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Discussion
Lackey (1949) and Rohlick (1949) have recently given a stimulating review and discussion of the place of protozoa in sewage digestion. Hawkes and Jenkins (1951) have also recently reviewed the problem. The process of organic degradation is imperfectly understood. The precise role of the different components of the active flora and fauna is not yet determined (Barker, 1946). The efficiency of degradation appears to be wholly dependent upon the presence of the correct microflora and fauna. The role of the protozoa in this process is greatly debated. Their function seems to be not so much to effect the degradation of the organic substrate, although they may contribute towards this, but rather to prey on the bacterial flora and thereby maintain its efficiency and to assist in flocculation. Work cited by Rohlick and other authors showed that degradation takes place more effectively in the presence than in the absence of protozoa. Satisfactory degradation has been obtained, however, in the absence of protozoa (Jenkins, 1951).
The organisms involved vary from one digestion plant to the next, but certain species appear with greater frequency than others and these are sometimes called anaerobic, although as Lackey points out many of them are truly only facultative anaerobes. The history of the present digester includes only one true anaerobe. Metopus es, one of the most common sewage ciliates. The other species occurring in the digester are facultative anaerobes, and it is therefore inferred that for a short time, following the addition of fresh meat waste, aerobic conditions must prevail. It is known that facultative anaerobes cannot divide in the absence of oxygen (Watson, 1944) and further that Vorticella normally forms a telotroch when oxygen is removed (Brand, 1946). (The effect of environmental factors, including oxygen lack, upon the life history of Vorticella microstoma will be described in a later paper.) It appears that in the digester the vorticellids encyst, due perhaps to intense bacterial decomposition associated with a falling redox potential. The Eh of the digester fluid was about −290mv. whereas the Eh of the oxidation pond was about 200mv. This may also be the case with the other species of facultative anaerobes. Hayes (1938) described encystment of Dileptus anser in intense bacterial infusion. Following aeration the vorticellids excyst. There is no need of further stimulus. Prorodon is wholly dependent upon the growth of Vorticella and the activity of this ciliate must therefore be confined within the trophic history of the vorticellids. It is interesting to compare the conditions of encystment and excystment of Vorticella microstoma and Prorodon microstoma. Vorticella encysts primarily in response to absence of bacterial food, but under strongly reducing conditions will encyst in the presence of bacteria. It excysts primarily in response to bacteria and oxygen, although better excystment is obtained with fresh medium which suggests the presence in stale medium of an inhibitory factor (Brand, 1923). The life history of Prorodon
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is determined by very similar factors. It encysts in the absence of food and excysts in the presence of bacteria. Under the present conditions the excystment of Vorticella followed within an hour or two of the addition of bacteria but Prorodon did not excyst until twenty-four hours or more later. For this reason there was generally a vigorous culture of Vorticella to precede the excystment of Prorodon when cysts of the two ciliates were in the same drop or culture to which bacteria had been added. Such conditions proved ideal for the development of Prorodon for this ciliate cannot be starved indefinitely. Its movements become increasingly sluggish, more like a normal slow swimming holotrich and quite unlike the vigorous darting movement of the normal trophic form. At such a stage it seems unable to attach itself to a vorticellid and so, even if vorticellids are present, it is unable to feed. It is possible that during temporary encystment the ciliate restores its kinetic energy, and when fully active and able to feed breaks out of the cyst membrane. In other words it is suggested that encystment is primarily determined by the energy level of the ciliate. With constant feeding and division, this remains high and encystment does not take place. As the number of vorticellids is diminished, the Prorodon encounters fewer food organisms and conversely there is a longer period between each “kill”. If this period is too prolonged the ciliate will encyst, provided it is of certain optimum size. It is at this stage that other factors, such as staling, must determine whether a temporary or permanent cyst is formed. In either case the ciliate which subsequently escapes from the cyst membrane is able, once again, to commence that swift darting motion which is essential for its trophic existence.
In its general behaviour and life history Prorodon closely resembles two other predacious ciliates, Didinium nasutum and Woodruffia metabolica, both of which prey largely on Paramecium. Structurally it is closely related to Didinium and its feeding behaviour is identical (Calkins, 1915). Didinium also excysts in response to bacteria although encystment is primarily due to crowding (Beers, 1946, 1947). The cysts of Didinium, like the permanent cysts of Prorodon, have an ectocyst and an endocyst but Didinium also has a thin endocyst which is dissolved after the ciliate has escaped from the outer membranes (Beers, 1945a). In Woodruffia there are only the two membranes, as in Prorodon, and there are also several types of cyst including permanent and temporary cysts. In Woodruffia division also takes place within a cyst membrane, and temporary cysts are formed in food-rich cultures in the same way as with Prorodon (Johnson and Evans, 1939, 1940). The chief difference between the cysts of Prorodon and Vorticella is that in Vorticella there is a definite excystment pore, as with Bursaria (Beers, 1948), a point not made clear by Brand (1923), although the pore is clearly shown in a drawing by Stein (Kent, 1880–1882). The mechanics of excystment in Vorticella are generally similar to those of Tillina (Beers, 1945b), in that the build up of hydrostatic pressure accompanying the activity of the contractile vacuole finally ruptures the cyst membrane.