HETEROKONTOPHYTA, BACILLARIOPHYCEAE	373

Fig. 17.7 Stereopair illustrating details of the external areolar pattern and structure in Mastogloia angulata. If a stereo viewer is not available, bring the photograph about

6 inches (15 cm) from your eyes and cross your eyes to view in three dimensions the loculi, each with a sieve membrane at the bottom of the loculi. (From Navarro, 1993.)

The girdle bands in some diatoms have the same sculpturing as the valve, but in many others sculpturing on the girdle and valve is different. In some cases, the girdle bands have no sculpturing.

Special pores (mucilage or slime pores) (Fig. 17.18) through which mucilage is secreted are known in many diatoms. In the pennate diatoms, these pores usually occur singly near one or both poles of the valve and generally occupy thickenings in the walls.

The valve surface can have extensions, called processes, whose main function appears to be to maintain contact between contiguous cells and to assist colony formation. These processes are given different names: Cornutate processes are horn-like; strutted processes are ones that have been reduced to a boss at the apex of a valve (Figs. 17.8, 17.38(c), (d)); spinulae are very small processes; awns or setae are hollow and elongated (Figs. 17.32, 17.44).

The frustule is composed of quartzite or hydrated amorphous silica that may also have small amounts of aluminum, magnesium, iron, and titanium mixed with it (Mehta et al., 1961; Lewin, 1962). Diatom frustules from marine plankton contain 96.5% SiO2 and 1.5% Al2O3 or Fe2O3 (Rogall, 1939). The inorganic component of the frustule is enveloped by an organic component or “skin” (Reimann et al., 1965), the latter composed of amino acids and sugars (Coombs and Volcani, 1968; Hecky et al., 1973) with the amino acid hydroxyproline and collagen present (Nakajima and Volcani, 1969).

Cell division and the formation of the new wall

The normal asexual method of reproduction is by division of one cell into two, the valves of the parent cell becoming the epithecas of the daughter cells with each daughter cell producing a new hypotheca (Figs. 17.9, 17.10). As a result of cell division, one of the daughter cells is of the same size as the parent cell, and the other is smaller. As the size of the cell decreases, so does the relative width to height and the morphology of the cell; in other words, the smaller cells are

374 CHLOROPLAST E.R.: EVOLUTION OF TWO MEMBRANES

Fig. 17.8 Scanning electron micrographs of centric diatoms. (a) Thalassiosira lacustris. Undulated hypovalve with strutted processes and part of epitheca showing distal pattern of band openings.

(b) Cyclotella striata. A frustule showing the epitheca, intercalary, and girdle bands. (c), (d) Thalassiosira gessneri. Valve showing undulation, aerolation, and subcentral and marginal rings of strutted processes (c). Higher magnification of labiate and strutted processes (d).

(e) Skeletonema costatum showing the marginal ring of strutted processes used in chain formation. ((a), (c), (d) from Hasle and Lange, 1989; (b) from Prasad et al., 1990; (e) from Medlin et al., 1993.)

Fig. 17.9 Diagrammatic representation of a diatom cell in

girdle view showing the reduction in size through two

divisions. (After Hendey, 1964.)

not geometrically proportional to the larger ones from which they arise.

Uptake of silica is confined to a period of the cell cycle following cytokinesis and prior to the separation of the two daughter cells (Sullivan, 1977). Cellular energy for silicification and transport comes from aerobic respiration without any direct involvement of photosynthetic energy (Martin-Jezequel et al., 2000).

Diatoms have an absolute requirement for silicon if cell division is to take place. In water, solid silica dissociates to produce undissociated silicic acid Si(OH)4:

SiO2 (solid) 2 H2O Si(OH)4

By increasing the concentration in solution with a pH less than 9, or decreasing the pH of a saturated solution, silicic acid will autopolymerize to form amorphous silica. Amorphous silica is the form of silicon in diatom cell walls. Although silicon is the second most abundant element in the Earth’s crust, its availability is limited by its solubility in water. The growth of marine diatoms can so deplete surface waters of silicon that their further growth is prevented (Hildebrand, 2004).

