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Home , Phacus, Trachelomonas, Trichocyst

Euglenophyta • In euglenophytes, a single class, Euglenophyceae, is recognized. Euglenophyta and Dinophyta • It includes about 800 , most of which are freshwater species, but some are estuarine and intertidal. • Predominantly unicellular, . • Only a third of the species have , and the rest are heterotrophic (both facultative and obligate). • The ability of to grow heterotrophically in the absence of is unique amongst the . • Autotrophic euglenophytes contain chlorophylls a and b and, thus, have been related to Chlorophyta, but this is the only shared character. • Carotenoid include β-carotene and diadinoxanthin, other xanthophylls Algae with one membrane of less prominent. • The energy storage product is paramylon, a polysaccharide found endoplasmic outside the chloroplast in the cytoplasm. Chrysolaminarin can also accumulate in a liquid form in vacuoles in the cytoplasm. reticulum

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• The shows an anterior bottle-shaped invagination called the reservoir (R) . At the base Eyespot, paraflagellar swelling, and phototaxis of the reservoir, there is a contractile vesicle • The eyespot (stigma) (E) is a collection (CV) as well as two flagella, but only one of them of orange-red lipid droplets, independent emerges through the neck. of the chloroplast. • Euglenoid cells have two basal bodies and one or two emergent flagella. The flagella having a • The eyespot is in the anterior part of the paraxonemal rod (paraxial rod) that runs the cell, curving to ensheath the neck of the length of the inside the flagellar reservoir on the dorsal side. membrane. • The eyespot has been reported to contain • The one emergent flagellum in Euglena has α-carotene and seven xanthophylls, helically arranged fibrillar hairs (no ) mainly β-carotene, or a β-carotene attached along the length of the flagellar derivative, echineone. membrane. • is asexual, sexual reproduction • The base of the longer flagellum bears a has not yet been observed. thickening (P), believed to be a • In order to survive when external conditions are photoreceptor, close to the stigma. unfavorable, some species produce resistant • The swelling is composed of a crystalline cysts surrounded by a thick mucilaginous sheath body next to the axoneme and inside the that they produce. flagellar membrane. • All euglenoid species with an eyespot and flagellar swelling exhibit phototaxis, A semidiagrammatic drawing of the fine structure of the usually swimming away from bright light anterior part of a Euglena cell. (C) Canal; (CER) (negative phototaxis) and away from chloroplast endoplasmic reticulum; (CV) contractile darkness toward subdued light (positive vacuole; (E) eyespot; (LF) long flagellum; (M) photoaxis) to accumulate in a region of mastigonemes; (MB) muciferous body; (Mt) low light intensity. A semidiagrammatic drawing of the fine structure of the microtubules; (N) nucleus; (P) paraflagellar swelling; anterior part of a Euglena cell. (E) eyespot; (LF) long (Pa) paramylon; (PG) pellicle groove; (Pl) ٣ ٤ flagellum; (P) paraflagellar swelling; (R) reservoir; (SF) plasmalemma; (PS) pellicle strip; (Py) ; (R) short flagellum. (A) Axoneme of flagellum; (B) paraxonemal rod of flagellum reservoir; (SF) short flagellum.

•The strips are arranged in parallel, are characteristic of the species. • Euglenoid cells are not surrounded by , but a pellicle that containing structural protein. • The pellicle has four main components: – the plasma membrane, – repeating proteinaceous units called strips, – subtending microtubules, and – tubular cisternae of endoplasmic reticulum. Euglena showing pellicle

Scanning electron micrographs showing metaboly in different : (B) Distigma, (C) Eutreptia and (D) Euglena

Scanning electron micrographs of euglenoids showing the helical pellicle of triqueter (lower left), Euglena acus (middle), and Lepocincilis ovata (upper right). ٥ ٦ •The locomotory flagellum or flagella emerge from an anterior invagination of the cell, which consists of a narrow tubular rigid portion, the canal, and a spherical or pyriform easily changes shape chamber, the reservoir. • The pellicle lines the canal but not the reservoir, the reservoir being • There are two basic types of flagellar movement in the class. the only part of the cell covered solely by the plasmalemma. • The first group (including the Eutreptiales and Euglenales) has the •The , in the anterior part of the cell next to the flagellum continually motile from base to apex, resulting in cell reservoir, has an osmoregulatory function, expelling excessive water gyration with the anterior end of the cell tracing a wide circle. taken into the cell. • The second group (including Peranema, Entosiphon, and • It empties into the reservoir, from which the water is carried out Sphenomonas) has the flagellum held out straight in front of the cell through the canal. with just the tip motile, resulting in smooth swimming or gliding locomotion in contact with the substratum or air–water interface.

