HETEROKONTOPHYTA, BACILLARIOPHYCEAE	391

Ecology

Diatoms comprise the main component of the open-water marine flora and a significant part of the freshwater flora. Attached diatoms can be characterized by the brown scums found on various kinds of substrata, as well as the fluffy brown growths caused by abundant epiphytic diatoms. The pennate diatoms are represented in about equal numbers in the freshwater and marine habitats, whereas the centric and gonoid diatoms are present predominantly in the marine environment. Generally speaking, in the marine environment the colder the water is, the greater the diatom population. The populations of diatoms in the open oceans usually have a large number of species with the total number of organisms being low, in contrast to the diatoms living close to the shore, where the total number of diatoms is very high, but the number of different species within the population is low. This situation is actually fairly typical of oligotrophic, unenriched waters (the open ocean) versus eutrophic, enriched waters (the coastal waters receiving enrichment from the land).

Marine environment

The maintenance of oceanic diatoms in the water column involves some adaptation of the cells to make them buoyant. The silicified wall of diatoms has a density of about 2.600, comprising up to 50% of the dry weight of the cell (see Smayda, 1970, for a review). The density of seawater varies from 1.021 to 1.028; therefore, planktonic diatoms must have cellular components that are lighter than seawater to achieve neutral or positive buoyancy. The density of the cytoplasm in marine organisms is slightly heavier than seawater, varying from 1.030 to 1.100. This leaves only the vacuole as a likely source of positive buoyancy in diatoms. In fact, most planktonic diatoms have a large vacuole whose contents, while being isotonic, are lighter than seawater. For example, the vacuolar sap of the planktonic diatom Ditylum brightwelli (Figs. 17.22(a), (b), 17.31(b), (c)) has a density of 1.0202, less than that of seawater. The vacuole of these diatoms contains lighter ions than the surrounding seawater: the concentration of Na is great relative to K (the

latter element is heavier by about 40%), there is a relatively high concentration of light NH 4 ions, and divalent relatively heavy ions, especially SO24 , are excluded. This ionic mechanism of buoyancy is probably important in planktonic diatoms as long as they are above a certain size (20 m diameter in Ditylum), below which the size of the vacuole is too small to provide the necessary lift to the cell. The ionic mechanism of buoyancy cannot be applied to the flotation of freshwater phytoplankton because of the low quantity of salts in the water. In freshwater, most colonial diatoms are encased in a gelatinous sac of low density which aids the flotation of the organisms. Settling in the water column is slowed by increasing the surface area relative to the volume, thereby increasing the drag of the cell in the water. In diatoms this can be accomplished by setae (Chaetoceros, Figs. 17.6(a), 17.32, 17.44, 17.45) or by cells shaped as discs (Coscinodiscus, Figs. 17.2(a), 17.6(b), 17.15, 17.38(a), (b)), ribbons (Fragilaria, Fig. 17.31(d), (e)), or elongate forms (Rhizosolenia, Fig. 17.33(c)). The aggregation of diatom cells into chains increases the settling rate of the cells because it decreases the surface area. Increasing the size of diatom cells results in an increase in the ascension rate. The largest diatom cells can ascend up to 8 meters per hour (Moore and Villareal, 1996).

Planktonic diatoms can vary in density during the day, moving up and down in the water column. This causes a constant flow of water over the surface of the diatom, enabling a better absorption of nutrients. Ditylum brightwelli (Figs. 17.22(a), (b), 17.31(b), (c)) is a diatom with a variation in sedimentation rates, the greatest rate of settling occurring during the latter half of the light period and the least settling occurring at the end of the dark period (Fisher and Harrison, 1996). The insignificance of fat deposits in buoying up cells can be seen here, where the cells contain the maximum amount of fat during the period of maximum settling (Andersen and Sweeney, 1977). Sedimentation of diatoms in the ocean continues in many cases until an increased density of the water, due to either a thermocline or greater water depth, causes them to remain steady in the water column. In the central north Pacific Ocean, there is a widespread deep (110to 130-m depth) chlorophyll layer containing pigments of living algae, not detritus. This chlorophyll maximum

392 CHLOROPLAST E.R.: EVOLUTION OF TWO MEMBRANES

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(a)		(b)
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Fig. 17.31 (a) Skeletonema costatum. (b), (c) Ditylum brightwelli. (b) Girdle view. (c) Valve view. (d), (e) Fragilaria virescens, valve and girdle view.

Fig. 17.32 Scanning electron micrograph of Chaetoceros peruvianus showing the setae and a labiate process. (From Pickett-Heaps et al., 1994.)

contains growing organisms and is not an accumulation of moribund cells (Venrick et al., 1973; Jeffrey, 1976). Diatoms make up a good proportion of these cells and are able to grow there by virtue

of an adaptation of the cells to the decreased light (Jeffrey and Vesk, 1977). The light that penetrates down to this depth is composed principally of low-intensity blue-green light with a maximum at about 480 nm. It has been shown with the marine diatom Stephanopyxis turris (Figs. 17.33(a), 17.38(c), (d)) that blue-green light increases the

HETEROKONTOPHYTA, BACILLARIOPHYCEAE	393

(f)

		(b)
(a)
				(c)
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						(d)

Fig. 17.33 (a) Stephanopyxis turris. (b) Navicula glaciei.

