INTRODUCTION TO THE SILICOFLAGELLATA

The silicoflagellates are mainly marine taxa that are found in the plankton, though some occur as coccoid cells and filaments. They are made up of two different groups called dictyophytes and pelagophytes which have been united by molecular phylogenies (e.g. Brown and Sorhannus 2010; Yang et al. 2012, see Figure 1 below). The main characters that unite them are the presence of chlorophyll c3 together with chlorophylls a and c, a distinctive girdle lamella in the chloroplast, and a chloroplast genome that is grain-like and scattered (not the typical ring-like chromosome). Furthermore, the chloroplasts have the same types of xanthophylls and never have eyespots.

THE DICTYOCHOPHYTES

The dictyochophytes tend to be unicellular without a wall, but they can have organic scales or a silica skeleton (text with tooltip) Silica is silicon dioxide (SiO2). of tubular elements formed externally to the plasmalemma (Figure 2). The cells may have one to many chloroplasts, usually discoid. Many taxa in this group are heliozoan in overall form. Dictyocha (Figure 2) forms a delicate silicaceous basket around which the frothy cytoplasm of the vegetative cell enwraps itself. They do form rounded motile swarmer cells (gametes?), but further details of a sexual life history are unknown. They can form blooms in the ocean plankton. Their star-like external skeletons preserve well so that they do have a fossil history.

Pedinellids are motile unicells that can have axopods (text with tooltip) An axopod is a thin pseudopodium that is permanent and stiff due to the presence of an internal microtubular structure. This is characteristic of the Actinopodotista. that either emerge from the whole cell or are localized around the flagellar insertion point. The anteriorly-directed (text with tooltip) An anteriorly-directed flagellum extends in the direction of the motion of the motile cell. The interpretation is that the flagellum functions by pulling the cell. tinsel flagellum has a paraflagellar rod that draws the flagellum into a ribbon (see Figure 3). The posterior whiplash flagellum can attach to a substrate as in the bicosoecids.

Rhizochromulines are free-floating heliozoan-type cells with long fine filopodia in the typical feeding stage. The motile stage (zoospore?) has a single anterior flagellum and one chloroplast.

PELAGOPHYTES

Pelagophytes can be plankters or periphyton. When they make motile cells, the anterior flagellum has bipartite rather than the typical heterokont tripartite tubular flagellar hairs. Pelagomonas (Figure 4) is a pervasive member of the picoplankton in the world’s oceans. Aureococcus, another picoplankter, is nonmotile and has been implicated in the formation of toxic “brown tides”, particularly along the mid-Atlantic coast (Bricelj and Lonsdale 1992, see Figure 5). Though the toxins do not affect humans, they can devastate filter-feeders like clams and cause the decline and death of sea grass. Gobler et al. (2010) suggest that the ability of Aureococcus to outcompete other co-occurring plankters in the presence of adequate organic matter may be due to their unusually large genome.

In systems such as Kristiansen (1982, cited in Bold and Wynne 1985) and Kristiansen (1990), the silicoflagellates were considered to be members of the chrysophytes. The relatively recent recognition of the differences between silicoflagellates and chrysophytes led first to a fragmentation of taxa (e.g. Patterson 1999). These began to be reshuffled following the recognition of the Pelagophyceae (Anderson et al. 1993). The relatively recent taxonomically broad molecular phylogenetic studies of the heterokonts have supported a new arrangement of taxa. They, in general, support the association of the Dictyochophyceae and the Pelagophyceae (e.g. Brown and Sorhannus 2010; Yang et al. 2012).