• Mitochondria are of typical algal type. Colorless euglenoids always have more mitochondria than do equivalent-sized green ones. ٧ ٨

• Euglena gracilis changes its shape two times per day when grown under the synchronizing effect of a daily light–dark cycle. • Muciferous bodies (MB) (or mucocysts) occur under the pellicle strips and – At the beginning of the light period, when photosynthetic capacity is low contain a water-soluble mucopolysaccharide. The muciferous bodies open to (as measured by the ability of the cells to evolve oxygen), the population the outside through pores between the strips of the pellicle. The muciferous of cells is largely spherical. bodies function in the formation of the stalk in Colacium, formation in , cyst formation and lubrication during euglenoid movement. – The mean cell length of the population increases to a maximum in the middle of the light period when photosynthetic capacity is greatest, and then decreases for the remainder of the 24-hour period. – The population becomes spherical by the end of the 24-hour period when the cycle reinitiates.

• These changes are also observed under dim light conditions and are therefore controlled by a biological clock and represent a circadian rhythm in cell shape.

• Some euglenoids have a flexible pellicle that allows the cells to undergo a flowing movement known as euglenoid movement. This type of movement occurs only when the cells are not swimming and results from the movement of the pellicle strips.

٩ ١٠ Euglena gracilis. (a)–(d). Sequence of shape changes photographed at 5-second intervals of a cell undergoing euglenoid movements.

Surviving unfavorable periods. • The cell rounds off and secretes a thick sheath of that Nucleus and nuclear division survives for months until the cell emerges by cracking the cyst. • The euglenoid nucleus is of the mesokaryotic • In conditions of partial desiccation or excessive light, the slime sheath type, having sometimes acts as a temporary cyst, cells emerging from the sheath – chromosomes that are permanently condensed as soon as conditions improve. during the mitotic cycle, – a nucleolus (endosome) that does not disperse • In certain genera (Euglena and Eutreptia), within the slime during nuclear division, layer leads to the formation of a palmelloid colony, which may form – no microtubules from chromosomes to pole extensive sheets of cells covering many square feet of mud surface. spindles, and – a nuclear envelope that is intact during nuclear division. • The chromosome number is usually high, and polyploidy probably occurs in some genera. • in euglenoids begins during early prophase with the nucleus migrating from the center of the cell to an anterior position. • Microtubules appear in the nucleus, but they do not attach to the chromosomes. • At metaphase, bundles of microtubules are among the chromosomes, and the nucleolus has started to elongate along the division axis. Scanning electron micrographs of the encystment of Eutreptiella gymnastica. The vegetative cell (a) • In anaphase, the intact nuclear envelope A semidiagrammatic drawing of an loses its flagella, forms a large number of paramylon grains, and begins to round up (b). The cell swells elongates along the division axis, the nucleolus anaphase nucleus of Euglena with and produces a mucilaginous covering (c). divides, and the daughter chromosomes disperse nuclear envelope intact, the nucleolus into the two daughter nuclei. pinching in two, and the chromosomes .not attached to spindle microtubules ١١ ١٢ • Paramylon granules which are often visible as colorless Chloroplasts and storage products or white particles in light microscopy are composed • The euglenoid chloroplasts are of a carbohydrate that although isomeric with surrounded by two membranes of the starch (amylon), was not chloroplast envelope plus one membrane stained with iodine. For this of chloroplast endoplasmic reticulum; the reason, they were termed latter membrane is not continuous with the paramylon granules. nuclear membrane. • They have since been • The chloroplasts are usually discoid or shown to be composed of a plate-like with a central pyrenoid. β-1,3 linked glucan. • The thylakoids are grouped into bands of • The liquid storage product, three, with two thylakoid bands traversing chrysolaminarin, can be an alternative storage product the pyrenoid. in some Euglenophyceae. • A shield of paramylon grains surrounds the pyrenoid, but outside the chloroplast, in phototropically grown cells. • Paramylon granules are distributed (a) Euglena gracilis. (b) Astasia klebsii. (c) Eutreptiella throughout the cytoplasm in marina. (d) Trachelomonas grandis. (e) Phacus triqueter. heterotrophically grown cells in the dark. (C) Chloroplast; (Ca) canal; (CV) contractile vacuole; (E) • Their shape is often characteristic of the eyespot; (Ev) envelope; (F) emergent flagellum; (FS) Euglena species that produces them. flagellar swelling; (M) mitochondrion; (N) nucleus; (P) Euglena rustica. Transmission electron micrograph paramylon grains or paramylon sheath around of a mid sagittal section of a cell. (Arrow) Paramylon ١٣ ١٤ chloroplast; (R) reservoir. granule; (cp) chloroplast; (Py) pyrenoid; (Nu) nucleus