(c) Rhizosolenia castracanei. (d) Tabellaria fenestrata.

(e) Tabellaria flocculosa. (f) Achnanthes exigua.

amount of chlorophyll up to 100% when compared to white light, with an appropriate increase in chloroplasts but with no change in the chlorophyll : carotenoid ratio. At the same time, bluegreen light enhances the photosynthetic fixation of carbon dioxide. Thus the diatoms living at great water depth receiving only blue-green light “switch on” a mechanism for more efficient photon capture of light and increased fixation of carbon dioxide. They are therefore able to remain viable and grow at a water depth that has a low amount of light.

Symbioses between nitrogen-fixing (diazotrophic) bacteria and cyanobacteria, and the diatom genera Rhizosolenia and Hemiaulus are common in warm oligotrophic seas (Villareal, 1989). Rhizosolenia castracanei (Fig. 17.33(c)) and R. imbricata var. shrubsolei form free-floating diatom mats in oligotrophic central oceanic regions, such as the Sargasso Sea and the North Pacific Gyre (where Rhizosolenia mats make up 98% of all living diatom silicon (Shipe et al., 1999)). In these areas the fixation of nitrogen by the endosymbionts contribute a significant amount of nitrogen to the ecosystem (Martinez et al., 1983). The Rhizosolenia cells migrate vertically in the column at a rate of several meters per hour and can be found to a depth of 150 meters (Richardson et al., 1996; Villareal and Carpenter, 1994). Cells that are

deprived of nitrogen are negatively buoyant and sink below the euphotic zone into waters that are relatively high in nutrients. After taking up nutrients, the cells become positively buoyant and move up into the euphotic zone and resume photosynthesis.

Freshwater environment

Many planktonic diatoms have regular annual fluctuations in growth that can be attributed to environmental conditions. Asterionella formosa is one of these diatoms (Fig. 17.16), a common freshwater planktonic diatom that forms large spring growths and smaller autumn ones (Fig. 17.34) (Lund, 1949, 1950). During the winter months, light and temperature limit the growth of the diatoms; lower temperature affects respiration more than photosynthesis so that the compensation point is lowered, and the possibility that photosynthesis will exceed respiration is increased. With the beginning of spring and consequent increase in temperature and light, there is a rapid growth of cells until the middle of spring, when another factor becomes limiting – this time the concentration of dissolved silica in the water; the growth of diatoms then results in much of the dissolved silica being incorporated into the siliceous frustules. Once the concentration of dissolved silica reaches 0.5 mg liter 1, most diatoms cease growth. Because the silica is now limiting, the number of diatoms falls off rapidly and increase only to a lesser degree in the fall when more

394 CHLOROPLAST E.R.: EVOLUTION OF TWO MEMBRANES

Fig. 17.34 (a) Seasonal distribution of live cells of

Asterionella in Lake Windermere, England, plotted on a logarithmic scale in the 0- to 5-m water column. The horizontal axis represents the months of 1947. (b) Numbers of Asterionella formosa (solid line) and the concentrations of dissolved silica and nitrate-nitrogen in mg liter 1 in the 0- to 5-m water column in Esthwaite Water, England. The white line represents the 0.5 mg liter 1 concentration of silica necessary for growth of diatoms. ((a) after Lund, 1949;

(b) after Lund, 1950.)

dissolved silica is available from the breakdown of the frustules of the spring population and the inflow of water containing silica from streams. There are other factors, such as grazing by invertebrates and fungal parasitism, that also check the growth of Asterionella, but they are only of secondary significance.

Enrichment of streams by run off from agricultural land, or input from industry, results in the diatom community being dominated by the “agricultural guild” (Fig. 17.35) (Richardson et al., 1996). The appearance of these diatom species in water samples is an indication of the deterioration of water quality.

The diatoms also make up a large part of the periphyton or the attached algae in freshwaters. These attached diatoms in streams have two opposing factors acting on their growth (Reisen and Spencer, 1970): (1) any increase in current retards the attachment of the diatoms to the substrate, and (2) the increase in current results in

an increase in the growth of the diatoms. This leads to the observation that the diatoms growing in fast-flowing streams usually have a higher total standing crop in the long term than those in slow-flowing streams, although initially the fastflowing water results in a lower number of diatoms being attached to the substrate. The diatoms in the fast-flowing water are, however, able to grow much faster and soon grow so rapidly that they overcome the disadvantage caused by low initial attachment numbers.

Attached diatoms in standing waters have good growth in spring, as do the planktonic diatoms, but do not show as marked a decrease in growth when the concentration of silica in water reaches 0.5 mg liter 1 or less. This can be seen by the growth of the diatom Tabellaria flocculosa (Fig. 17.33(e), (f )) on stems of the reeds Phragmites communis and Schoenoplectus lacustris (Fig. 17.36) (Knudson, 1957). In the May–June period, when the dissolved silica in the water is usually less than 0.5 mg liter 1, there is a decrease in the number of diatoms, but not as great as that of planktonic diatoms, probably because of leaching of silica from the stems of the host plants. A second difference between planktonic and epiphytic diatom seasonal growth is that in the epiphytic diatoms, maximum growth normally occurs in the winter, possibly owing to the secretion by the host of some organic products that are used by epiphytic diatoms.