DICTYOCHOPHYCEAE

These golden-brown algae are characterized by tentacles or rhizopodia on basically amoeboid vegetative cells (Moestrup, 1995; Preisig, 1995). Amoeboid cells are relatively rare among the algae, being mostly restricted to the Dictyochophyceae and the Xanthophyceae (Hibberd and Chretiennot-Dinet, 1979). The algae in the Dictyochophyceae have been previously classified in the Chrysophyceae, although molecular evidence shows them to be most closely related to the Pelagophyceae (CavalierSmith et al., 1995) or Eustigmatophyceae (Daugbjerg and Andersen, 1997).

Classification

The Dictyochophyceae can be divided into three orders (Preisig, 1995):

Order 1 Rhizochromulinales: marine and freshwater unicells with tentacles.

Order 2 Pedinellales: unicells with a long anterior flagellum and a second flagellum reduced to a basal body, usually three to six chloroplasts (if chloroplasts are present), marine and freshwater.

Order 3 Dictyocales: marine unicells with an external silicified skeleton.

Rhizochromulinales

This order contains the more primitive organisms in the order (O’Kelly and Wujek, 1995). Rhizochromulina (Fig. 14.1(a), (b)) has amoeboid

non-flagellated vegetative cells with many fine beaded-filipodia and a single golden-brown chloroplast (Hibberd and Chretiennot-Dinet, 1979). The fusiform zoospore has a single tinsel flagellum with a second basal body in the protoplasm (Fig. 14.1(b)). Chrysoamoeba (Fig. 14.1(d)) lives as a solitary amoeba for the greater part of its life cycle, transforming into swimming cells with a single long flagellum only for short periods. In Phaeaster (Fig. 14.1(c)), the anterior portion of the cell is drawn out into rhizopodia.

Pedinellales

The pedinellids are unique in containing genera that are phototrophic (Apedinella (Fig. 14.2(c)), Pseudopedinella), mixotrophic (able to photosynthesize and take up organic compounds) (Pedinella) (Figs. 14.2(a), (b), 14.3, 14.4), and phagotrophic (Pteridomonas, Ciliophrys). The phagotrophic genera have vestigial chloroplasts and evolved from genera with chloroplasts (Sekiguchi et al., 2002). The organisms in this order have three interconnected microtubules (triads) that course from the nuclear envelope through tentacles (if they are present) to the plasma membrane (Fig. 14.2(a)) (Daugbjerg, 1996). A long apical flagellum is extended into a lateral wing by a paraxonemal rod (Figs. 14.2, 14.3(b)). The apical flagellum is inserted in a pit and there is a second flagellum that is reduced to a basal body. The basal bodies are at a slight angle to each other. The cells are radially symmetrical with a large central nucleus and a posterior Golgi apparatus. There are usually three to six chloroplasts present

if the organism is phototrophic (whereas in the Chrysophyceae there are usually only one or two chloroplasts). Some genera, such as Pedinella (Fig. 14.4(d), (e)) and Apedinella (Fig. 14.2(c)), have scales attached to the plasma membrane by microligaments (Koutoulis et al., 1988).

A posterior trailing stalk, associated with a system of vacuoles, can be present. Pedinella (Figs. 14.2(a), 14.4) cells rotate while swimming, trailing the stalk behind. The stalk appears to be sticky, and swimming cells will often adhere to a substrate by means of the stalk. Pedinella can undergo phagotrophy and ingest other small cells. Bacteria are passed down the flagellum and adhere to the plasma membrane, just outside the row of tentacles. Secretions of the muciferous bodies in this region provide an adhesive that sticks the cells to the plasmalemma. Usually within a minute, the bacteria become enveloped in a sheet of cytoplasm that is extruded from the cell.

Dictyocales

The Dictyocales or silicoflagellates are a group of cosmopolitan marine flagellates, presently represented by only one extant genus Dictyocha, although an abundance of taxa have been described from fossil siliceous skeletons (Henriksen et al., 1993).

Silicoflagellate cells have one emergent flagellum and a skeleton of hollow siliceous rods outside of the protoplasm. The skeleton can be a simple ring, an ellipse, or a triangle, but it is often much more complete. In Dictyocha fibula (Fig. 14.5), the skeleton is composed of a series of peripheral polygons surrounding a central hexagon. The nucleus is in the center of the protoplasm with a number of cytoplasmic processes extending from the central mass. The chloroplasts are usually in the cytoplasmic processes (van Valkenburg, 1971a, b). Most species have chloroplasts that are derived from secondary endosymbioses, although

Fig. 14.2 (a) Pedinella hexacostata in the light and electron microscope. (b) Structure of the winged flagellum of P. hexacostata. (c) Apedinella spinifera. (A) Axoneme of flagellum;

(B) paraxonemal rod of flagellum; (CV) contractile vacuole;

(F) flagellum with hairs; (FV) food vacuole; (G) Golgi body;

(L) leucosin; (M) mitochondrion; (MB) muciferous body;

(N) nucleus; (Pe) peduncle; (Pu) pusule; (S) scale; (SS) spined scale; (SV) scale vesicle; (T) tentacle. ((a), (b) after Swale, 1969; (c) after Throndsen, 1971.)

there has been a report of chloroplasts derived from haptophytes through tertiary endosymbioses (Daugbjerg and Henriksen, 2001).

In Dictyocha speculum, the skeleton-bearing cells multiply vegetatively by mitotic division (Fig. 14.6) (Hendriksen et al., 1993). Cells connected by bridges develop and give rise to large spherical cells without skeletons that become multinucleate. Uninucleate swarmers with a single flagellum develop in the large spherical cells. The swarmers are released and grow into large multinucleate

Fig. 14.6 Growth stages of the

silicoflagellate Dictyocha speculum.

(Adapted from Henriksen et al.,

1993.)

Fig. 14.5 (a) A drawing of the fine structure of Dictyocha fibula. (b), (c) Side and front views of the skeleton of Dictyocha. (ar) Apical ring; (br) basal ring; (c) chloroplast; (cb) cell boundary; (m) mitochondrion; (n) nucleus;

(p) pseudopodium; (rs) radial spine;

(s) silica skeleton; (sb) supporting bar. (After van Valkenburg, 1971a,b.)