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Algae with one membrane ofchloroplast endoplasmic

reticulum

Euglenophyta and Dinophyta

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Euglenophyta• In euglenophytes, a single class, Euglenophyceae, is recognized. • It includes about 800 species, most of which are freshwater species,

but some are estuarine and intertidal. • Predominantly unicellular, flagellate. • Only a third of the species have chloroplasts, and the rest are

heterotrophic (both facultative and obligate).• The ability of Euglena to grow heterotrophically in the absence of

plastids is unique amongst the algae. • 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 less prominent.

• The energy storage product is paramylon, a polysaccharide found outside the chloroplast in the cytoplasm. Chrysolaminarin can also accumulate in a liquid form in vacuoles in the cytoplasm.

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A semidiagrammatic drawing of the fine structure of theanterior part of a Euglena cell. (C) Canal; (CER) chloroplast endoplasmic reticulum; (CV) contractile vacuole; (E) eyespot; (LF) long flagellum; (M) mastigonemes; (MB) muciferous body; (Mt)microtubules; (N) nucleus; (P) paraflagellar swelling; (Pa) paramylon; (PG) pellicle groove; (Pl) plasmalemma; (PS) pellicle strip; (Py) pyrenoid; (R) reservoir; (SF) short flagellum.

• The cell shows an anterior bottle-shaped invagination called the reservoir (R) . At the base of the reservoir, there is a contractile vesicle (CV) as well as two flagella, but only one of them emerges through the neck.

• Euglenoid cells have two basal bodies and one or two emergent flagella. The flagella having a paraxonemal rod (paraxial rod) that runs the length of the flagellum inside the flagellarmembrane.

• The one emergent flagellum in Euglena has helically arranged fibrillar hairs (no microtubules) attached along the length of the flagellarmembrane.

• Reproduction is asexual, sexual reproduction has not yet been observed.

• In order to survive when external conditions are unfavorable, some species produce resistant cysts surrounded by a thick mucilaginous sheath that they produce.

(A) Axoneme of flagellum; (B) paraxonemal rod of flagellum

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Eyespot, paraflagellar swelling, and phototaxis• The eyespot (stigma) (E) is a collection

of orange-red lipid droplets, independent of the chloroplast.

• The eyespot is in the anterior part of the cell, curving to ensheath the neck of the reservoir on the dorsal side.

• The eyespot has been reported to contain α-carotene and seven xanthophylls, mainly β-carotene, or a β-carotene derivative, echineone.

• The base of the longer flagellum bears a thickening (P), believed to be a photoreceptor, close to the stigma.

• The swelling is composed of a crystalline body next to the axoneme and inside the flagellar membrane.

• All euglenoid species with an eyespot and flagellar swelling exhibit phototaxis, usually swimming away from bright light (negative phototaxis) and away from darkness toward subdued light (positive photoaxis) to accumulate in a region of low light intensity.

A semidiagrammatic drawing of the fine structure of theanterior part of a Euglena cell. (E) eyespot; (LF) long flagellum; (P) paraflagellar swelling; (R) reservoir; (SF) short flagellum.

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• Euglenoid cells are not surrounded by cell wall, 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

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Scanning electron micrographs of euglenoids showing the helical pellicle of Phacus triqueter (lower left),Euglena acus (middle), and Lepocincilis ovata (upper right).

•The strips are arranged in parallel, are characteristic of the species.

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

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• 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 the only part of the cell covered solely by the plasmalemma.

• The contractile vacuole, in the anterior part of the cell next to the reservoir, has an osmoregulatory function, expelling excessive water taken into the cell.

• It empties into the reservoir, from which the water is carried out through the canal.

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

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• There are two basic types of flagellar movement in the class. • The first group (including the Eutreptiales and Euglenales) has the

flagellum continually motile from base to apex, resulting in cell gyration with the anterior end of the cell tracing a wide circle.

• The second group (including Peranema, Entosiphon, and Sphenomonas) has the flagellum held out straight in front of the cell with just the tip motile, resulting in smooth swimming or gliding locomotion in contact with the substratum or air–water interface.

