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Description of Silicoflagellata

The Silicoflagellata (known in botanical literature as the Dictyochophyceae Silva 1982) comprises three small (c. 30 extant species) but distinctive groups of free-living protists sometimes counted among the chrysomonads (chrysophytes): the silicoflagellates sensu stricto, the pedinellids, and the rhizochromulinids. These organisms, according to recent research, form a clade that is most closely related to pelagomonads and diatoms (Saunders et al. 1995). In some classifications (Cavalier-Smith 1993; Saunders et al. 1995) silicoflagellates, pedinellids, and pelagomonads are each afforded class status. Silicoflagellata are small to medium size unicellular protists (range 5-50, exceptionally to 500 µm), usually occurring either as flagellates or as axopodial amoebae. As in most other members of the heterokont/stramenopile clade, the youngest (anterior or only) flagellum on swimming cells bears tripartite tubular hairs. The defining characteristic is the presence of cytoplasmic microtubules that arise from differentiated pads (presumed nucleating sites) on the nuclear envelope. These microtubules extend into the axopodia, although they may be present even when axopodia are not expressed. The axopodial microtubules are either not united into axonemes or they form axonemes consisting of three microtubules; this feature distinguishes Silicoflagellata from the actinophryid amoebae, in which the axopodial axonemes contain numerous, highly organized microtubules. No root microtubules are associated with the kinetid in flagellate cells, a feature that links Silicoflagellata with pelagomonads and separates them from other heterokont/stramenopile taxa. Also lacking is the transitional helix typically found distal to the transition zone between kinetosomes and flagella in other heterokonts, but two rings (or a helix of two gyres) may be present proximal to the transition zone. This feature also links Silicoflagellata and pelagomonads. Most species of Silicoflagellata are photosynthetic, containing chloroplasts with thylakoid membranes in stacks of three and the chrysomonad pigments (chlorophylls a and c, fucoxanthin), but a few pedinellids are known that lack chloroplasts. Silicoflagellates sensu stricto form complex siliceous external skeletons, which fossilize well. From these palynomorphs, a silicoflagellate fossil record including several form genera and extending back to the Cretaceous is known. Other Silicoflagellata are naked or have a covering of organic scales. For these taxa, no fossil record is known. Silicoflagellata are common in marine planktonic and benthic habitats. Blooms are usually benign, but a few have been associated with fish kills although toxins have not been identified. Some species of pedinellids are known from fresh water. Species of Silicoflagellata may have complex asexual life histories including several different trophic stages and cysts, but sexual reproduction has yet to be observed. Accounts of the structure, reproduction and diversity of Silicoflagellata have been given by Moestrup and Thomsen (1990), Moestrup (1995) and O'Kelly and Wujek (1995).

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