Chapter 17

Heterokontophyta

BACILLARIOPHYCEAE

The Bacillariophyceae or the diatoms probably evolved from a scaly member of the Chrysophyceae (similar to the organisms in the Parmales) or Bolidophyceae (Guillou et al., 1999). The diatoms are unicellular, sometimes colonial algae found in almost every aquatic habitat as free-living photosynthetic autotrophs, colorless heterotrophs, or photosynthetic symbiotes (Schmaljohann and Röttger, 1978). They may occur as plankton or periphyton, with most brownish-green films on substrates such as rocks or aquatic plants being composed of attached diatoms. The cells are surrounded by a rigid two-part box-like cell wall composed of silica, called the frustule. The chloroplasts contain chlorophylls a, c1, and c2 with the major carotenoid being the golden-brown fucoxanthin, which gives the cells their characteristic color.

In discussing diatoms and silica, there is often confusion over terminology in regard to silicon. Silicon is the element. Silica is a short convenient designation for silicon dioxide (SiO2) in all of its crystalline, amorphous, and hydrated or hydroxylated forms. Silicate is any of the ionized forms of monosilicic acid [Si(OH)4] (Iler, 1979).

Cell structure

The two-part frustule surrounds protoplasm that has a more or less central nucleus suspended in a system of protoplasmic threads. The chloroplasts occupy most of the cell (Figs. 17.17, 17.46) usually

as two parietal plastids although sometimes as numerous discoid plastids. The storage product, chrysolaminarin, occurs in vesicles in the protoplasm.

Cell wall

The characteristic feature of the Bacillariophyceae is their ability to secrete an external wall composed of silica, the frustule. It is constructed of two almost equal halves, the smaller fitting into the larger like a Petri dish (Figs. 17.1, 17.9, 17.10, 17.41). The outer of the two half-walls is the epitheca and the inner the hypotheca. Each theca is composed of two parts, the valve, a more or less flattened plate, and the connecting band, attached to the edge of the valve. The two connecting bands, one attached to each valve, are called the girdle (von Stosch, 1975). Sometimes the connecting bands themselves are called girdle bands (Fig. 17.41). Occasionally there are one or more additional bands between the valve and the girdle, which are called intercalary bands. When an appreciable part of the edge of the valve is bent inward, this portion is called the mantle or valvejacket. The girdle bands, often furnished with minute teeth, hold the valves together by their edges. The value margin thus butts onto the end of the girdle band and is usually connected to it by a pectinaceous film. If this film is destroyed, the valve and girdle bands separate.

The siliceous material of the frustule is laid down in certain regular patterns that leave the wall ornamented. According to Hendey (1964), the ornamentation of diatoms can be divided into four basic types: (1) centric and radial, where the

370 CHLOROPLAST E.R.: EVOLUTION OF TWO MEMBRANES

Fig. 17.1 Light microscopical drawing of valve (a) and girdle (b) views of the diatom Mastogloia. (c) Drawing of a transverse section of M. grevillei in the transmission electron microscope. (Ch) Chloroplast; (CN) central nodule;

(E) elongate chamber of a septum; (GB) girdle band;

(I) intercalary band; (IBE) intercalary band of the epitheca; (IBH) intercalary band of the hypotheca; (LT) locule tubule;

(O) oil; (R) raphe; (S) stria. ((c) adapted from Stoermer et al., 1965.)

structure is arranged according to a central point, for example, Coscinodiscus (Fig. 17.2(a)); (2) trellisoid, where the structure is arranged uniformly over the surface without reference to a point or line, for example, Eunotia (Fig. 17.2(b)); (3) gonoid, where the structure is dominated by angles, for example, Triceratium (Fig. 17.2(c)); (4) pennate,

Fig. 17.2 The basic patterns of ornamentation in the Bacillariophyceae. (a) Centric and radial (example Coscinodiscus). (b) Trellisoid, with structure arranged margin to margin (example Eunotia). (c) Gonoid, with structure supported by angles (example

Triceratium). (d) Pennate, symmetrical about an apical line (example Navicula).

(After Hendey, 1964.)

HETEROKONTOPHYTA, BACILLARIOPHYCEAE	371

where the structure is symmetrically arranged upon either side of a central line – for example, Navicula (Fig. 17.2(d)).

Some pennate diatoms have a raphe system composed of the raphe (a longitudinal slot in the theca), divided into two parts by the central nodule (Figs. 17.3, 17.4, 17.5). Each half of the raphe terminates in a swelling of the wall called the polar nodule. The ornamentation in the pennate diatoms is bilaterally symmetrical around the raphe. In those pennate diatom valves that do not have a raphe system, there is instead an unornamented area running down the center of the valve, which is called the pseudoraphe (Fig. 17.3). The raphe is not a simple cleft in the wall but is instead an S-shaped slit that is wider at the outer and inner tissue, and thinner in the middle partition region (Fig. 17.5).

Besides the raphe, there are basically two types of wall perforations within the Bacillariophyceae: the simple pore or hole, and the more complex loculus or areola (Figs. 17.6, 17.12) (Hendey, 1964; Ross and Sims, 1972). The pore consists of a simple hole within a usually homogeneous silicified wall

Fig. 17.3 (a) A cell with a raphe system (Pinnularia viridis). (cn) Central nodule; (pn) polar nodule; (r) raphe. (b) A cell with a pseudoraphe (pr) (Tabellaria fenestrata).

Fig. 17.4 Climaconeis colemaniae. Light and scanning electron micrographs of the frustule. The valve contains linear striae, each with 6–8 poroid aerolae. The valve contains a raphe opening. Two pores occur in the area of the central nodule. (From Prasad et al., 2000.)

372 CHLOROPLAST E.R.: EVOLUTION OF TWO MEMBRANES

Fig. 17.5 Scanning electron micrographs of sections of the frustule of Haslea nipkowii. (From Poulin et al., 2004.)

Fig. 17.6 The types of openings in frustule walls. (a) Hole or pore (Chaetoceros didymos var. anglica). (b) Loculus opening outward (Coscinodiscus linatus). (c) Loculus opening inward (Thalassiosira wailesii). (h) Hole; (lp) lateral pore or pass pore; (sm) sieve membrane; (sp) sieve pore. (After Hendey, 1971.)

that is frequently strengthened by ribs and costae (Figs. 17.5, 17.27). If the pore is occluded by a plate, then it is called a poroid. The loculus consists of a usually hexagonal chamber in the wall that is separated from other loculi by vertical spacers, which

often have pores in them to allow for communication between loculi (Fig. 17.6(b), (c)). At one end of the loculus is a sieve membrane (pore membrane, velum, cribrum) (Figs. 17.6, 17.7). The sieve membrane can be on the outside (an inwardopening loculus) or on the inside (an outwardopening loculus) (Fig. 17.6(b), (c)). The structure of the valve wall with loculi thus resembles a honeycomb. Pores or loculi (punctae) in a single row are referred to as stria (plural striae) (Figs. 17.1, 17.4).