HETEROKONTOPHYTA, CHRYSOPHYCEAE	341

Fig. 10.10 (a) Uroglena conradii. (b), (c) Anthophysa vegetans.

(d) Chromulina conica. (cer) Chloroplast endoplasmic reticulum; (cv) contractile vacuole; (e) eyespot; (fv) food vacuole; (l) leucosin; (lf) long flagellum; (lp) leucoplast; (mt) microtubules; (n) nucleus; (sf) short flagellum. ((a), (d) after Schiller, 1929; (b) after Pringsheim, 1946; (c) after Belcher and Swale, 1972.)

Order 2 Parmales: cells with siliceous walls composed of five or eight plates.

Order 3 Chrysomeridales: motile naked cells with laterally inserted flagella and an eyespot, motile cells similar to those in the brown algae.

Classification

Three of the orders of the Chrysophyceae (Preisig, 1995) will be considered here:

Order 1 Chromulinales: cells with the flagella inserted into the anterior portion of the cell.

Chromulinales

All of the organisms in this order have a unicell with two apically inserted flagella somewhere in their life history. One of the flagella is tinsel with mastigonemes and directed forward, while the second flagellum is whiplash (lacking mastigonemes) and is inserted at approximately 90° to the tinsel flagellum (e.g., Ochromonas,

342 CHLOROPLAST E.R.: EVOLUTION OF TWO MEMBRANES

Figs. 10.2, 10.3). In some of the genera (e.g., Chromulina, Fig. 10.10(d)) the whiplash flagellum is reduced to a stub. There are usually two parietal chloroplasts, a central nucleus, and a large posterior chrysolaminarin vesicle.

Within the order there is a progression from the unicellular to the colonial form (Fig. 10.10) as exemplified by Uroglena (Fig. 10.10(a)) and Anthophysa (Fig. 10.10(b), (c)). Some of the genera, such as Dinobryon (Fig. 10.11), have cells surrounded by a lorica. In Chrysococcus (Fig. 10.5), the cell is surrounded by a lorica that has pores in it.

In the Chromulinales there is a frequent tendency toward loss of photosynthetic activity and adaptation to various forms of phagotrophy and chemo-organotrophy. Associated with this is a diminution of the chloroplast as in Anthophysa (Fig. 10.10(b), (c)), which has a leucoplast with a pigmented eyespot. The end of this reduction is represented by Paraphysomonas (Fig. 10.4), which has no trace of an eyespot or plastid. Along with the reduction in the chloroplast have arisen adaptations for feeding, such as in Anthophysa, which beats its long flagellum, creating a water current toward the cells, which brings with it bacteria from as far away as 200 m. The bacteria strike the anterior end of the cells and are ingested at the point where they touch. The bacteria sink into the cytoplasm, which closes behind them, and are completely in the cell within 2 to 3 seconds of contact (Belcher and Swale, 1972).

In Dinobryon cylindricum, sexual reproduction is heterothallic and dioeceous, and morphologically and physiologically anisogamous (Fig. 10.11) (Sandgren, 1981). Cells of the second or third tier of the colony (the basal cell being the oldest) are the best potential gamete-producing cells. Gametes are produced mainly in exponentially growing populations. Female cells release a chemical erogen that causes male cells to divide once. One of the male cells remains in the lorica, whereas the other swims away as a naked male gamete, instead of attaching to the lorica mouth and building a new lorica as a vegetative cell would. The non-loricate male gamete (which is structurally similar to an Ochromonas cell) swims to the female cell in its lorica, the flag-

ella become oriented parallel to each other, and fusion occurs at the anterior end of each cell. Plasmogamy results in a quadriflagellate planozygote that fills the lorica. Nuclear fusion does not occur. After 30 minutes, the flagella have been lost and the zygote creeps to the lorica mouth by amoeboid movement. A thin cellulose encystment vesicle is formed as a lorica extension, the zygote rounds up, and a binucleate statospore (Fig. 10.12) is formed. In D. divergens, most statospores settle to the lake bottom and germinate early the next year (Sheath el al., 1975). Statospore germination occurs by the formation of a cellulosic vesicle at the statospore pore, followed by a presumably meiotic cleavage into four daughter cells that migrate into the vesicle. The Ochromonas-like swarmers escape to form new vegetative cells.

Asexually produced statospores of Dinobryon cylindricum may occur in exponential or stationary-phase populations, depending upon the clones involved (Sandgren, 1980, 1981). Asexual statospores (cysts) are formed at a much lower frequency (0.05% or less) than sexual statospores unless the population is placed in a nitrogen-depleted environment where asexual encystment can reach 4%. The continual production of a low number of the resistant statospores by a Dinobryon population acts as a hedge against a rapid unfavorable change in environmental conditions that would kill the vegetative cells.

