These organisms are important members of the plankton in both fresh and marine waters, although a much greater variety of forms is found in marine members. Generally the Dinophyceae are less important in the colder polar waters than in warmer waters. The highly elaborate Dinophysales (Fig. 7.47(d), (e)) are essentially a tropical group.

A typical motile dinoflagellate (Figs. 7.1, 7.2) consists of an epicone and hypocone divided by the transverse girdle or cingulum. The epicone and hypocone are normally divided into a number of thecal plates, the exact number and arrangement of which are characteristic of the particular genus (Figs. 7.1, 7.3, 7.20(b), 7.24(b)). There is a longitudinal sulcus running perpendicular to the girdle. The longitudinal and transverse flagella emerge through the thecal plates in the area where the girdle and sulcus meet. The longitudinal flagellum projects out from the cell, whereas the transverse flagellum is wave-like and is closely appressed to the girdle. The cells can be photosynthetic or colorless and heterotrophic. Photosynthetic organisms have chloroplasts surrounded by one membrane of chloroplast E.R., which is not continuous with the outer membrane of the nuclear envelope. Chlorophylls a and c 2 are present in the chloroplasts, with peridinin and neoperidinin being the main carotenoids. About half the Dinophyceae that have been examined by electron microscopy have pyrenoids in the chloroplasts (Dodge and Crawford, 1970). The storage product is starch, similar to the starch of higher plants (Vogel and Meeuse, 1968), which is found in the cytoplasm. An eyespot may be present. The nucleus has permanently condensed chromosomes and is called a dinokaryotic or mesokaryotic nucleus.

Cell Structure

Theca

The thecal structure of motile Dinophyceae consists of an outer plasmalemma beneath which lies a single layer of flattened vesicles (Figs. 7.2, 7.3(c), 7.5) (Dodge and Crawford, 1970 ; Sekida et al., 2004). These vesicles, which normally contain cellulosic plates, give the theca its characteristic structure. The actual form and arrangement of the thecal plates varies from none in the phagotrophic Oxyrrhis marina, to very thick plates with flanges at the edges in Ceratium (Figs. 7.11, 7.48, 7.49) and Peridinium spp. (Figs. 7.2, 7.10).

It was that eccentric British soldier of fortune Col. Meinertzhagen, in his Birds of Arabia, who expressed the sentiment that prefaces should be kept short because few people ever read them. Accordingly, I would like to take a brief opportunity to express my gratitude to the people who offered encouragement and assistance during the preparation of this book. I would like to thank Adele Strauss Wolbarst, Robert Cnoops, Charmaine Slack, Sophia Skiordis, Caroline Mondel, Jill Keetley- Smith, Heather Edwards, Gail Arbeter, and the Lending Library at Boston Spa, England, for help while most of this manuscript was being prepared at the University of the Witwatersrand. For general encouragement while at Pahlavi (Shiraz) University and for providing assistance during the last turbulent and chaotic year of imperial rule in Iran, while the manuscript was being finished, I would like to thank Mark Gettner, Brian Coad, and Mumtaz Bokhari. When photographs or drawings have been taken directly from the original material, this is indicated by stating in the legend that it is from the original work. Most of the drawings have been redrawn to suit my tastes, and these drawings are indicated by stating that the work is after the original.

In some cases I have made drawings from photographs or have incorporated a number of drawings in one, in which case I state that the finished drawing is adapted from the original work or works. I have used the metric system in this book, and the fine-structural illustrations are expressed in micrometers (μm) and nanometers (nm).

Eustimatophytes are yellow-green unicells that occur in freshwater, brackish water, and seawater as well as in the soil. The cells are similar to those in the Xanthophyceae, but differ in having an eyespot outside the chloroplast (Fig. 12.1) (the eyespot in the Xanthophyceae is in the chloroplast) (Hibberd and Leedale, 1970). Other characteristics of the class include a basal swelling of the tinsel flagellum adjacent to the eyespot, only chlorophyll a, chloroplasts without girdle lamellae and no peripheral ring of DNA, and chloroplast endoplasmic reticulum not connected to the nuclear envelope (Schnepf et al., 1996).