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• Muciferous bodies (MB) (or mucocysts) occur under the pellicle strips and contain a water-soluble mucopolysaccharide. The muciferous bodies open to the outside through pores between the strips of the pellicle. The muciferousbodies function in the formation of the stalk in Colacium, lorica formation in Trachelomonas, cyst formation and lubrication during euglenoid movement.

• 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.

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

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• Euglena gracilis changes its shape two times per day when grown under the synchronizing effect of a daily light–dark cycle.– At the beginning of the light period, when photosynthetic capacity is low

(as measured by the ability of the cells to evolve oxygen), the population of cells is largely spherical.

– 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.

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Nucleus and nuclear division• The euglenoid nucleus is of the mesokaryotic

type, having – chromosomes that are permanently condensed

during the mitotic cycle, – a nucleolus (endosome) that does not disperse

during nuclear division, – no microtubules from chromosomes to pole

spindles, and – a nuclear envelope that is intact during nuclear

division. • The chromosome number is usually high, and

polyploidy probably occurs in some genera.• Mitosis 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.

• In anaphase, the intact nuclear envelope elongates along the division axis, the nucleolus divides, and the daughter chromosomes disperse into the two daughter nuclei.

A semidiagrammatic drawing of an anaphase nucleus of Euglena with nuclear envelope intact, the nucleolus pinching in two, and the chromosomes not attached to spindle microtubules.١٢

Surviving unfavorable periods.• The cell rounds off and secretes a thick sheath of mucilage that

survives for months until the cell emerges by cracking the cyst.• In conditions of partial desiccation or excessive light, the slime sheath

sometimes acts as a temporary cyst, cells emerging from the sheath as soon as conditions improve.

• In certain genera (Euglena and Eutreptia), cell division within the slime layer leads to the formation of a palmelloid colony, which may form extensive sheets of cells covering many square feet of mud surface.

Scanning electron micrographs of the encystment of Eutreptiella gymnastica. The vegetative cell (a) loses its flagella, forms a large number of paramylon grains, and begins to round up (b). The cell swells and produces a mucilaginous covering (c).

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Chloroplasts and storage products• The euglenoid chloroplasts are

surrounded by two membranes of the chloroplast envelope plus one membrane of chloroplast endoplasmic reticulum; the latter membrane is not continuous with the nuclear membrane.

• The chloroplasts are usually discoid or plate-like with a central pyrenoid.

• The thylakoids are grouped into bands of three, with two thylakoid bands traversing the pyrenoid.

• A shield of paramylon grains surrounds the pyrenoid, but outside the chloroplast, in phototropically grown cells.

• Paramylon granules are distributed throughout the cytoplasm in heterotrophically grown cells in the dark.

• Their shape is often characteristic of the Euglena species that produces them.

Euglena rustica. Transmission electron micrographof a mid sagittal section of a cell. (Arrow) Paramylongranule; (cp) chloroplast; (Py) pyrenoid; (Nu) nucleus

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(a) Euglena gracilis. (b) Astasia klebsii. (c) Eutreptiellamarina. (d) Trachelomonas grandis. (e) Phacus triqueter. (C) Chloroplast; (Ca) canal; (CV) contractile vacuole; (E) eyespot; (Ev) envelope; (F) emergent flagellum; (FS) flagellar swelling; (M) mitochondrion; (N) nucleus; (P)paramylon grains or paramylon sheath around chloroplast; (R) reservoir.

• Paramylon granules which are often visible as colorless or white particles in light microscopy are composed of a carbohydrate that although isomeric with starch (amylon), was not stained with iodine. For this reason, they were termed paramylon granules.

• They have since been shown to be composed of a β-1,3 linked glucan.

• The liquid storage product, chrysolaminarin, can be an alternative storage product in some Euglenophyceae.

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• Paramylon belongs to a group of naturally occurring polysaccharides whichare considered bioactive compounds.

• The main interest in β-glucans stems from their ability to act as nonspecific immune system stimulants, by binding to a specific site on monocytes/macrophages and granulocytes.

• They have been successfully used in aquaculture to strengthen the nonspecific defense of many important species of fishes and shrimps by injection, immersion, or in the feed.

• Sulfated derivatives of Euglena paramylon, in particular, have shown anti-HIV (human immunodeficiency virus) activity.

• Moreover, it has been suggested that this polysaccharide has a cholesterol-lowering effect when incorporated in the diet of either humans or animals, and moderates the postprandial blood glucose and insulin response in humans.

