Protozoa abe non-cellular which distinguishes them from the adult forms of all other animals, and most are microscopic in size. Free-living protozoa are found in every variety of habitat. Many such as Dinoflagellates, Foraminifera, and Radiolaria are important elements of marine plankton and after death their shells form the oozes of the ocean floor and subsequently the fossil beds of limestone deposits. In fresh water, Rhizopods, Flagellates, and Ciliates form the protozoan population and favourable conditions will result in a thick bloom of these forms. Protozoa are also found in hot-springs, salt-lakes, sewage filters and in the soil.

Distribution Of Protozoa

Free-living protozoa in an encysted state are able to withstand the most adverse conditions for long periods. This fact combined with their microscopic size and the many avenues of transport which are open to them ensure that there can be no effective geographical barrier against their distribution. This is borne out by the numerous species which are found from the tropics to Greenland and from Europe to the Antarctic. So, when protozoan species are absent in an area it is usually attributable to unfavourable habitats.

Ciliates are found in both soil and fresh water but there are only a few species common to both habitats. In New Zealand, for example, more than 100 species of ciliates are known from soil and fresh water but only five of these occur in both habitats. In cases where a genus of ciliates is common to both environments the soil species are usually distinct from the freshwater species. Generally speaking, soil species are smaller than freshwater species and their small size is doubtless related to the restricted space in which most soil protozoa live. The different species of soil protozoa in New Zealand occur in characteristic groupings which can be correlated with soil type. Such a correlation excludes the possibility of explaining the distribution by random scattering. In soils of closely related types the species of the different systematic groups; amoebae, testaceans, holotrichous and spirotrichous ciliates show a common pattern or ratio although the actual species involved are not necessarily the same.

By using this pattern or ratio as a basis it is possible to compare soils under similar conditions in different parts of the world for although the species involved may vary from one geographical area to the next, the pattern will remain the same. Thus, the faunal patterns of samples taken under Podocarp vegetation in New Zealand and South Africa are very similar although only three species are common to both faunas.

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Related soil types under different conditions of vegetation, weathering and podzolization can also be compared, and the effect of the soil processes upon the character of the fauna can be judged. With increased leaching and podzolization for example the total number of species in the soil tends to fall but this reduction appears to affect the ciliate fraction less than the rhizopod fraction. Again, wherever there is accumulation of litter or plant matter associated with acidity, rhizopods, especially testaceans, occur in large numbers.

There are some cases where not only the faunal pattern but also the species involved are identical for soils of similar type. Thus a ciliate, Phacodinium metschicoffi, which was recovered from a beech litter from Curious Cove, Marlborough Sounds, was again recorded from beech litter from the Taupo area. Again several testacean species found in sphagnum and peat of Campbell Island were encountered in sub-alpine soils from the Ruahines. In this case then not only did the faunal pattern of the sub-antarctic soils resemble that of the sub-alpine soils but species found nowhere else were common to both faunas.

The Role of Protozoa in the Soil

Like all biological energy systems the soil organic cycle is an open system, that is, energy is constantly both being introduced and lost. That part of the organic cycle with which the protozoa are concerned takes place within the interstices and pore spaces between the various soil particles, organic and inorganic. Here in the microscopic architecture within and between soil particles or in holes formed by worms or plant roots and fissures left where the soil has parted live the soil micro-organisms. Bacteria, actinomyces, fungi and algae represent the plant kingdom; protozoa, mematodes, rotifers, gastrotrichs, tardigrades, enchytraeids (pot worms), mites and insect larvae represent the animal kingdom. To all these organisms moisture is essential for their activity and this is retained in the spaces from drainage loss partly by capillary action and partly by the absorbtive power of the soil colloids. As the soil dries out the colloids form a thin film around the particles binding them together.

It is for this reason that protozoa and other micro-organisms are not immediately visible if soil is examined directly under the microscope. When, however, a drop of water is added to the soil and it is examined some hours later active protozoa and other organisms will be found. Because the first ciliate protozoa to be observed appear to have recently excysted, which they are able to do within several hours, it has generally been assumed that protozoa are not active when confined to the colloidal film. When sufficient moisture is present however they may be seen active in the pore spaces and fissures where the water is held for a time.