The eyespot (Figs. 12.1, 12.2) is a large orangered body at the anterior of the motile cell and is completely independent of the chloroplast. It consists of an irregular group of droplets with no membrane around the whole complex of droplets. The flagellar sheath is extended to form a T-shaped flagellar swelling at the base of the tinsel flagellum (Figs. 12.1, 12.2). This swelling is always closely appressed to the plasmalemma in the region of the eyespot. In turn, in the eyespot there is a large droplet closely applied to the plasmalemma in the area of the flagellar swelling.

The chloroplasts of the Eustigmatophyceae have chlorophyll a and β-carotene, with the two major xanthophylls being violaxanthin and vaucheriaxanthin (Whittle and Casselton, 1969 ; Antia and Cheng, 1982), the only difference in pigments compared to the Xanthophyceae being the presence of violaxanthin and the absence of antheraxanthin. Violaxanthin is the major lightharvesting pigment in the Eustigmatophyceae (Owens et al., 1987).

The Eustigmatophyceae is a monophyletic group (Andersen et al., 1998). Most of the species produce zoospores with only a single emergent flagellum (Pleurochloris magna, Fig. 12.1(d) ; Polyhedriella helvetica, Fig. 12.1(b))(Hibberd and Leedale, 1972), but there is a second basal body present, indicating that the cells had a biflagellate ancestor. The emergent flagellum is tinsel with microtubular hairs, and the flagellum is inserted subapically. Two of the algae in the class, Ellipsoidion acuminatum and Pseudocharaciopsis texensis (Fig. 12.2) (Lee and Bold, 1973), have zoospores with a long forward tinsel flagellum and a short posteriorly directed smooth flagellum.

It is possible to write whole books on the relationships between algae and the environment. In this chapter I have chosen a few subjects that have generated the most interest in the past couple of decades.

Toxic Algae

Algae can be harmful in two basic ways (Hallegraeff et al., 2003 ; Lassus et al., 2016).

The Rhodophyta (red algae) and Chlorophyta (green algae) form a natural group of algae in that they have chloroplasts surrounded by only the two membranes of the chloroplast envelope. The endosymbiotic theory of chloroplast evolution, first proposed by Mereschkowsky in 1905, is the one most widely accepted for the evolution of the chloroplast (Fig. III.1). According to this theory, a cyanobacterium was taken up by a phagocytic organism into a food vesicle. Normally the cyanobacterium would be digested by the flagellate, but by chance a mutation occurred, with the flagellate being unable to digest the cyanobacterium. This was probably a beneficial mutation because the cyanobacterium, by virtue of its lack of feedback inhibition, secreted considerable amounts of metabolites to the host flagellate. The flagellate in turn gave the cyanobacterium a protected environment, and the composite organism was probably able to live in an ecological niche where there were no photosynthetic organisms (i.e., a slightly acid body of water where free-living cyanobacteria do not grow; see Chapter 2). Pascher (1914) coined terms for this association; he called the endosymbiotic cyanobacteria cyanelles; the host, a cyanome; and the association between the two, a syncyanosis. In the original syncyanosis, the cyanelle had a wall around it. Because the wall slowed the transfer of compounds from the cyanelle to the host and vice versa, any mutation that resulted in a loss of wall would have been beneficial and selected for in evolution. As evolution progressed, these two membranes became the chloroplast envelope, the cyanome cytoplasm took over the formation of the storage product and the polyhedral bodies containing ribulose-1,5-bisphosphate carboxylase/oxygenase differentiated into the pyrenoid.

Most of the genes from the endosymbiotic cyanobacterium were transferred to the host nucleus while a small number of these genes were maintained in the resulting plastid and gave rise to the plastid genome with its associated proteinsynthesizing system. The products of many of the cyanobacterial genes transferred to the nucleus were then retargeted to the plastid to keep it functional. Approximately 3000 nuclear genes in plants encode plastid proteins, whereas the chloroplast genome contains between 100 and 120 genes. The nucleus is also capable of sensing the state of the chloroplast and to react to maintain chloroplast homeostasis.