• Since Euglena gracilis can accumulate large quantities of paramylon when grown in the presence of an utilizable carbon source, it could represent an alternative source of this compound to Saccharomyces cerevisiae (baker’s yeast), which is currently exploited industrially for its extraction.

• Moreover, Euglena has been investigated as a potential protein source, and a promising dietary supplement due to its content of highly nutritious proteins and PUFAs, and to the simultaneous production of antioxidant compounds, such as β-carotene, vitamin C, and vitamin E.

importance of Paramylon

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Nutrition• The Euglenophyceae have a number of modes of nutrition,

depending on the species involved. • No euglenoid has yet been demonstrated to be fully

photoautotrophic – capable of living on a medium devoid of all organic compounds (including vitamins).

• All green euglenoid flagellates so far studied are photoauxotrophic–needing at least one vitamin. Euglena gracilis has an absolute requirement for vitamin B12.

• Vitamin B12-starved cells increase in cell volume, sometimes to 10 times the size of control organisms, the cells in the final stage of vitamin B12 starvation often being poly-lobed, poly-nucleate, and containing more than the normal number of chloroplasts per cell.

• During vitamin B12 starvation, total cellular RNA and protein increase 400% to 500% compared with controls.

• During vitamin B12 starvation the protein increase 400% to 500% and the total DNA increases about 180%.

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• As Euglena cells age, they become immobile and spherical, with a tendency to form enlarged “giant” cells and to accumulate orange to black pigment bodies.

• Aging also results in the formation of larger numbers of lysosomes and microbodies with an increase in the degradation of organelles.

• 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.

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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; (g) Golgi; (ir) ingestion rods; (lf) leading flagellum; (m) mitochondrion; (n) nucleus; (p) paramylon; (tf) trailing flagellum

• Three orders of euglenoids are presented here.• Order 1 Heteronematales: two emergent flagella, the longer

flagellum directed anteriorly and the shorter one directed posteriorlyduring swimming; cells have special ingestion organelle.– Peranema trichophorum is a euglenoid that ingests other cells and

detritus. Here the colorless cells have a special ingestion organelle, and are phagocytic, taking up food particles whole and digesting them in food vesicles.

Classification

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• Order 2 Eutreptiales: two emergent flagella, one directed anteriorlyand the other laterally or posteriorly during swimming; no special ingestion organelle.– Eutreptia and Eutreptiella are estuarine or marine genera, while

Distigma is characteristic of acid freshwaters.

Eutreptia DistigmaEutreptiella

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• Order 3 Euglenales: two flagella, only one of which emerges from the canal; no special ingestion organelle.

• Common genera in the order are the green photosynthetic Euglena, Trachelomonas, and Phacus, as well as the colorless osmotrophicAstasia.

• 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.

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

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Dinophyta(Pyrrophyta)

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• Dinoflagellates are unicellular, biflagellate organisms.• The term "dinoflagellate" 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.

٢٣ AV, amphiesmal (thecal) vesicle; TP, thecalplate within an amphiesmal vesicle; .

• Dinoflagellates are surrounded by a complex covering called the amphiesma, which consists of the plasma membrane and an underlying single layer of flattened vesicles (amphiesmal vesicles = "thecal vesicles").

• Based on their amphiesma, dinoflagellates are separated into two major subgroups

– In thecate, or ‘armoured’, forms, the amphiesmal vesicles contain cellulosicplates (= 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.

Representative drawings of different types ofamphiesmal arrangements in the Dinophyceae. The outer plasma membrane (Pl). The cellulosic thecal plate (T). vesicle (V).

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• The exact number and arrangement of thecal plates in armored dinoflagellates are characteristic of the particular genus, therefore they are used for taxonomic differentiation.

• Also, the form of the thecal plates varies from spherical forms like Peridinium to elongate horn-like forms such as Ceratium.

(a ventral view of Cachoninaniei showing the arrangement of thecal plates:

(b) Dorsal and ventral views showing the thecal plates of Ceratium

(b) Gymnodinium neglectum

• 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 theca is completely shed (ecdysis) at cell division, followed by the formation of a thickened pellicle around the cell to form an ecdysal cyst.

