Ecology of the amoebae

Ecology of the Amoebae
Page updated 27/2/03
Amoebae are everywhere. Common genera such as Acanthamoeba and Naegleria are present from the Antartica (Brown et al, 1982) to the Artic. They are to be found in caves (Gittleson & Hoover, 1970), mines (Johnson & Rang, 1993), and some deep aquifers systems (Novarino et al, 1997), but not in others (Kilvington, 1991). Common marine amoebae such as Platyamoeba and Vannella are found from coastal rock-pools, on the fronds of seaweeds (Armstrong et al, 2000), to the open ocean even in the middle of the Pacific ocean (Vors et al, 1995). Many genera, even non-cysts forming species have been found at altitudes in the air (Kingston & Warhurst, 1969; Rivera et al, 1987). They are especially abundant in soils, where a number of parameters have been identified as being of importance in the determination of numbers of amoebae:-
Moisture. It is intuitively obvious that a moist soil is likely to contain more active amoebae that dry soil and this is borne out to a large extent. Saturated soils of course present other problems such as low oxygen concentrations which might limit the apparent numbers of amoeba since most enumeration methods take place in aerobic conditions and so favour those amoeba that are oxygen tolerant (not all are of course).
Temperature Within the temperature ranch naturally encountered in the environment, a correlation between amoeba numbers and temperature is generally found (Anderson & Rogers, 1995; Anderson, 1996; Anderson, 1997;).
Porosity. Amoeba exist in the fine films of water that lie on and between the particles that make up soil. Amoeba feed by actively moving about (under the direction of chemotaxis) and contacted bacteria (mainly) on which they feed. This being the case, movement is crucial and if the spaces are too small, large amoeba will be physically excluded. This is particularly true for very fine silts such as those found in mangrove swamps and here evidence for pore selection has been found (Elliot & Bamforth, 1975).
Organic content. Higher organic content supports a higher bacterial population that in turn supports higher numbers of amoebae, but in addition to this, the higher organic content also permits the greater retention of moisture in the soil which as we have already seen tends to support higher numbers of amoeba.
Amoeba as bacterial predators Other prey items
Habitats Some amoebae species have been isolated from very different habitats where they are often quite numerous. Paratetramitus jugosus () for example was first isolated from arable soil and then discovered to be abundant in saline lagoons (Read et al, 1983)
Freshwater vs Marine amoeba Many genera of amoeba have been described from both marine and freshwater environments and in some cases it is claimed that the same species occupies both environments (e.g Vannella mira). It is known that some genera display a surprising ability to acclimatise at least in the short term to substantial changes in salinity (Oshima et al, 1986). Acanthamoeba has been repeatedly isolated from marine areas disrupted by human activity (principally sewage dumping) and is it supposed that they have been deposited there but are destined to survive for a short period before being out-competed by other protists or succumbing to the stress of high salinity. Whither or not the same amoeba can survive in both fresh and salt water is of course tied up in the debate as to what constitutes an amoebal species and this has recently been made even more controversial with a report that suggests that Vannella and Platyamoeba are one and the same group but that further, there are distinct marine and freshwater groups that contain species previously assigned to both genera. At the moment is seems most likely that marine species and freshwater species are likely not to mix competitively. They may be isolated from each others environment where they meet (e.g. Acanthamoeba in salt water) but it seems likely that they are not able to compete in the foreign environment. Perhaps freshwater species are isolated from marine environments more often than salt water types are isolated from freshwater because they form resistant cysts and possibly more important because there is a constant flow of freshwater species into the sea. Freshwater Lakes & Ponds The surface. Often as one cultures smaller amoebae especially Vannella, Naegleria and Acanthamoeba, rafts of amoeba can be seen locomoting about at the air-liquid interface and it is argued that this occurs in nature (Preston et al, 2001). Amoeba clinging to this interface will benefit by the accumulated bacteria and other prey items here but may be subjected to increased UV damage - a problem faced by many protists (Sommaruga & Buma, 2000). The Water Column. Many amoeba have "floating" forms with long appendages whose function it is presumed is to maintain the amoeba in suspension so that the amoeba can be dispersed. These forms may not actively feed as actively or as effectively as they would on a surface as has been shown for various flagellates and ciliates (Zubkov & Sleigh, 2000). Just as amoeba are found to be concentrated in macro-aggregates in seawater (Artolozaga et al, 2000), amoeba may also be concentrated within particles in the water column of freshwater bodies. The Lake Bottom. Conditions at the bottom of large ponds and lakes do not change much throughout the year but dramatic changes in abundance of particular amoebae have been noted (O'Dell, 1979). Changes in Acanthamoeba density were observed with a peak in August, September and another in November, these peaks may have been caused by predation on algae or cyanobacteria expected generally to increase in the summer. It is known that Acanthamoeba prey on cyanobacteria (Wright et al, 1981). In the same period, populations of Vahlkampfia and Naegleria peaked in May. It was noticed that these peaks did not correspond to any single variable measured factor and were concluded to result from a non-characterised combination of variables (O'Dell, 1979). In a very systematic study of a lake in North-Western Russia (Smirnov & Goodkov, 1996), 39 species were identified morphologically in the sediments of the lake bottom and 11 new species were recorded. This gives some indication of the abundance and complexity of the amoebal community at the lake bottom specifically and the general incompletion of the biology of the amoeba in general. Large amoeba of the genera Amoeba, Chaos, Saccamoeba and Trichamoeba were observe on top of the sediment layer in this lake and this has been the stated natural habitat of the club-shaped Peloxyma, although possibly this genera favours anoxic sediments. A shallow lake (1.5M) has also been studied with respect to the seasonal abundance of amoeba and this
pH and amoebae Amoebae have been isolated from extremely acidic environments (Johnson & Rang, 1993 ; Amaral Zettler et al, 2002). In some cases it is not certain if these amoebae exist as trophozoites or merely survive there as cysts.
Consumption of Bacteria by Amoebae
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