Algae with two membranes of chloroplast endoplasmic reticulum (chloroplast E.R.) have the inner membrane of chloroplast E.R. surrounding the chloroplast envelope. The outer membrane of chloroplast E.R. is continuous with the outer membrane of the nuclear envelope and has ribosomes on the outer surface (Fig. V.1).

The algae with two membranes of chloroplast E.R. evolved by a secondary endosymbiosis (Fig. V.1) (Lee, 1977 ; Keeling, 2013) when a phagocytic protozoan took up a eukaryotic photosynthetic alga into a food vesicle. Instead of being phagocytosed by the protozoan, the photosynthetic alga became established as an endosymbiont within the food vesicle of the protozoan. The endosymbiotic photosynthetic alga benefited from the acidic environment in the food vesicle that kept much of the inorganic carbon in the form of carbon dioxide, the form needed by ribulose bisphosphate/carboxylase for carbon fixation (see Part IV for further explanation). The host benefited by receiving some of the photosynthate from the endosymbiotic alga. The food vesicle membrane eventually fused with the endoplasmic reticulum of the host protozoan, resulting in ribosomes on the outer surface of this membrane, which became the outer membrane of the chloroplast E.R. Through evolution, ATP production and other functions of the endosymbiont's mitochondrion were taken over by the mitochondria of the protozoan host, and the mitochondria of the endosymbiont were lost. The resulting cytology is characteristic of the extant algae in the Chlorarachniophyta and Cryptophyta, which have a nucleomorph representing the degraded endosymbiotic nucleus, as well as storage product produced in what remains of the endosymbiont cytoplasm.

The type of chloroplast E.R. that exists in the Heterokontophyta and the Prymnesiophyta resulted from further reduction. The nucleomorph was completely lost and storage product formation was taken over by the host. The resulting cell had two membranes of chloroplast envelope surrounding the chloroplast. Outside of this was the inner membrane of chloroplast E.R. that was the remains of the plasma membrane of the endosymbiont. Outside of this was the outer membrane of chloroplast E.R. which was the remains of the food vesicle membrane of the host. Most of the protein synthesis of the endosymbiont was taken over by the nucleus of the host (Martin, 2010).

The Synurophyceae are closely related to the Chrysophyceae (Ariztia et al., 1991). The Synurophyceae differ, however, from the Chrysophyceae in the following: the Synurophyceae have chlorophylls a and c 1 (Andersen and Mulkey, 1983), the flagella are inserted into the cell approximately parallel to one another (Fig. 11.1), there is a photoreceptor near the base of each flagellum, there is no eyespot, and the contractile vacuole is in the posterior portion of the cell (Lavau et al., 1997 ; Andersen et al., 1999). Chloroplast endoplasmic reticulum is present in a few species, but absent in most. The cells usually are covered by bilaterally symmetrical scales.

In the Synurophyceae, scales composed of silica commonly occur outside the cell (Figs. 11.1, 11.2, 11.3). The scales are bilaterally symmetrical and are formed in a silica deposition vesicle. The membrane of the silica deposition vesicle (the silicalemma) controls the shape of the scale along with proteins and glycoproteins that adhere the developing scale to the silicalemma (Schultz et al., 2001). The presence of germanium in the medium results in inhibition of scale formation (Klaveness and Guillard, 1975). The scales are carried in the scale vesicle to the plasma membrane where the plasma membrane and the scale vesicle fuse, releasing the scales outside the cell (Beech et al., 1990). The scales are held next to the cell in an organic envelope (Ludwig et al., 1996), which is either hyaline or yellow-brown, the latter appearance being due to the impregnation of iron salts. The scales of the Synurophyceae are commonly composed of a number of parts, such as the dome, shield, and bristle of Mallomonas (Lavau and Wetherbee, 1994) (Figs. 11.2, 11.3(c), (d)). The scales of the Synurophyceae are overlapped precisely so that the anterior end of one scale overlaps the right margin of the scale to its left (Leadbeater, 1990). The scales are cemented together to form a scale case by the organic envelope. This precise arrangement of scales differs from the loosely arranged scales of the Chrysophyceae.

Analysis of lake sediments often reveals the presence of the silicified scales of the Synurophyceae as well as the silicified frustules of diatoms (Smol et al., 1984 ; Dixit et al., 1999).