Nutrition importance of Paramylon • Paramylon belongs to a group of naturally occurring polysaccharides which • The Euglenophyceae have a number of modes of nutrition, are considered bioactive compounds. • The main interest in β-glucans stems from their ability to act as nonspecific depending on the species involved. immune system stimulants, by binding to a specific site on • No euglenoid has yet been demonstrated to be fully monocytes/macrophages and granulocytes. photoautotrophic – capable of living on a medium devoid of all • They have been successfully used in aquaculture to strengthen the nonspecific defense of many important species of fishes and shrimps by organic compounds (including vitamins). injection, immersion, or in the feed. • All green euglenoid so far studied are photoauxotrophic • Sulfated derivatives of Euglena paramylon, in particular, have shown anti- –needing at least one vitamin. Euglena gracilis has an absolute HIV (human immunodeficiency virus) activity. • Moreover, it has been suggested that this polysaccharide has a cholesterol- requirement for vitamin B12. lowering effect when incorporated in the diet of either humans or animals, and moderates the postprandial blood glucose and insulin response in humans. • Vitamin B12-starved cells increase in cell volume, sometimes to • Since Euglena gracilis can accumulate large quantities of paramylon when 10 times the size of control organisms, the cells in the final stage of grown in the presence of an utilizable carbon source, it could represent an alternative source of this compound to Saccharomyces cerevisiae (baker’s vitamin B12 starvation often being poly-lobed, poly-nucleate, and yeast), which is currently exploited industrially for its extraction. containing more than the normal number of chloroplasts per cell. • Moreover, Euglena has been investigated as a potential protein source, and • During vitamin B12 starvation, total cellular RNA and protein a promising dietary supplement due to its content of highly nutritious proteins and PUFAs, and to the simultaneous production of antioxidant increase 400% to 500% compared with controls. compounds, such as β-carotene, vitamin C, and vitamin E. • During vitamin B12 starvation the protein increase 400% to 500% and the total DNA increases about 180%. ١٥ ١٦

Classification • Three orders of euglenoids are presented here. • Order 1 Heteronematales: two emergent flagella, the longer flagellum directed anteriorly and the shorter one directed posteriorly during swimming; cells have special ingestion . •As Euglena cells age, they become immobile and spherical, with a tendency – Peranema trichophorum is a euglenoid that ingests other cells and to form enlarged “giant” cells and to accumulate orange to black pigment detritus. Here the colorless cells have a special ingestion organelle, and bodies. are phagocytic, taking up food particles whole and digesting them in • Aging also results in the formation of larger numbers of lysosomes and food vesicles. microbodies with an increase in the degradation of . • The older cells undergo a shift from carbohydrate to fat oxidation.

• The euglenoids belong to the osmotrophic acetate flagellates, having the ability to grow heterotrophically in the dark by utilizing substrates such as acetic (CH3COOH) and butryic acids (CH3CH2CH2-COOH) and the corresponding alcohols (e.g., ethanol). The two most commonly used substrates are acetate and ethanol.

Peranema trichophorum. (a) General cell structure. (b),(c) Two stages in the ingestion of a cell of Euglena (stippled cell). (c) Canal; (cv) contractile vacuoles; (cy) rim of cytosome; (fv) food vesicle; ١٧ ;g) Golgi; (ir) ingestion rods; (lf) leading flagellum; (m) mitochondrion; (n) nucleus; (p) paramylon) ١٨ (tf) trailing flagellum • Order 3 Euglenales: two flagella, only one of which emerges from • Order 2 Eutreptiales: two emergent flagella, one directed anteriorly the canal; no special ingestion organelle. and the other laterally or posteriorly during swimming; no special • Common genera in the order are the green photosynthetic Euglena, ingestion organelle. Trachelomonas, and Phacus, as well as the colorless osmotrophic – Eutreptia and Eutreptiella are estuarine or marine genera, while Astasia. Distigma is characteristic of acid freshwaters. • Colacium libellee is a member of this order that establishes itself in the rectum of damselfly nymphs during the winter in colder lakes. During the warm summer months the damselfly nymphs and C. libellee live separately.

Distigma Eutreptia Eutreptiella

(a) Larva of the damselfly Ischnura verticalis with a plug of Colacium libellee in the rectum. (b), (c) Colony and single ١٩ .swimming cell of Colacium vesiculosum ٢٠

• are unicellular, biflagellate organisms. • The term "" means "whirling flagella". • 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. • Dinoflagellates are less important in the colder polar waters than in warmer waters.

Dinophyta (Pyrrophyta)

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• The exact number and arrangement of thecal plates in armored • Dinoflagellates are surrounded by a complex covering called dinoflagellates are characteristic of the particular , therefore the amphiesma, which consists of the plasma membrane and an they are used for taxonomic differentiation. underlying single layer of flattened vesicles (amphiesmal vesicles = • Also, the form of the thecal plates varies from spherical forms "thecal vesicles"). like Peridinium to elongate horn-like forms such as Ceratium. • Based on their amphiesma, dinoflagellates are separated into two major subgroups –In thecate, or ‘armoured’, forms, the amphiesmal vesicles contain cellulosic plates (= thecal plates), resulting in a rigid and inflexible wall. –In athecate, or ‘unarmoured’ (naked) dinoflagellates, the cells have empty amphiesmal vesicles (without plate-like structures) and may be fragile, easily distorted and difficult to preserve.