Protozoa are capable of using a wide range of food sources. They may live on dissolved carbohydrates and proteins, on bacteria, on other protozoa whether of the same or different species or on organic debris formed by page 105 plants or fungal mycelia. Their powers of synthesis are however limited and generally like higher animals they require vitamins or growth factors for normal growth, besides a carbon and nitrogen source. Recently Faure-Fremiet has described a typical association of micro-organisms, in this case the sulphur bacterium, Beggotia, and its protozoan predators. The bacterium grows in a thick mat on bottom mud deriving its energy from the oxidation of hydrogen sulphide. When a colony develops it is invaded by a succession of protozoa, some feeding on the bacteria and some on each other, until finally the colony is consumed.

Protozoa in the soil are largely predators on bacteria, and it has been shown that there is an inverse ration between the numbers of the two groups of organisms. Because of this relationship it was once thought that protozoa were responsible for limiting bacterial activity. At the turn of the century the beneficial effects of partial sterilization of soil by steam or chemicals was realized. Russell and Hutchinson in a celebrated paper suggested that these beneficial effects were due to the removal of a factor limiting bacterial activity, and consequently plant growth, and that this factor was the protozoan fauna. Recently samples of steam sterilized soil were examined to investigate the effect on the protozoan fauna. The sterilized top-soil was found to have only 40% of the species present in the unsterilized top-soil but the number of active protozoa present in the sterilized soil was far greater than in the unsterilized.

From these results it is obvious that while Russell and Hutchinson were correct in surmising that the number of protozoan species would be drastically reduced, they were in error when they attributed the heightened activity of the bacteria to the absence of protozoa. For not only are protozoa present in the sterilized soil but they are present in very great numbers and their activity is stimulated by the sterilization just as much as the activity of the bacteria. Both show an initial inhibition of growth followed by greatly stimulated growth of that part of the fauna and flora which was not killed by steaming.

It is now recognized that the effect of steaming soil is more extensive than its biological effects and that physical and chemical characteristics of the soil are also affected. These include water-holding capacity, capillary properties, destruction of the colloidal film surrounding soil particles, increased concentration of the soil solution, and increased solubility of soil constituents, e.g. Ca, Mg, Mn, and phosphate and also to a lesser extent K, Si, and Al. The biological effects are an initial inhibition both of bacterial activity and seed germination followed by greatly stimulated bacterial and plant growth. The present view of sterilization is that the destruction of organisms removes possible disease-causing or antibiotic organisms and releases plant nutrients both by the autolysis of microbial protoplasm and by reducing microbial competition for nutrients. Deficiencies of trace elements can be removed by partial sterilization. The whole phenomenon may be viewed as a gross reorganization of the soil equilibrium and the stimulated bacterial, protozoan, and plant activity as resultants of the new equilibrium reached.

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The view of Russell and Hutchinson then which treats bacterial activity as an inverse function of protozoan activity is erroneous. It is based upon a misconception of bacterial-protozoan relations. Both are directly affected by the soil equilibrium and any change in it which may occur. Although protozoa will undoubtedly reduce the number of bacteria in an environment they are unlikely to permanently affect the pulse of bacterial activity since any marked reduction of the bacterial flora will result in the death or encystment of the protozoa followed by renewed bacterial activity. Further by constantly preying on the bacterial flora they maintain it at the highest state of efficiency. They flocculate and condense dispersed particles and bring together into their own bulk energy sources which can then be used by larger organisms such as nematodes or else form centres of activity for fresh bacterial colonies.