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

• In most dinophyceae, outside of the plasma membrane there is a glycocalyx composed of acidic polysaccharides.

Scales• Although relatively rare, scales occur outside the plasma membrane

in some Dinophyceae

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• A typical motile dinoflagellate consists of an epicone (epitheca) and hypocone (hypotheca) divided by the transverse girdle or cingulum.

• 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.

Flagella• Generally, dinoflagellates have a transverse flagellum that fits into

the transverse girdle and a longitudinal flagellum that projects out from the longitudinal sulcus.

A representation of thecal plates on a typical armored dinoflagellate .

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

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• Fibrillar hairs may cover the entire longitudinal flagellum.

• The transverse flagellum has a helical shape and attached to the cell body except for the tip .

• The transverse flagellum consists of – (1) an axoneme whose form

approximates a helix, – (2) a striated strand that runs parallel to

the longitudinal axis of the axoneme but outside the loops of the coil, and

– (3) a flagellar sheath that encloses both 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 have been drawn in.

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• The transverse flagellum is thought to be responsible for driving the cell forward and it also brings about rotation, while the longitudinal flagellum is the propelling and steering flagellum.

• The axoneme of transverse flagellum is a left-handed screw; waves propagated from the attached end toward the free end.

• The flagellum causes a direct forward movement while at the same time causing the cell to rotate.

• The flagellar beat always proceeds counterclockwise when seen from the cell apex.

• The longitudinal flagellum swings in a narrow orbit acting as a rudder.

• The longitudinal flagellum can also reverse the swimming direction: it stops beating, points in a different direction by bending, and then resumes beating.

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• The dinoflagellates are one of the fastest swimmers among the algae

• Marine dinoflagellates frequently move into deeper, nutrient-rich waters at night and better illuminated waters near the surface during the day.

• The diel (over a 24-hour period) migrations of dinoflagellates are 5 to 10 m in relatively quiet waters.

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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 osmoregulatoryfunction similar to that of a contractile vacuole.

(a) Drawing of Amphidinium carteri. The smallepicone is separated from thehypocone by the encircling girdle,which contains the transverseflagellum. The peripheral chloroplast(C) is shown only at the sides of thecell so the positions of the nucleus(N), mitochondria (M), pyrenoid(Py), and the pusule (P) associatedwith the flagellar canals can be seen.(b) Longitudinal section through apusule showing the flagellar poreconstriction (C), the pusule vesicles(V), and the flagellum (F).(c) Transverse section of a pusuleshowing the flagellum within theflagellar canal (Fc) and the pusulevesicles (V) opening into the canal.

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Chloroplasts and pigments• 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 c2 are present in the chloroplasts, with peridinin and neoperidinin being the main carotenoids.

• About half of the Dinophyceae have pyrenoids 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 Peridiniumsp., a dinoflagellate showing many of the features of the class. (C) Chromosome; (CE) chloroplast envelope; (CER) chloroplast endoplasmic reticulum; (E) epicone;(G) Golgi apparatus; (Gr) girdle; (H) hypocone; (L) lipid globule; (LF) longitudinal flagellum; (Nu) nucleolus; (P) trichocyst pore; (S) starch; (T) trichocyst; (TF) transverse flagellum; (TP) thecal plate.٣٢

Phototaxis and eyespots• Less than 5% of the Dinophyceae contain eyespots (e),

and those that do are mostly freshwater species; yet the eyespots are among the most complex in the algae.

• An eyespot is not necessary for a phototactic response.

Woloszynskia tenuissima. Ventral view showing the two flagella, the girdle (g), and the eyespot (e).

(a) A ventral view of a cell of Glenodinium foliaceum showing the location of the eyespot (e). (b) Threedimensional diagram of the eyespot area. The two flagella arise just above the lamellar body (l). The eyespot (e) lies beneath the sulcus. (mt) Microtubular roots; (b) banded root; (t) thecal plate.

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NucleusThe nucleus of the dinoflagellata is unusual where is has

many odd features unique to the group. • The DNA does not seem to have histones (basic

proteins which the DNA coils around). • Dinoflagellates have more DNA in their nucleus than

other eukaryotes, so much so that the nucleus often fills half the volume of the cell .This implies that a large amount of the DNA is genetically inactive (structural DNA) in dinoflagellates.