(a ventral view of Cachonina (b) Gymnodinium neglectum (b) Dorsal and ventral views showing niei showing the arrangement the thecal plates of Ceratium of thecal plates: • In many dinoflagellates, cell division usually involves sharing of the mother cell thecal plates between the daughter cells, with the daughter cells producing the new thecal plates that they lack. • However, in certain genera the is completely shed (ecdysis) Representative drawings of different types of amphiesmal arrangements in the Dinophyceae. The at cell division, followed by the formation of a thickened pellicle outer plasma membrane (Pl). The cellulosic thecal plate around the cell to form an ecdysal cyst. (T). vesicle (V). ٢٣ ٢٤ AV, amphiesmal (thecal) vesicle; TP, thecal plate within an amphiesmal vesicle; . • A typical motile dinoflagellate consists of an epicone (epitheca) and hypocone (hypotheca) divided by the transverse girdle or cingulum. • In most dinophyceae, outside of the plasma membrane there is a • There is a longitudinal sulcus running perpendicular to the girdle. glycocalyx composed of acidic polysaccharides. • The longitudinal and transverse flagella emerge through the thecal plates in the area where the girdle and sulcus meet. Scales • Although relatively rare, scales occur outside the plasma membrane Flagella in some Dinophyceae • Generally, dinoflagellates have a transverse flagellum that fits into the transverse girdle and a longitudinal flagellum that projects out from the longitudinal sulcus.

Dinoflagellate scales. (a) Oxyrrhis marina. (b) Heterocapsa niei. (c) Katodinium rotundatum.

Scrippsiella trochoidea.Scanning electron micrographs ٢٥ ٢٦ A representation of thecal plates on a showing the thecal plates in ventral (a) and dorsal (b) typical armored dinoflagellate . view.

• The transverse flagellum is thought to be responsible for driving the cell forward and it also brings about rotation, • Fibrillar hairs may cover the entire while the longitudinal flagellum is the propelling and longitudinal flagellum. steering flagellum. • The axoneme of transverse flagellum is a left-handed • The transverse flagellum has a helical screw; waves propagated from the attached end toward shape and attached to the cell body the free end. except for the tip . • The flagellum causes a direct forward movement while at • The transverse flagellum consists of the same time causing the cell to rotate. – (1) an axoneme whose form • The flagellar beat always proceeds counterclockwise when seen from the cell apex. approximates a helix, • The longitudinal flagellum swings in a narrow orbit acting – (2) a striated strand that runs parallel to as a rudder. the longitudinal axis of the axoneme but • The longitudinal flagellum can also reverse the swimming outside the loops of the coil, and direction: it stops beating, points in a different direction – (3) a flagellar sheath that encloses both by bending, and then resumes beating. axoneme and striated strand. • On one side of the axoneme, a unilateral row of fibrillar hairs is attached to the flagellar sheath. • Contraction of the striated strand leads to supercoiling of the axoneme.

Diagram of part of the transverse flagellum of Peridinium cinctum. Only a portion of the fibrillar hairs ٢٧ ٢٨ have been drawn in.

Pusule •A pusule (P) is a sac-like structure that opens by means of a pore into the flagellar canal (Fc) and probably has an osmoregulatory function similar to that of a contractile vacuole.

• The dinoflagellates are one of the fastest swimmers (a) Drawing of Amphidinium carteri. The small epicone is separated from the among the algae hypocone by the encircling girdle, • Marine dinoflagellates frequently move into deeper, which contains the transverse flagellum. The peripheral chloroplast nutrient-rich waters at night and better illuminated waters (C) is shown only at the sides of the near the surface during the day. cell so the positions of the nucleus (N), mitochondria (M), pyrenoid • The diel (over a 24-hour period) migrations of (Py), and the pusule (P) associated dinoflagellates are 5 to 10 m in relatively quiet waters. with the flagellar canals can be seen. (b) Longitudinal section through a pusule showing the flagellar pore constriction (C), the pusule vesicles (V), and the flagellum (F). (c) Transverse section of a pusule showing the flagellum within the flagellar canal (Fc) and the pusule vesicles (V) opening into the canal.

٢٩ ٣٠ Chloroplasts and pigments Phototaxis and eyespots • The cells can be photosynthetic or colorless and heterotrophic. • Less than 5% of the Dinophyceae contain eyespots (e), • Photosynthetic organisms have and those that do are mostly freshwater species; yet the chloroplasts surrounded by one membrane of chloroplast E.R., which is not continuous eyespots are among the most complex in the algae. with the outer membrane of the nuclear • An eyespot is not necessary for a phototactic response. envelope. • Chlorophylls a and c2 are present in the chloroplasts, with peridinin and neoperidinin being the main carotenoids. • About half of the Dinophyceae have in the chloroplasts. • The storage product is starch, similar to the starch of higher plants, which is found in the cytoplasm.

