Toxic Diatoms
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CHAPTER 12 Toxic Diatoms VERA L. TRAINER, BARBARA M. HICKEY, AND STEPHEN S. BATES INTRODUCTION based on biomass and numbers of species; there are esti- mated to be greater than 50,000 species in the aquatic eco- A tragic event in 1987 changed forever the way diatoms system, and up to 18,000 in the marine environment (Fryxell are viewed. Most diatoms are benefi cial to the oceans’ eco- and Hasle, 2003). systems and ultimately to human health. However, a unique Some diatoms are found as single cells; others form long food poisoning in eastern Canada led to the illness of more “chains” by linking to the adjacent cell, either by abutting than 100 people and the death of four elderly individuals. end to end or by joining their protruding spines or setae. The potent neurotoxin domoic acid (DA) was found to be Like all plants, diatoms contain chlorophyll and other pig- the culprit, and its source was the marine diatom Pseudo- ments that capture the energy of sunlight to convert carbon nitzschia multiseries. This event resulted in intense scientifi c dioxide and water molecules into carbohydrates via photo- interest in this genus, because it was the fi rst time that a synthesis. Their survival also requires nutrients such as diatom had been shown to produce a toxin. The clinical nitrogen, phosphorus, silicon, and trace metals. Diatoms are syndrome was subsequently called amnesic shellfi sh poison- a major source of the world’s oxygen, producing amounts ing (ASP) because of one of its reported symptoms, memory equivalent to all the tropical rain forests (Field et al., 1998). loss. Here, we introduce the general biology of diatoms and They are also responsible for 40% to 45% of the total pro- focus specifi cally on toxic diatoms of the genus Pseudo- duction of organic carbon compounds in the oceans (Mann, nitzschia. We also discuss DA and its mode of action, 1999), or 20% to 25% for the entire Earth. As such, they are describe oceanographic factors that lead to the growth of this a signifi cant source of food for other organisms within the toxic diatom, and include specifi c examples of Pseudo- marine food web and play an important role in the biogeo- nitzschia blooms and related physiology. chemical cycling of elements (Sarthou et al., 2005). Because of their large cell size and rapid sinking rate, they also play an important role as part of the “biological pump” that GENERAL BIOLOGY OF DIATOMS removes carbon dioxide from the atmosphere and places it at depth, thus helping to moderate the increasing levels of Diatoms are microscopic single-celled plants that live in carbon dioxide associated with climate change. freshwater and marine ecosystems, from the poles to the The name diatom comes from the Greek word diatomos tropics. They are found in sea ice and snow; some are even (dia = “through” + temnein = “to cut”), meaning “cut in blown into the air. Benthic diatoms live in or on the sedi- two.” This is because a diatom cell is composed of two ments and on the surface of other plants (epiphytic), animals overlapping halves (thecae) that fi t together like a petri dish: (epizootic), or rocks (epilithic). Those that live in the water the upper half is called the “epitheca” and the lower half the column, together with the dinofl agellates (Chapter 13), other “hypotheca” (Fig. 12-1). The theca is composed of an upper fl agellates, and blue-green algae (cyanobacteria) (Chapter (epi-) or lower (hypo-) valve, plus girdle bands that encircle 15), among others, make up the phytoplankton (Greek the middle of the cell to hold the two halves together. Each phyton = “plant,” planktos = “wanderer”), also called micro- valve possesses a slit, called a “raphe,” that runs along the algae. Diatoms are the most abundant of the phytoplankton whole length of the valve and is reinforced on the interior C Copyright © 2008 by Academic Press. Oceans and Human Health 219 All rights of reproduction in any form reserved. Ch012-P372584.indd 219 12/7/2007 1:53:59 PM 220 Oceans and Human Health Epivalve Raphe canal Fibulae Epitheca Girdle bands Hypotheca Poroids Interstriae Raphe canal Hypovalve Hypovalve FIGURE 12-1. Silica frustules of Pseudo-nitzschia multiseries shown diagrammatically as one cell overlapping part of another, with cell length much reduced for illustration purposes. Raphe is shown on edge of upper valve (epivalve) and opposite corner of lower valve (hypovalve). Section of epitheca removed to show raphe canal, interstriae, three to four rows of poroids in the stria areas, and several poroids on the inner side of girdle bands that encircle the frustule. Diagram at left shows cross-section of two overlapping cells, with raphe canal in diagonally opposite position. Re- drawn from MacPhee et al. (1992). side with bridges, called “fi bulae.” The valves are orna- they are no longer able to divide, and they die. This