The first event in statospore formation is vegetative cell enlargement so that the cell begins to bulge out of the lorica. The Golgi form vesicles that fuse to form a continuous silica deposition vesicle in the peripheral cytoplasm. The silica deposition vesicle has a pore that eventually will be the statospore pore. Precipitation of silica proceeds in the silica deposition vesicle to form the silicified statospore wall. The portion of the cytoplasm that is exterior to the wall retracts through the pore in the cyst wall into the statospore interior. The small amount of cytoplasm outside the wall forms the collar around the pore. A plug is formed in the cyst wall, sealing the statospore protoplasm. The mature statospore has two nuclei, two plastids, and a rich supply of energy reserves in the form of oil and chrysolaminarin.

HETEROKONTOPHYTA, CHRYSOPHYCEAE	343

Fig. 10.11 The life cycle of Dinobryon. (Adapted from

Sheath et al., 1975; Sandgren, 1980, 1981.)

Chrysophaera (Fig. 10.13) is an example of a colonial alga in the order. This organism has a dominant spherical non-motile stage with cells adhering to each other and the substrate in irregular clusters. The smallest individuals are unicellular, but individuals with up to 256 cells within a common mucilaginous envelope are not uncommon. All the cells within the envelope have a long tinsel flagellum and a short stub of the second flagellum (Belcher, 1974). When a colony is placed in distilled water, it begins to swell within

Fig. 10.12 Scanning electron micrograph of a cyst of

Dinobryon cylindricum. Bar 5 m. (From Sandgren, 1983.)

344 CHLOROPLAST E.R.: EVOLUTION OF TWO MEMBRANES

Fig. 10.13 The life cycle of

Chrysosphaera magna.

(C) Chloroplast; (E) eyespot;

(L) leucosin vesicle; (LF) basal body of long flagellum; (M) muciferous body; (Mt) mitochondria;

(ME) mucilaginous envelope;

(N) nucleus; (O) oil; (SF) short flagellum. (After Belcher, 1974.)

15 minutes, and the mucilage gelatinizes on one side. The zoospores are released a few minutes later and swim for about 2 minutes before they shed their flagella, settle down, form a mucilaginous envelope, and become identical in appearance to the youngest coccoid stages.

Parmales

The Parmales consist of small cells, generally 2 to 5 m in diameter, each with a chloroplast (Marchant and McEldowney, 1986) and a silicified cell wall composed of five to eight plates (Figs. 10.14, 10.15). In the Pentalaminaceae there are five wall plates whereas in the Octolaminaceae there are eight wall plates (Booth and Marchant, 1987). Members of the Parmales occur at concentrations of 105 cells per liter in polar and subpolar marine waters, making them one of the more abundant groups in these waters.

Four different shapes of plates occur around the cells (Figs. 10.14, 10.15) (Booth and Marchant, 1987). A shield plate or round plate is a circular plate with or without a central knob or process. A triradiate plate has three arms equally spaced, each arm fitting between two shield plates. A ventral plate is a round plate of greater diameter than that of shield plates; it is found in Triparma on the opposite side of the sphere from the triradiate plate. Girdle plates are three oblong plates in Triparma, juxtaposed end to end to form a ring around the ventral plate between it and the other four plates. Ornamentation of various types (papillae, wings, spines, keels) occurs on the plates.

Chrysomeridales

These algae have zoospores with the flagella more or less laterally inserted into the cell body. There

HETEROKONTOPHYTA, CHRYSOPHYCEAE	345

(a)

(b)

(c)

Fig. 10.14 Three genera in the Parmales. (a) Pentalamina.

(b) Tetraparma. (c) Triparma. (G) Girdle plate; (R) round plate;

(S) shield plate; (T) triradiate plate; (V) ventral plate. (After Booth and Marchant, 1987.)

Fig. 10.15 (a), (b) Triparma laevis. (c) Triparma strigata. (From Kosman et al., 1993.)

is an eyespot in the chloroplast and the accessory pigment violaxanthin is present. The similarities in the zoospores of the algae in this order have led to the speculation that the Phaeophyceae (brown algae) probably evolved from an alga similar to Giraudyopsis in this order (Fig. 10.16) (O’Kelly, 1989; Saunders et al., 1997). The algae in the Chrysomeridales, however, lack the unilocular and plurilocular sporangia, as well as the plasmodesmata and alginates characteristic of the

Phaeophyceae.	Fig. 10.16	Giraudyopsis stelliger. (After Dangeard, 1966.)