• Chromosomes in its nucleus are not scattered, but are attached to the nuclear membrane.

• Upon division, the nuclear membrane does not break down as in plants and animals.

• The chromosomes remain condensed during mitosis and even during interphase, though they do unwind for replication of the DNA.

• Such permanently condensed organization of chromatin (chromosomes) is different from either prokarytoic or eukaryotic cells.

• This unusual nuclear situation is termed mesokaryotic or dinokaryotic in the belief that it was transitional between prokaryotic and eukaryotic structure, but it is now thought to be a uniquely derived feature of the Dinogflagellata .

A dinoflagellate nucleus. Note the Condensed chromosomes with characteristic banding pattern

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Trichocysts• Trichocysts (T) are membrane-bound

organelles containing proteinaceousthreadlike structures found in a number of species.

• In these species, the cell wall is perforated by pores (P) through which trichocysts are discharged or projected from the cell under stimulation, often hundreds per cell.

• The actual benefit of trichocysts to the cell (if any) is still obscure. – They could be a mechanism for quick

escape as the cells move sharply in the opposite direction of discharge,

– or they have a protective or evasive function since they could be able to directly “spear” a naked intruder.

Accumulation body• This is a large vesicle containing the

remains of digested organelles.

Light and electron microscopical drawings of Peridiniumsp., a dinoflagellate showing many of the features of the class. (C) Chromosome; (CE) chloroplast envelope; (CER) chloroplast endoplasmic reticulum; (E) epicone;(G) Golgi apparatus; (Gr) girdle; (H) hypocone; (L) lipid globule; (LF) longitudinal flagellum; (Nu) nucleolus; (P) trichocyst pore; (S) starch; (T) trichocyst; (TF) transverse flagellum; (TP) thecal plate.

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Resting spores or cysts or hypnospores and fossil Dinophyceae• The resting spore or cyst of most dinoflagellates is morphologically

distinct from the parent cell.• The cell walls of the cysts are highly resistant to decay and contain

dinosporin, a chemical similar to sporopollenin in the pollen of higher plants.

• The cysts are extremely resistant and are preserved in ancient sediments where they are of value in paleoecological studies.

• Many extant resting spores are identical to fossilized cysts.

Dinoflagellate cysts. (a) Alexandrium catenella. (b)Calciodinellum operosum. (c) Scrippsiellatrophoidea. (d) Gonyaulax grindleyi. (e) Polykrikos schwartzii. (f) Gymnodinium catenatum.

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• The process of encystment or resting spore formation is regulated by a complex interaction of – day length, – temperature, and – nutrient concentration

• Melatonin levels increase by several orders of magnitude 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.

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Toxins• Some Dinophyceae have the ability to produce very potent toxins

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.

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Toxins have been divided into different classes based on the syndromes associated with exposure to them, such as

1. Diarrhetic shellfish poisoning. This occurs primarily in temperate regions and is caused by species of the planktonic dinoflagellates Exuviaella, Dinophysis, and 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.

Prorocentrum

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• Diarrhetic shellfish poisoning is caused by the polyether carboxylic acidsokadaic acid, macrolide toxins, and yessotoxin.

• The polyether carboxylic acid okadaic acid is initially formed inside the dinoflagellate cell as dinophysistoxin-4. This weakly’sulfated derivative of okadaic acid is not toxic to the dinoflagellate cell.

• Dinophysistoxin-4 is either excreted by the dinoflagellate cell, or is released on death of the cell.

• Dinophysistoxin- 4 is hydrolyzed in the medium to okadaic acid diol ester, which is lipid soluble and can pass through cell membranes. Thus, okadaicacid diol ester can be taken up by cells which further hydrolyze the compound to okadaic acid.

• Research has shown that okadaic acid with a free acid moiety is the toxic form of the compound.

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2 Ciguatera fish poisoning. This occurs primarily in tropical regions with the common causative agent being Gambierdiscus.

• The dinoflagellate contains gambieric acids, ciguatoxins, and maitotoxins, that are very potent Ca2+ channel (pathways for calcium entry into cells) activators that result in breakdown of the cell membrane.

• Gambierdiscus is epyphytic on macroalgae that are eaten by herbivorous fish and shellfish, which, in turn, are eaten by humans. In French Polynesia alone, approximately 1000 cases of ciguatera fish poisoning are reported every year.