Light and electron microscopical drawings of Peridinium sp., a dinoflagellate showing many of the features of the class. (C) Chromosome; (CE) chloroplast envelope; (a) A ventral view of a cell of Glenodinium foliaceum showing (CER) chloroplast endoplasmic reticulum; (E) epicone; Woloszynskia tenuissima. Ventral view the location of the eyespot (e). (b) Threedimensional diagram (G) Golgi apparatus; (Gr) girdle; (H) hypocone; (L) lipid showing the two flagella, the girdle (g), and the of the eyespot area. The two flagella arise just above the globule; (LF) longitudinal flagellum; (Nu) nucleolus; (P) eyespot (e). lamellar body (l). The eyespot (e) lies beneath the sulcus. (mt) trichocyst pore; (S) starch; (T) trichocyst; (TF) .transverse flagellum; (TP) thecal plate ٣١ .Microtubular roots; (b) banded root; (t) thecal plate ٣٢

Nucleus Trichocysts The nucleus of the dinoflagellata is unusual where is has many odd features unique to the group. • Trichocysts (T) are membrane-bound organelles containing proteinaceous • The DNA does not seem to have histones (basic threadlike structures found in a proteins which the DNA coils around). • Dinoflagellates have more DNA in their nucleus than number of species. other , so much so that the nucleus often • In these species, the cell wall is fills half the volume of the cell .This implies that a perforated by pores (P) through which large amount of the DNA is genetically inactive trichocysts are discharged or (structural DNA) in dinoflagellates. projected from the cell under • Chromosomes in its nucleus are not scattered, but are stimulation, often hundreds per cell. attached to the nuclear membrane. • The actual benefit of trichocysts to the • Upon division, the nuclear membrane does not break cell (if any) is still obscure. down as in plants and animals. – They could be a mechanism for quick • The chromosomes remain condensed during mitosis escape as the cells move sharply in and even during interphase, though they do unwind the opposite direction of discharge, for replication of the DNA. – or they have a protective or evasive • Such permanently condensed organization of function since they could be able to chromatin (chromosomes) is different from either directly “spear” a naked intruder. prokarytoic or eukaryotic cells. A dinoflagellate nucleus. Note the Light and electron microscopical drawings of Peridinium • This unusual nuclear situation is Condensed chromosomes sp., a dinoflagellate showing many of the features of the termed mesokaryotic or dinokaryotic in the belief with characteristic banding pattern Accumulation body class. (C) Chromosome; (CE) chloroplast envelope; (CER) chloroplast endoplasmic reticulum; (E) epicone; that it was transitional between prokaryotic and • This is a large vesicle containing the (G) Golgi apparatus; (Gr) girdle; (H) hypocone; (L) lipid eukaryotic structure, but it is now thought to be a remains of digested organelles. globule; (LF) longitudinal flagellum; (Nu) nucleolus; (P) uniquely derived feature of the Dinogflagellata . trichocyst pore; (S) starch; (T) trichocyst; (TF) ٣٣ .transverse flagellum; (TP) thecal plate ٣٤

Resting spores or cysts or hypnospores and fossil Dinophyceae • The process of encystment or resting spore formation is •The resting spore or cyst of most dinoflagellates is morphologically regulated by a complex interaction of distinct from the parent cell. – day length, • The cell walls of the cysts are highly resistant to decay and contain – temperature, and dinosporin, a chemical similar to sporopollenin in the pollen of – nutrient concentration higher plants. • The cysts are extremely resistant and are preserved in ancient sediments where they are of value in paleoecological studies. • Melatonin levels increase by several orders of magnitude • Many extant resting spores are identical to fossilized cysts. during encystment and may function in preventing oxidation of the lipids in the cyst • Cysts are recognizable because they lack chromatophores and have a microgranular brown cytoplasm and a red eyespot (if the organism normally has an eyespot).

• Calcification of cysts in some genera occurs by the deposition of calcium carbonate crystals in the narrow space between the cell wall and the plasma membrane. • The cysts of Ceratium hirundinella contain an outer silicon layer. Dinoflagellate cysts. (a) Alexandrium catenella. (b)Calciodinellum operosum. (c) Scrippsiella ٣٥ ٣٦ trophoidea. (d) Gonyaulax grindleyi. (e) Polykrikos schwartzii. (f) Gymnodinium catenatum. Toxins have been divided into different classes based on the syndromes associated with exposure to them, such as Toxins 1. Diarrhetic shellfish poisoning. This occurs primarily in temperate regions and is caused by species of the planktonic dinoflagellates Exuviaella, • Some Dinophyceae have the ability to produce very potent toxins Dinophysis, and Prorocentrum. which cause the death of fish and shellfish during red tides when there are dinoflagellate blooms that color the water red. • The dinoflagellates become lodged in the gills of the shellfish, and when shellfish are eaten by humans or animals, poisoning results. • All of the dinoflagellates that have been demonstrated to produce toxins contain chloroplasts, indicating that the ability to produce toxins may have been derived from endosymbiotic cyanobacteria.

