Cercozoa and Neomonada

These two phyla include many taxa of small free-living heterotrophic species <20 ^m, many under 10 ^m, with zero, one, two, three or four cilia. Most species previously were designated as 'nanoflagellates' or 'zoo-flagellates' and 'amoebo-flagellates'. The Neomonada are known mostly from marine samples and are not considered here.

The Cercozoa have tubular cristae and many have peroxisomes. Ciliated cells are variably amoeboid. There is no cytostome region, but filopodia are common. Some are gliding species in water films with a trailing cilium. There are anaero-tolerant genera which appear active in anoxic soils. Most species probably form cysts with a cell wall. Two classes of Cercozoa are important to soil ecology, the Sarcomonadea which includes the heterotrophic amoeboid-ciliated genera (e.g. Cercomonas and Heteromita), and the Filosea which includes the filose testate amoebae (such as euglyphid testamoebae) and several amoeboid reticulate genera (Fig. 1.15). The Filosea have a surface test with secreted siliceous plates. Although the correct phylogeny and system-atics of the Cercozoa are uncertain, soil nanoflagellates are very abundant and unavoidably important in nutrient cycling. There is clearly more work required on understanding the biology and diversity of edaphic Cercozoa. The systematics of the Cercozoa are still evolving, and many genera and families will be reshuffled. A useful key to most sarcomonad genera was provided in Lee's illustrated guide (2001, pp. 1303-1307). The Sarcomonadea in the soil include many of the tiny ciliated species (<20 ^m, but mostly under 10 ^m) that are so abundant in fertile soil. They are primarily bacterivorous, with vari-

filopodia of a cercozoan cell spread thin and flat against the substrate. (Certain genera tentatively assigned to Cercozoa.) Scale bar 10 ^m.

able ability for osmotrophy. Selection of prey bacteria is probably common and depends on the bacteria's dimensions, shape, secondary metabolites, capsule size and composition. Filaments of bacteria are too long to ingest, and long or wide bacteria may be too difficult to phagocytose. Filose pseudopodia stretch through the soil to explore pores and surfaces, and participate in directing chemotaxis. Multiple branched or unbranched filopodia (<2 ^m diameter) can explore pores that are inaccessible to the cell, by stretching out long hair-like strands of pseudopodia. Edaphic species are mostly gliding or amoeboid on surfaces, but in sufficient water they round up into a swimming cell rapidly (seconds) to change microhabitat. Gliding species usually possess an anterior cilium with a trailing posterior cilium. The sarcomonads offer little morphology for identification with light microscopy, and they are poorly described. Identification requires electron microscopy and gene sequence analysis, which is lacking for many soil isolates. Species may be confused with bodonids, Colpodellidae or Neomonada. Genera that may be Cercozoa and are encountered in soils include Adriamonas (Pseudodendromonadida), Allantion, Allas, Amastigomonas, Apusomonas, Artodiscus, Cercomonas, Heteromita, Multicilia, Parabodo, Proleptomonas (with chytrid-like behaviour), Sainouron and Tetracilia.

The dimorphids are encountered infrequently in soils but have 2-4 cilia and axopodia that are retracted if disturbed in swimming. They possess extrusomes and are bacterivorous, cytotrophic or ingest microdetritus.

The Filosea include testate amoebae with filose pseudopodia such as the euglyphids which secrete siliceous scales for their test (Fig. 1.16), and others such as Pseudodifflugia which bind soil mineral particles with a secreted cement to form the test. Both are common in soil litter. They were traditionally within the Testacealobosea but separated based on ultrastructure and molecular phylogenies (Table 1.3). Their biology and feeding habits are similar to those of other testate amoebae described above (see Amoebae).

Species descriptions for testate amoebae traditionally are based on details of test morphology. The terrestrial species are smaller than similar aquatic species, and some variation in test size occurs naturally in the soil. This variation may reflect an adaptation to drier periods as well as resource limitations. For example, Trinema lineare (Filosea) makes a smaller test under low soil moisture conditions (Laminger, 1978). Many species form a smaller test opening in drier periods, probably to minimize desiccation. Schonborn (1983) observed that the presence of spines and horns on the test can vary with culture conditions. Foissner (1987) states that identical test morphology can be found between samples, ranging in size from small to very large, raising the question of whether these could be different species or simply biogeographical variations. There are many reports of observed changes in test morphology occurring in the laboratory under culture conditions or naturally in the soil in response to changing environmental conditions and food availability

Urzeala Batatura
Fig. 1.16. Filose testate amoebae (Filosea, Cercozoa). (A) Euglypha with overlapping siliceous scales. (B) Trinema with large and small scales embedded in the test. Scale bar 25 ^m.
Table 1.3. Families and selected genera of testate edaphic Filosea.

Orders

Families

Selected genera

Euglyphida

Euglyphidae

Euglypha, Tracheleuglypha, Placocista, Assulina, Trachelocorythion

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