• The term ciguatera is derived from the Spanish term “cigua” for the turban shell, which was commonly eaten before the illness developed.

• 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

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• 3 Paralytic shellfish poisoning. This is caused by species of Alexandrium (A. catanella, A. acatenella, A. excavatum, A. tamarensis), Pyrodinium bahamense, and Gymnodinium catenatum.

• These dinoflagellates produce a group of toxins that are derivatives of saxitoxin.

• Saxitoxins are potent neurotoxins acting upon voltage-gated Na+ -channels, preventing influx of Na+, thereby preventing the generation of an action potential.

Gymnodinium catenatumPyrodinium bahamenseAlexandrium catanella

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• Alexandrium excavatum is a toxic red-tide dinoflagellate.

• The vegetative cells divide to produce motile gametes that subsequently fuse.

• Fusing pairs swim poorly and settle in the water.

• The motile quadriflagellate planozygotesswim for a few days before losing their flagella and thecal plates, and encyst to 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 meiosis 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. (b) Scanning electron micrograph showing the smooth surface of a hypnospore.

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• The amount of toxin in cells of Alexandrium is relatively low when nutrients are in ready supply.

• The toxin concentration is highest under conditions of phosphorus deficiency, possibly because free amino acids (precursors of toxins) accumulate under conditions of phosphorus deficiency.

• Cells of dinoflagellates that produce toxins are avoided by grazing copepods.

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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.

2 Wind: A strong, offshore wind aids upwelling, whereas a gentle onshore wind 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 dinoflagellates and can prevent the development of red tides.

3 Light intensity: There is usually a period of bright, sunny, calm weather before outbreaks.

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 cause of these blooms. It is thought that preceding blooms of diatoms 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), and also allow the production of organic nutrients such as vitamin B12, which are important for their growth.

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Dinoflagellates and oil and coal deposits• Blooms of dinoflagellates have most likely been

responsible for some of the oil deposits of the world, 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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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,

emitting a bluish-green (maximum wavelength at 474 nm) flash of light of 0.1-second duration when the cells are stimulated.

• The luminescent wake of a moving ship is usually caused primarily by Dinophyceae.

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• The compound responsible for bioluminescence is luciferin, which is oxidized with the aid of the enzyme luciferase, resulting in the emissions of light.

• Luciferin and luciferase are terms for a general class of compounds, and not of a specific chemical structure.

• Bioluminescence occurs in many organisms in many different phyla, ranging from bacteria to dinoflagellates to jellyfish and brittle stars to worms, fireflies, molluscs, and fish.

• In bacteria, luciferin is a reduced flavin; in insects it is a (benzo)thiazolenucleus; and in dinoflagellates it is a tetrapyrrole.

• Likewise, luciferase has different structures, although all luciferases share the 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:

A possible partial structure of dinoflagellate luciferin.

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• Associated with dinoflagellate luciferin is a luciferin-binding protein (LBP) which sequesters luciferin at alkaline pH and releases it under acidic conditions.

• It has been postulated that the flash of bioluminescent light may occur simply by a lowering of the pH from 8.0 to 6.5.

• Agitation of cells depolarizes the vacuolar membrane, allowing a flux of protons (H+) and acidification of the peripheral cytoplasm.

• Lowering the pH causes two pH-dependent reactions to occur: (1) release of luciferin from its binding protein at acidic pH, and (2) activation of luciferase followed by emission of a photon of blue-green

light.

Dinoflagellates can emit light in three modes:(1) they can flash when stimulated mechanically, chemically, or electrically; (2) they can flash spontaneously; and (3) late at night they can glow dimly.

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• There are two theories concerning the adaptive value of bioluminescence to dinoflagellates, both of which relate to nighttime grazing of the dinoflagellates:

1 “Burglar alarm” hypothesis. In this hypothesis dinoflagellates generate a signal identifying the location of invertebrate grazers (e.g. copepods) to individuals two levels up the food chain from the dinoflagellates. Bioluminescence generated by dinoflagellates serves to attract predators of the grazers of the dinoflagellates.