The Si(OH)4 in marine waters is about 6 ppm. In the global ocean, about 97% of the dissolved Si is present as Si(OH)4, with the pH of seawater buffered at about 8.0 by the CO2-carbonate system (Del Amo and Brzezinski, 1999). However, in freshwater lakes where pH values reach up to 10, Si(OH)4 is only about 23% of the total dissolved silicon, most of the rest being present as ionized

HETEROKONTOPHYTA, BACILLARIOPHYCEAE	375

Fig. 17.10 Diagrammatic representation of the stages of frustule formation, including girdle band formation, in

Gomphonema parvulum. (a) The nucleus divides, and the cell elongates. (b) There is invagination of the plasma membrane. (c) Formation of the silicalemma occurs by the fusion of Golgi-derived vesicles. (d) Silica is deposited within the silicalemma to form the valves; vesicles accumulate in the area of the first girdle band. (e) The first girdle band is formed within a vesicle and secreted to the outside. The internal membrane of the silicalemma becomes the new plasmalemma, and the old plasmalemma and the outer membrane of the silicalemma are lost. (f)–(i) The second and third girdle bands are formed. (After Dawson, 1973.)

silicic acid SiO(OH)3 according to the following formula:

Si(OH)4 H2O SiO(OH)3 H

Silica is taken up into the diatom cell as Si(OH)4 by a protein as an active silicon transporter requiring metabolic energy (Hildebrand et al., 2004; Wetherbee et al., 2004). Silicic acid transport is coupled to sodium in marine diatoms and sodium and perhaps potassium in freshwater diatoms. The transport in marine species has the characteristics of a sodium/silicic acid symporter

376 CHLOROPLAST E.R.: EVOLUTION OF TWO MEMBRANES

with a Si(OH)4:Na ratio of 1:1. At least five types of silicic acid transporter (SIT) genes have been isolated. Germanium (germanic acid) and silicic acid are competitive inhibitors of each other but germanium can not be incorporated into the diatom cell wall.

Prior to cell division, the cell elongates, pushing the epitheca away from the hypotheca, and the nucleus divides. After the protoplasm has divided (Fig. 17.10) into two by the invagination of the plasmalemma, the Golgi bodies produce translucent vesicles which collect beneath the plasmalemma. These vesicles fuse to form the silicalemma or membrane of the silica deposition vesicle (Li et al., 1989). The vesicle gradually expands and assumes the shape of a new valve (Schmid and Volcani, 1983). Two silica deposition vesicles are formed per cell with silica deposited in each to form two new hypovalves (Brzezinski and Conley, 1994). The silicon is probably packaged by Golgi into vesicles that are transported to the silica deposition vesicle by microfilaments in the cytoplasm. At the silica deposition vesicle, the small, silica-laden vesicles fuse with the silicalemma, adding membrane material to the silicalemma, and releasing silica to the interior of the silica deposition vesicle (Fig. 17.10). The silica deposition vesicle determines the ultimate form of the silicified frustule (Fig. 17.15). Silica is deposited as amorphous silica in the form of spheres 30–50 nm in diameter (Figs. 17.11, 17.12) (Crawford et al., 2001).

The silica deposition vesicles are acidic, the low pH facilitating fast nucleation and aggregation of silica particles (Vrieling et al., 1999). The acidic environment also protects the newly formed valves from dissolution before coverage by an organic casing prior to secretion.

The silica deposition vesicles contain peptides called silaffins (Fig. 17.13) that cause the precipitation of silicic acid into silica nanospheres (Figs. 17.12, 17.14) (Kroger et al., 1999, 2000; Kroger and Sumper, 2004). Different diatom species have different silaffins that, in turn, have different polyamine chains (Fig. 17.13) attached to the silaffins. Different silaffins produce different sized silica nanospheres and it may be that the specific silaffin controls the frustule ornamentation, a characteristic of the individual diatom species.

Fig. 17.11 The formation of a sphere of hydrated

amorphous silica from molecules of monosilicic acid.

The discovery of silaffins has generated considerable commercial interest since silica-based materials such as resins, molecular sieves, and catalysts are widely used in industry. The production of the manufactured silica-based materials currently requires extremes of temperature, pressure, and pH. In contrast, biosilicification with silaffins proceeds at ambient temperatures and pressures.

A class of glycoproteins, called frustulins, is also associated with the silica deposition vesicle. The frustulins are associated with deposition of the organic casing around the silicified frustule (Perry and Keeling-Tucker, 2000).

When the deposition of silica is complete, the inner membrane of the silicalemma becomes the plasmalemma of the daughter cell, and original plasmalemma and the external membrane of the silicalemma are lost (Fig. 17.10). After the epitheca and the hypotheca are formed, Golgi vesicles now collect and fuse to form the silicalemma of the girdle band. The girdle band is deposited outside the cell in the area between the hypovalve and epivalve when silica deposition is complete. If there are further girdle bands, they are formed in the same manner (Dawson, 1973).

Although amorphous silica is slowly soluble at the pH of natural waters, the silica frustule of