occurs mented with striae (strips containing rows of small poroids), in clonal cultures (i.e., started with one cell) of Pseudo- alternating with narrower riblike strips called “interstriae.” nitzschia and other pennate diatoms. However, cells of many The entire diatom shell is called the “frustule” and is com- sizes, including large cells, are found in the ocean; most posed of silica (SiO2 = glass) that is covered by an organic diatoms are able to restore their large size via sexual repro- membrane through which nutrients pass for cell growth. duction. In centric diatoms, the same cell can produce both Based on how the silica ribs on the valve radiate, diatoms small fl agellated male gametes plus large nonmotile female (division Heterokontophyta, class Bacillariophyceae) are gametes (i.e., they are homothallic and oogamous). placed into two orders: radially symmetric (round) cells are In pennate diatoms, clones of opposite mating type are called centric diatoms and longitudinally symmetric (long, needed (i.e., they are heterothalic), and they produce non- narrow) cells are pennate (from the Latin penna, meaning fl agellated gametes of the same size (i.e., they are isoga- “feather”) diatoms. Pseudo-nitzschia species are pennate mous). Thus, mating will not occur in a clonal culture; a diatoms. clone of opposite mating type must be added. In both types Centric diatoms have fl agellated gametes but are other- of diatoms, the gametes fuse, and an oval (in centrics) or wise swept passively by currents. Pennate diatoms are elongated (in pennates) structure, called an auxospore, is capable of limited motility. With Pseudo-nitzschia, single produced. A large initial cell is formed within the fully cells, or an entire chain of cells, can be seen gliding across expanded auxospore and is eventually released, thus restor- a solid surface (such as a microscope slide), sometimes stop- ing the large cell size of the diatom cell. The sexual repro- ping and then reversing direction. The role of this motility duction of Pseudo-nitzschia species can affect bloom in Pseudo-nitzschia is uncertain because these cells are dynamics (population growth) and perhaps even toxicity found mostly suspended in the water column. Motility is (e.g., Davidovich and Bates, 1998). For a more detailed facilitated by a raphe, the longitudinal slit that runs along an description of the evolution, morphology, sexuality, system- edge of the valve face (Fig. 12-1). Mucus is secreted through atics, taxonomy, and biology of diatoms, see Round et al. the raphe and attaches to particles or substrata; as the diatoms (1990) and Hasle and Syvertsen (1997). move, they leave a trail of this sticky substance. The details Because they are abundant, geographically widespread, of what causes the motive force are still somewhat mysteri- live in a wide variety of environments, and are generally ous, but fi laments of the protein actin are thought to change well preserved, modern and fossilized diatoms have many shape, thus moving the cell along. uses by human society (Stoermer and Smol, 1999). Fossil- Diatoms encounter a serious problem because their frus- ized diatom frustules form diatomaceous earth, also called tule is composed of solid silica and the hypotheca is slightly diatomite, which is commonly used as a fi lter medium in smaller than the epitheca. During vegetative cell division, swimming pools and aquaria, a mild abrasive in metal the daughter cell formed on the side of the hypotheca is thus polishes and toothpaste, cat litter, insulating material, slightly smaller than the parent cell. As cell division contin- chromatographic separation material such as silica gel, a ues, the average cell size of the population therefore mechanical insecticide (it works by cutting, and thus dehy- C decreases. Eventually, the cells can become so small that drating the insects), an absorbent for toxic spills, and in the Ch012-P372584.indd 220 1/7/2008 3:10:58 PM Toxic Diatoms 221 manufacture of dynamite, among other uses. Fossilized and are discussed elsewhere (Fryxell and Hasle, 2003). diatoms are also used in paleolimnology and paleoceanog- However, a few (<20) diatom species cause harm by produc- raphy to reconstruct past climate changes. Modern diatoms ing toxins that affect molluscan shellfi sh, marine mammals, are used as an indicator of eutrophication (i.e., the enrich- birds, and even humans. Pseudo-nitzschia was the fi rst ment of water by nutrients, especially nitrogen and phospho- diatom genus known to produce a compound that is toxic to rus, causing an accelerated growth of algae and leading to humans and other animals. This naturally occurring toxin is undesirable disturbances in the balance of organisms and in called domoic acid (DA), and the syndrome of DA poisoning water quality) and acidifi cation (i.e., the negative conse- in humans is called ASP. The remainder of this chapter will quences of acid rain).