2 “Startle” hypothesis. In this hypothesis, mechanical stimulation of a bioluminescent dinoflagellate by a grazer produces a flash of light that startles an invertebrate grazer, such as a copepod, and causes the copepod to swim away.

• Whichever theory is correct, experiments have shown that copepods consume only half as many dinoflagellates at night, indicating that the bioluminescence is serving as a deterrent to grazing.

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RhythmsMany Dinophyceae exhibit rhythmic processes, with circadian (which

means literally about (circa) a day (diem)) characteristics.1. It produces light via bioluminescence if the cells are stimulated by

shaking or stirring. • The greatest luminescence will be produced in the middle of the dark

period, whereas toward morning, flashes will gradually become smaller and a greater stimulus will be required

• 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 continuous light.

2. The photosynthesis, measured, either as oxygen production or carbon dioxide fixation is also found to be rhythmic and the rhythm is circadian and continues under conditions of continuous light.

• The maximum rate of photosynthesis occurs, as one would expect, in the middle of the day.

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 “dawn.”

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4. A fourth type of rhythm involves the vertical migration of dinoflagellate cells in the water column.

• Before dawn, the cells rise to the surface, where they form dense clouds (aggregations), and before night fall, 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,.

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• 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 hours (Fig. 7.45).

• A biological clock, may control all of these processes.• It appears that the part of the cell that may be the controlling agent is

the plasma membrane because there is a rhythmic reorganization of the plasma membrane over a 24-hour period in synchronized cells

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 reset the clock will phase-shift all four rhythms simultaneously, suggesting that all of these overt rhythms are driven by a single pacemaker.

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Heterotrophic dinoflagellates• An estimated half of the more than 2000 living dinoflagellate species

lack chloroplasts and are exclusively heterotrophic. • In addition, many dinoflagellates that contain chloroplasts are

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;

Drawing of the ingestion of food organisms (other algae, bacteria) by Noctiluca. (a) The tentacle (T) isin an extended configuration. Any food organisms (FO) that collide with the mucus-covered tentacle tip, stick to the tentacle. (N) Nucleus; (OP) oral pouch. (b) The tentacle bends back toward the oral pouch. (c) 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.

tentacle

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– (2) pallium feeding where the prey is engulfed by a cytoplasmic veil – the palium – with digestion of the food taking place outside of the dinoflagellate cell;

The heterotrophic dinoflagellate Protoperidinium conicum feeding on the diatom Corethron hystria. Initially the dinoflagellate attaches to the prey by a long thin filament (a). Next a pseudopod extends along the filament (b) and engulfs the prey (c), which is digested.

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– (3) peduncle feeding (myzocytosis) involving the uptake of intracellular material of the prey through a cytoplasmic extension – the peduncle – leaving the plasma membrane and extracellularmaterial of the prey behind;

– (4) osmotrophy or the uptake of dissolved substances

(a) Light micrograph of the dinoflagellate Gymnodinium fungiforme (G) ingesting the protoplasm of 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 electron micrograph of a zoospore of Pfiesteria pisciicida showing the peduncle.٥٦

Symbiotic dinoflagellates• Symbiotic dinoflagellates (zooxanthellae) occur

in almost all species of tropical and reef-building corals, jellyfish, and sea anemones (Cnidaria).

• The dinoflagellates are coccoid spheres in the 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 organisms that live inside dinoflagellate cells.

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Classification• 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

(b),(c) Prorocentrum micans, side (b) and front (c) views.Scanning electron micrographs of Prorocentrumhoffmanianum.

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• Order 2 Dinophysiales: cell wall divided vertically into two halves, cells with elaborate extensions of the theca.

• One of the more complex organisms in this order is Ornithocercus(Fig. 7.56(d), (e)).

(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 sulcallist; (LS(pm)) posterior moiety of the left sulcal list; (LUG) left upper girdle list; (RS) right sulcal list; (RLG and RUG) right lower and upper girdle list; (g) girdle; (p) pore; (w) wing.

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• Order 3 Peridiniales: motile cells with an epicone and hypoconeseparated by a girdle, relatively thick theca.

• 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

Ceratium tripos٦٠

• Order 4 Gymnodiniales: motile cells with an epicone and hypoconeseparated by a girdle; theca thin or reduced to empty vesicles.

• The life cycle of Gymnodinium pseudopalustre is a representative of the order.