Prorocentrum

Fig. 7.30 Scanning electron micrographs of dinoflagellates that cause diarrhetic shellfish poisoning. (a) Dinophysis acuminata. (b) Dinophysis fortii. (c) Prorocentrum lima with the arrows pointing to pores in the theca.

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2 Ciguatera fish poisoning. This occurs primarily in tropical regions with • Diarrhetic shellfish poisoning is caused by the polyether carboxylic acids the common causative agent being Gambierdiscus. okadaic acid, macrolide toxins, and yessotoxin. • The polyether carboxylic acid okadaic acid is initially formed inside the • The dinoflagellate contains gambieric acids, ciguatoxins, and maitotoxins, dinoflagellate cell as dinophysistoxin-4. This weakly’sulfated derivative of that are very potent Ca2+ channel (pathways for calcium entry into cells) okadaic acid is not toxic to the dinoflagellate cell. activators that result in breakdown of the . • Dinophysistoxin-4 is either excreted by the dinoflagellate cell, or is released • Gambierdiscus is epyphytic on macroalgae that are eaten by herbivorous on death of the cell. fish and shellfish, which, in turn, are eaten by humans. In French • Dinophysistoxin- 4 is hydrolyzed in the medium to okadaic acid diol ester, Polynesia alone, approximately 1000 cases of ciguatera fish poisoning which is lipid soluble and can pass through cell membranes. Thus, okadaic are reported every year. acid diol ester can be taken up by cells which further hydrolyze the • The term ciguatera is derived from the Spanish term “cigua” for the turban compound to okadaic acid. shell, which was commonly eaten before the illness developed. • Research has shown that okadaic acid with a free acid moiety is the toxic form of the compound. • The typical course of ciguatera fish poisoning is diarrhea for two days, followed by general weakness for one to two days. • Occasionally the condition is fatal.

turban shell ٣٩ ٤٠

• Alexandrium excavatum is a toxic red-tide dinoflagellate. • 3 Paralytic shellfish poisoning. This is caused by species of • The vegetative cells divide to produce Alexandrium (A. catanella, A. acatenella, A. excavatum, A. motile gametes that subsequently fuse. tamarensis), Pyrodinium bahamense, and Gymnodinium catenatum. • Fusing pairs swim poorly and settle in the • These dinoflagellates produce a group of toxins that are derivatives water. of saxitoxin. • The motile quadriflagellate planozygotes • Saxitoxins are potent neurotoxins acting upon voltage-gated Na+ - swim for a few days before losing their channels, preventing influx of Na+, thereby preventing the flagella and thecal plates, and encyst to generation of an action potential. form resting cysts (hypnospores) that can survive for 5 to 15 years. • The hypnospores contain storage products and have a thick, three-layer wall. • Cyst germination is controlled by a biological clock with a 12 month maturation period. • Each hypnospore undergoes with two cell divisions to produce haploid vegetative cells, completing the life cycle.

Left: Hypnospores (resting spores) of Alexandrium. (a) Light micrograph of a hypnospore surrounded by mucus. ٤١ b) Scanning electron micrograph showing the smooth) ٤٢ surface of a hypnospore. Alexandrium catanella Pyrodinium bahamense Gymnodinium catenatum A number of factors have been suggested as the cause of red tides: 1 High surface-water temperatures: Dinoflagellates favor warm water, and are generally more abundant near the surface. This does not necessarily mean that they occur only in warm seas, because the surface of the sea in normally cool areas may be warmed up during periods of hot, calm weather. • The amount of toxin in cells of Alexandrium is 2 Wind: A strong, offshore wind aids upwelling, whereas a gentle onshore wind relatively low when nutrients are in ready supply. concentrates the bloom near the coast. On the other hand, heavy weather and strong winds disperse the bloom. Storms also result in the death of • The toxin concentration is highest under dinoflagellates and can prevent the development of red tides. conditions of phosphorus deficiency, possibly 3 Light intensity: There is usually a period of bright, sunny, calm weather before outbreaks. because free amino acids (precursors of toxins) accumulate under conditions of phosphorus 4 Nutrients: Red tides usually occur after an upwelling has stopped, but the nutrients brought to the surface do not, themselves, appear to be the direct deficiency. cause of these blooms. It is thought that preceding blooms of may impoverish the water and reduce one or more of the inorganic nutrients to a level favorable for the growth of dinoflagellates (but too low for the diatoms), • Cells of dinoflagellates that produce toxins are and also allow the production of organic nutrients such as vitamin B12, avoided by grazing copepods. which are important for their growth.

