after twenty weeks. The rhizopod fauna does not appear until after the majority of ciliate species have disappeared. The effect of a heavy inoculation of bacteria into a mixed culture of protozoa causes the death of certain species, such as Euplotes aediculatus and the rapid growth of other species such as Paramecium caudstum, Cyclidum glaucoma and Vorticella microstoma which encyst or die when the bacteria are wholly consumed. Other species such as Vorticella striata neither die nor divide rapidly. Continued heavy inoculations of bacteria cause Veritcella microstoma to form unstable cysts due it is suggested to the accumulation of bacterial metabolites and the strongly reducing conditions. These cysts excyst when aerated. They are larger than the normal resting cyst which is formed on the exhaustion of food. Only a few species such as Colpoda steinii, Colpoda inflata and Colpoda cucullus survive anoxia for any length of time at room temperatures. These facultative anaerobes continue to feed and move although they cannot divide or excyst. Rarely they form unstable cysts. Other ciliates which also survive for some time are Stentor roeseli and Vorticella microstoma both normally sessile forms. The former becomes detached and swims freely in the medium with the aid of its peristomal cilia. Vorticella also becomes detached and forms a telotroch or swarmer which does not settle unless oxygen is present. The telotroch will survive in the absence of oxygen a little over two days. It cannot feed and it is suggested that the limited survival of these two ciliates is due partly to the exhaustion of their food reserves. Other species such as Stylonychia mytilus a common fresh water species and Halteria graudinella are extremely sensitive to anoxia. Colpoda steinii, Colpoda inflata and Paramecium caudatum are very resistant to high carbon dioxide tensions and C. steinii will continue to move though not feed or divide at very high carbon dioxide tensions. Halteria grandinella is moderately resistant but Coleps hirtus and Stylonychia mytilus are extremely sensitive. Vorticella microstoma in response to high carbon dioxide tensions forms a telotroch which survives a comparatively long time. The trophic ciliate is much more sensitive. Stentor also becomes detached in high carbon dioxide tensions. Colpoda steinii will grow in salinities up to 3% NaCl but division is progressively inhibited and unstable cysts are formed. Excystment takes place only if the environment is hypotonic to the ciliate. Growth of Colpoda steinii is most rapid at about 27 [degrees] c. At high and low temperatures division is inhibited and unstable cysts are formed. This unstable cyst is similar to the reproductive cyst but unlike that cyst remains inactive until the inhibiting factor, e.g. temperature or salinity, is removed. The factors which inhibit division also inhibit excystment and for this reason it is presumed that a common morphogenetic mechanism, identified with the 'activity' system found in other organisms, underlies them both. An interpretation of the physiology of Colpoda upon this assumption is used to explain the common effect of diverse environmental factors. Encystment and excystment are considered two distinct processes, contrary to the suggestion of Bridgeman (1948) who considered them complementary. Excystment of Vorticella microstoma may be stimulated by very low oxygen tensions but is normally dependent upon the presence of bacteria. Activation of the encysted Vorticella leads to two processes: the differentiation of the telotroch and the escape of the ciliate from the cyst membrane. Following imbibition of water the ciliate bursts through the cyst pore the aboral end foremost. The posterior ciliary wreath is normally differentiated after the ciliate escapes from the cyst membrane and assumes the elongated body form of the telotroch. Sometimes the ciliate fails to escape and a fully differentiated telotroch is formed within the cyst membrane. Following excystment the telotroch becomes free swimming and finally settles. It swims with the aboral end and the posterior ciliary wreath directed anteriorly. Telotrochs are normally formed during excystment or by division but anoxia or high carbon dioxide tensions will also cause the trophic ciliate to form a telotroch. It does not encyst under these conditions contrary to prevailing opinion (Brand, 1923). A consideration of the bionomics of ciliates shows that their ecology is determined in their behaviour, life history and physiology. Soil species, such as Colpoda, are distinguished by their small size, their tolerance of a wide range of soil conditions and the efficiency of their cyst physiology. Fresh water species are excluded from soil either because they are not tolerant of such environmental conditions as high carbon dioxide tensions, e.g. Coleps hirtus, or because they have a poorly developed cyst physiology, e.g. Paramecium.