Physiological Character of Soil Protozoa

Soil protozoa are characteristically small. Few species are over 100μ in size and over half of those most common in New Zealand soils are 50μ or less. This enables them to survive in the smallest drops of moisture and the thinnest moisture films. They are very hardy and can withstand extreme conditions of temperature, dessication, low oxygen and high carbon dioxide tensions. Their resistance to unfavourable conditions depends largely upon their ability to encyst. In the ciliate genus Colpoda, one of the most common and characteristic soil protozoa, there are basically two types of cyst. In one, under favourable conditions, cell division takes place, two, four or eight daughter cells being formed. The second type of cyst is a protective cyst formed in the absence of food and by crowding of the ciliates. Encystment may take place spontaneously, as with the reproductive cyst; or when the cysts are washed in a hypotonic solution such as distilled water or in response to some stimulant such as bacteria, hay infusion, yeast extract or alcohol. These protective cysts may be dried, heated to high temperatures (100° C.) or kept below freezing point and are still viable after months or even years of inactivity.

In the presence of food the soil protozoa can grow very rapidly. The reproductive rate of Colpoda steinii at room temperature is just under 4 per diem and of Vorticella microstoma, a wide-spread soil and sewage form, about 8. Not all bacterial strains are suitable as food for protozoa. So it is found that if loops of a suitable food bacterium are added to a culture of mixed bacteria and a single protozoan species (e.g. Colpoda steinii) the protozoan eats only the edible bacterium and consequently the non-edible bacteria increase in numbers. In this way protozoa have a selective effect on a mixed bacterial population. When only inedible bacteria are present the protozoa will encyst.

The flocculation of bacterial cells and other particles by protozoan activity has been recorded for a number of species, including the soil flalgellate Oikomonas termo and the coprophilic ciliate Balantiophorus page 107 minutus, also recorded from soil. In this latter case flocculation is achieved by mucus secreted by the ciliate.

In the complete absence of oxygen some free-living protozoa die within an hour or two, e.g. the fresh-water ciliate Stylonychia mytilus. Some survive for a time but do not feed, e.g. the normally sessile Vorticella microstoma which becomes free-swimming. Others feed normally under such conditions and can therefore survive indefinitely although the cells do not divide. To this last group belong Colpoda steinii and Trichopelma sphagnetorium, two common soil ciliates. Some protozoa are very sensitive to high carbon dioxide tensions whereas others such as Colpoda steinii and Colpoda inflata are very resistant. These two factors, oxygen lack and high carbon dioxide tension, are particularly relevant to soil conditions for they are both liable to occur in the capillary spaces where the protozoa live.

It can be seen from these few examples that the systematic distinction of the soil protozoan fauna is paralleled by morphological and physiological characteristics which make them particularly well suited for life in the soil. In all they combine a marked tolerance of varying environmental conditions with the ability to react quickly to favourable or unfavourable factors. These same characteristics favour them in other similar habitats and coprophilic and sewage protozoa consist largely of typical soil species. It is also interesting to find that the soil ciliate Colpoda steinii is an accidental parasite of slugs and snails, inhabiting particularly the pulmonary cavity where it divides without forming its typical reproductive cyst. Infection takes place through the slug eating material covering or containing the ciliate cysts and the parasite eventually causes the death of its host. This is a rare record of accidental parasitism by a free-living ciliate and is an indication of the adaptability of the protozoa.

Conclusions

The distribution of soil protozoa is plainly not fortuitous. There are a large number of highly tolerant species which thrive under a wide variety of soil conditions but there are certain species which are found only in favoured habitats. This shows that soil conditions will influence the number and ratio of the different protozoan groups and will also favour the occurrence of characteristic species. On comparing the species list of freshwater and soil protozoa of any locality it is clear that there is specialization and adaptation of soil protozoa as a whole in addition to the modifications of particular species.

As members of the soil population, protozoa hold an intermediate place between the bacteria on the one hand and the higher plants and animals on the other. Most protozoa feed chiefly on bacteria preferring some species to others and thereby influence both bacterial numbers and bacterial activity. In their turn they are consumed either by other protozoa or microscopic carnivores or else upon their death they become fresh centres of bacterial activity. All this contributes to the turnover of organic matter in the soil and as such the protozoa play their part in the soil organic cycle.