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Bioluminescence • Bioluminescence (chemiluminescence), in which energy from an exergonic chemical reaction is transformed into light energy • Many marine, but no freshwater dinoflagellates are capable of bioluminescence. • The Dinophyceae are the main contributors to marine bioluminescence, Dinoflagellates and oil and coal deposits emitting a bluish-green (maximum wavelength at 474 nm) flash of light of • Blooms of dinoflagellates have most likely been 0.1-second duration when the cells are stimulated. responsible for some of the oil deposits of the world, • The luminescent wake of a moving ship is usually caused primarily by Dinophyceae. including the North Sea oil deposit. • Petroleum deposits and ancient sediments contain 4α- methylsteroidal hydrocarbons, which probably originated from 4α-methylsterols in dinoflagellates

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• The compound responsible for bioluminescence is luciferin, which is • Associated with dinoflagellate luciferin is a luciferin-binding oxidized with the aid of the enzyme luciferase, resulting in the emissions of protein (LBP) which sequesters luciferin at alkaline pH and light. releases it under acidic conditions. • It has been postulated that the flash of bioluminescent light may • Luciferin and luciferase are terms for a general class of compounds, and not occur simply by a lowering of the pH from 8.0 to 6.5. of a specific chemical structure. • Agitation of cells depolarizes the vacuolar membrane, allowing a flux + • Bioluminescence occurs in many organisms in many different phyla, ranging of protons (H ) and acidification of the peripheral cytoplasm. from bacteria to dinoflagellates to jellyfish and brittle stars to worms, fireflies, • Lowering the pH causes two pH-dependent reactions to occur: molluscs, and fish. (1) release of luciferin from its binding protein at acidic pH, and • In bacteria, luciferin is a reduced flavin; in insects it is a (benzo)thiazole (2) activation of luciferase followed by emission of a photon of blue-green nucleus; and in dinoflagellates it is a tetrapyrrole. light.

Dinoflagellates can emit light in three modes: (1) they can flash when stimulated mechanically, chemically, or electrically; A possible partial structure of dinoflagellate luciferin. (2) they can flash spontaneously; and • Likewise, luciferase has different structures, although all luciferases share the (3) late at night they can glow dimly. feature of being oxygenases (enzymes that add oxygen to compounds). • In the basic reaction of bioluminescence, a luciferin is oxidized by a luciferase, resulting in an electronically excited product (P)* which emits a photon (hv) on decomposition:

٤٧ ٤٨ Rhythms • There are two theories concerning the adaptive value of Many Dinophyceae exhibit rhythmic processes, with circadian (which bioluminescence to dinoflagellates, both of which relate to nighttime means literally about (circa) a day (diem)) characteristics. grazing of the dinoflagellates: 1. It produces light via bioluminescence if the cells are stimulated by 1 “Burglar alarm” hypothesis. In this hypothesis dinoflagellates generate a shaking or stirring. signal identifying the location of invertebrate grazers (e.g. copepods) • The greatest luminescence will be produced in the middle of the dark to individuals two levels up the food chain from the dinoflagellates. period, whereas toward morning, flashes will gradually become Bioluminescence generated by dinoflagellates serves to attract smaller and a greater stimulus will be required predators of the grazers of the dinoflagellates. • The rhythm is circadian, as shown by the persistence of changes in brightness of luminescence when the cells are kept in the dark or in 2 “Startle” hypothesis. In this hypothesis, mechanical stimulation of a continuous light. bioluminescent dinoflagellate by a grazer produces a flash of light that startles an invertebrate grazer, such as a copepod, and causes the 2. The , measured, either as oxygen production or carbon copepod to swim away. dioxide fixation is also found to be rhythmic and the rhythm is circadian and continues under conditions of continuous light. • Whichever theory is correct, experiments have shown that copepods • The maximum rate of photosynthesis occurs, as one would expect, in consume only half as many dinoflagellates at night, indicating that the the middle of the day. bioluminescence is serving as a deterrent to grazing.

3. A third rhythm with a circadian period is that of cell division; all cell division occurring during 30 minutes when cultures are in a light–dark cycle. • When the light–dark cycle is 12 : 12, then this 30 minutes spans ٤٩ ”.dawn“ ٥٠

•In Lingulodinium polyedrum, there exists a control over luminescence, photosynthesis, and cell division, so that each process reaches a maximum and then declines in an orderly fashion during each 24 4. A fourth type of rhythm involves the vertical migration of hours (Fig. 7.45). dinoflagellate cells in the water column. •A biological clock, may control all of these processes. • It appears that the part of the cell that may be the controlling agent is • Before dawn, the cells rise to the surface, where they the plasma membrane because there is a rhythmic reorganization of form dense clouds (aggregations), and before night fall, the plasma membrane over a 24-hour period in synchronized cells they again sink to lower depths

• In the marine environment, this vertical migration exposes the cells to several gradients: –(1)Nutrients are more concentrated at lower depths while surface waters are often practically devoid of nutrients. –(2)Temperatures at the surface exceed those in deeper waters. –(3)Variations in light intensities. –(4)differences in washout by the tidal waters in shallow waters,.

Fig. 7.45 Circadian rhythmicity has been identified in at least four distinct biological processes of the unicellular Lingulodinium polyedrum. Although the four rhythms peak at different times of the 24-hour day, as represented here diagrammatically, they are all synchronized with the circadian clock. Perturbations that ٥١ ٥٢ reset the clock will phase-shift all four rhythms simultaneously, suggesting that all of these overt rhythms are driven by a single pacemaker.

Heterotrophic dinoflagellates –(2) pallium feeding where the prey is engulfed by a • An estimated half of the more than 2000 living dinoflagellate species cytoplasmic veil – the palium – with digestion of the lack chloroplasts and are exclusively heterotrophic. • In addition, many dinoflagellates that contain chloroplasts are food taking place outside of the dinoflagellate cell; capable of mixotrophy where a portion of their nutrients is obtained heterotrophically, particularly when nutrient conditions are low in the environment • The different modes of heterotrophy are: –(1) phagotrophy through the direct engulfment of prey;

tentacle

The heterotrophic dinoflagellate Protoperidinium conicum feeding on the Corethron hystria. Initially the dinoflagellate attaches to the prey by a long thin filament (a). Next a pseudopod extends along Drawing of the ingestion of food organisms (other algae, bacteria) by Noctiluca. (a) The tentacle (T) is the filament (b) and engulfs the prey (c), which is digested. in an extended configuration. Any food organisms (FO) that collide with the mucus-covered tentacle tip, (to the tentacle. (N) Nucleus; (OP) oral pouch. (b) The tentacle bends back toward the oral pouch. (c stick٥٣ ٥٤ The cytosome (C) at the base of the oral pouch opens, the tentacle tip is inserted into the cytosome, and the food organisms are swept into a food vacuole. Symbiotic dinoflagellates –(3) peduncle feeding (myzocytosis) involving the uptake of intracellular material of the prey through a cytoplasmic extension • Symbiotic dinoflagellates (zooxanthellae) occur – the peduncle – leaving the plasma membrane and extracellular in almost all species of tropical and reef-building corals, jellyfish, and sea anemones (Cnidaria). material of the prey behind; • The dinoflagellates are coccoid spheres in the –(4) or the uptake of dissolved substances symbiotic state and have been assigned to the genus Symbiodinium. • The host exerts strong control over the translocation of metabolites from the dinoflagellate endosymbiont, resulting in 98% of the carbon fixed by the endosymbiont being released to the host. • The host animal cells secrete the amino acid taurine which causes the dinoflagellate to release photosynthate outside the cell for absorption by the animal cells.

• In the flatworm Amphiscolops langerhansi the symbiotic dinoflagellate is Amphidinium klebsii

• In addition to Dinophyceae living symbiotically inside other organisms, there are other (a) Light micrograph of the dinoflagellate Gymnodinium fungiforme (G) ingesting the protoplasm of organisms that live inside dinoflagellate cells. Dunaliella salina (D). The peduncle (P) of G. fungiforme has attached to D. salina with the protoplasm of D. salina passing through the enlarged and extended peduncle into the dinoflagellate. (b) Scanning ٥٥ ٥٦ electron micrograph of a zoospore of Pfiesteria pisciicida showing the peduncle.

• Order 2 Dinophysiales: cell wall divided vertically into two halves, cells with elaborate extensions of the theca. Classification • One of the more complex organisms in this order is (Fig. 7.56(d), (e)). • There is a single class in the Dinophyta, the Dinophyceae. • Four orders are considered here. • Order 1 Prorocentrales: cell wall divided vertically into two halves; no girdle; two flagella borne at cell apex. • Prorocentrum is an example of the order

(d),(e) Ornithoceros magnificus, a righthand view (d) and an “exploded” view (e) of the theca. (DA) Dorsal accessory moiety of the left sulcal list; (LLG) left lower girdle list; (LS(am)) anterior moiety of the left sulcal list; (LS(pm)) posterior moiety of the left sulcal list; (LUG) left upper girdle list; (RS) right sulcal list; (RLG and Scanning electron micrographs of Prorocentrum (b),(c) Prorocentrum micans, side (b) and front (c) views. RUG) right lower and upper girdle list; (g) girdle; (p) pore; (w) wing. hoffmanianum.

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• Order 4 Gymnodiniales: motile cells with an epicone and hypocone separated by a girdle; theca thin or reduced to empty vesicles. • Order 3 Peridiniales: motile cells with an epicone and hypocone • The life cycle of Gymnodinium pseudopalustre is a representative of separated by a girdle, relatively thick theca. the order. • The algae in this order have the classic dinoflagellate structure with an epicone and hypocone and two furrows, the transverse girdle and the longitudinal sulcus. • Ceratium is widely distributed genus.

Scrippsiella trochoidea.Scanning electron micrographs showing the thecal plates in ventral (a) and dorsal (b) view

٥٩ ٦٠ Ceratium tripos